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THE RARE EARTHS




  THE RARE EARTHS

  THEIR OCCURRENCE, CHEMISTRY,
  AND TECHNOLOGY

  BY
  S. I. LEVY
  B.A. (CANTAB.), B.SC. (LOND.), A.I.C.

  LATE HUTCHINSON RESEARCH STUDENT OF ST. JOHN’S COLLEGE
  CAMBRIDGE

  WITH ILLUSTRATIONS

  LONDON
  EDWARD ARNOLD
  1915

  [All rights reserved]




PREFACE


During the thirty years which have elapsed since Dr. Auer’s application
of the rare earths to the production of artificial light, the
incandescent mantle industry has developed to an extent which gives it a
prominent place among those chemical industries which may be considered
essential to modern civilisation. This technical development has in turn
assisted and stimulated the scientific examination of the elements of
this group, with the result that ordered and accurate knowledge is
beginning to replace the confused and uncertain data which had been
collected by earlier workers in the field. These advances have served to
emphasise the scientific interest and importance of the rare earth
group, and the difficulty of bringing it into relation with the other
elements. The relatively scant attention devoted to the study of this
province of inorganic chemistry by teachers and students in England is
probably due no less to the difficulty in classification, and the
uncertainty with regard to the homogeneity and individuality of the
various members of the family--an uncertainty by no means entirely
removed even now--than to the fact that the very extensive literature on
the subject is somewhat confused and difficult of access, especially to
those unfamiliar with the French and German languages.

The present work is intended to give a general but fairly comprehensive
account of the rare earth group. In accordance with general usage, the
elements zirconium and thorium have been included, though these are now
recognised as falling outside the limits of the rare earth group proper.
The inclusion of titanium, which chemically is so far removed from the
cerium and yttrium elements, has been considered desirable, not only on
account of its general occurrence in the rare earth minerals, and its
position in Group IVB with zirconium, cerium, and thorium, but also on
account of its increasing chemical and technical interest, and its use
in the ordinary quantitative laboratory operations.

Though the nature of the matter embraced has rendered the division into
three parts desirable, the whole subject has been treated primarily from
the chemical standpoint. In view, however, of the occurrence of
considerable quantities of monazite within the British Empire, and of
the possibility that in the near future the Brazilian fields will not
remain the sole source of thorium nitrate, stress has been laid on the
technical aspect, which is more especially developed as regards the
production of monazite and the incandescent mantle industry in Chapters
VII and XVII-XX.

In the preparation of Part I full use has been made of Dana’s
indispensable ‘System of Mineralogy,’ as well as of the encyclopædic
‘Handbuch’ of Hintze, whilst for Part II the excellent monograph of R.
J. Meyer, in Abegg’s ‘Handbuch,’ Vol. III, Div. I, and the work of the
same author and Hauser, ‘Die Analyse der seltenen Erden und der
Erdsäuren,’ Vols. XIV-XV of ‘Die Chemische Analyse,’ have been of
service.

I have great pleasure in expressing my gratitude to Mr. A. Hutchinson,
of Pembroke College, Cambridge, who has kindly read for me the
manuscript of Part I, and suggested improvements; to Dr. H. J. H.
Fenton, of Christ’s College, who has given me similar assistance in Part
II; and to Dr. S. Ruhemann, of Gonville and Caius College, who has read
Parts II and III. I am also greatly indebted to Mr. E. J. Holmyard, of
Sidney Sussex College, who helped me with the preparation of Part II;
and to Mr. H. M. Spiers, of Gonville and Caius College, who read the
proofs for me with special thoroughness and care.

I have also to thank Professor Soddy and his publishers, Messrs.
Longmans, Green & Co., for kind permission to reproduce from ‘The
Chemistry of the Radio-Elements’ the diagram on p. 138.

  S. I. LEVY.




CONTENTS


  PART I
  _OCCURRENCE OF THE RARE EARTHS_

  CHAPTER                                                           PAGE

      I. THE NATURE OF THE MINERALS AND THEIR MODE OF OCCURRENCE       1

     II. THE SILICATES                                                30

         (_a_) _Silicates of Yttrium and Cerium Metals_--Cerite;
               Gadolinite, Glowing of Minerals; Allanite,
               Hellandite, Thalénite and Thortveitite; etc.

         (_b_) _Silicates of Thorium and Zirconium_--Thorite,
               Zircon, Naegite; etc.

         (_c_) _Mixed Silicates_--Eudialyte, Beckelite; etc.

    III. THE TITANO-SILICATES AND TITANATES                           52

         (_a_) _Titano-silicates_--Yttrotitanite, Titanite; etc.

         (_b_) _Titanates_--Yttrocrasite, Delorenzite, Ilmenite;
               etc.

     IV. THE TANTALO-COLUMBATES                                       60

         (_a_) _Containing no Titanium Dioxide_--Samarskite
               (Annerödite), Plumboniobite, Yttrotantalite,
               Fergusonite, Sipylite; etc.

         (_b_) _Containing Titanium Dioxide_--Æschynite; the
               isodimorphous series Euxenite, Polycrase,
               Blomstrandine, Priorite; Risörite, Wiikite; etc.

      V. THE OXIDES AND CARBONATES                                    72

         (_a_) _Oxides_--Uraninite, Thorianite, Baddeleyite;
               Rutile, Anatase and Brookite; etc.

         (_b_) _Carbonates_--Lanthanite; Parisite (Synchisite),
               Cordylite; etc.

     VI. THE PHOSPHATES AND HALIDES                                   82

         (_a_) _Phosphates_--Monazite, Xenotime (Hussakite); etc.

         (_b_) _Halides_--Yttrocerite, Yttrofluorite; etc.

    VII. THE MONAZITE SANDS                                           90

   VIII. RADIOACTIVITY OF THE MINERALS                                99


  PART II
  _THE CHEMISTRY OF THE ELEMENTS_

     IX. GENERAL PROPERTIES OF THE CERIUM AND YTTRIUM GROUPS         111

      X. GENERAL METHODS OF SEPARATION                               142

     XI. THE CERIUM GROUP--CERIUM                                    156

    XII. THE CERIUM GROUP (CONTINUED)--LANTHANUM, PRASEODYMIUM,
         NEODYMIUM, AND SAMARIUM                                     168

   XIII. THE TERBIUM GROUP                                           184

    XIV. THE ERBIUM AND YTTERBIUM GROUPS--YTTRIUM AND SCANDIUM       194

     XV. THE GROUP IVA ELEMENTS--TITANIUM                            219

    XVI. THE GROUP IVA ELEMENTS (CONTINUED)--ZIRCONIUM AND
         THORIUM                                                     238


  PART III
  _THE TECHNOLOGY OF THE ELEMENTS_

   XVII. THE INCANDESCENT MANTLE INDUSTRY--HISTORICAL AND GENERAL
         INTRODUCTION                                                265

  XVIII. THE CHEMICAL TREATMENT OF MONAZITE                          275

    XIX. THE MANUFACTURE OF MANTLES FROM COTTON AND RAMIE            291

     XX. ARTIFICIAL SILK--ITS PRODUCTION AND USE IN THE MANTLE
         INDUSTRY                                                    301

    XXI. OTHER TECHNOLOGICAL USES OF THE CERIUM AND YTTRIUM
         ELEMENTS, ZIRCONIUM AND THORIUM                             313

   XXII. THE INDUSTRIAL APPLICATIONS OF TITANIUM AND ITS COMPOUNDS   325

         INDEX                                                       342




TABLE OF ABBREVIATIONS EMPLOYED IN THE REFERENCES


  _Abstr. Chem. Soc._           Abstracts in Journal of the Chemical
                                Society.

  _Amer. Chem. J._              American Chemical Journal.

  _Amer. J. Sci._               American Journal of Science.

  _Annalen_                     Justus Liebig’s Annalen der Chemie.

  _Ann. Chim. Phys._            Annales de Chimie et de Physique.

  _Astrophys. J._               Astrophysical Journal.

  _Ber._                        Berichte der Deutschen chemischen
                                Gesellschaft.

  _Berz. Jahres._               Berzelius’ Jahresbericht über die
                                Fortschritte der Chemie und Mineralogie.

  _Bull. Imp. Inst._            Bulletin of the Imperial Institute.

  _Bull. Soc. chim._            Bulletin de la Société chimique de
                                France.

  _Bull. Soc. franc. Min._      Bulletin de la Société française de
                                Minéralogie.

  _Bull. Soc. franc. Photog._   Bulletin de la Société française de
                                Photographie.

  _Bull. U. S. Geol. Survey_    Bulletin of the United States Geological
                                Survey.

  _Cass. Mag._                  Cassier’s Magazine.

  _Centr. Min._                 Centralblatt für Mineralogie, Geologie
                                und Paläontologie.

  _Chem. Eng._                  Chemical Engineer.

  _Chem. Ind._                  Chemische Industrie.

  _Chem. News_                  Chemical News.

  _Chem. Zeitg._                Chemiker Zeitung.

  _Chem. Zentr._                Chemisches Zentralblatt.

  _Compt. rend._                Comptes rendus hebdomadaires des Séances
                                de l’Académie des Sciences.

  _Dingl. Polyt. J._            Dingler’s Polytechnisches Journal.

  _D. R. P._                    Deutsche Reichspatentschrift.

  _E._                          English Patent Specification.

  _Elect. chem. Ind._           Electrochemical Industry (since 1904,
                                Electrochemical and Metallurgical
                                Industry).

  _F._                          Brevet d’Invention de la République
                                Française.

  _J. Amer. Chem. Soc._         Journal of the American Chemical
                                Society.

  _J. Gasbel._                  Journal für Gasbeleuchtung.

  _J. Gaslighting_              Journal of Gaslighting.

  _J. Ind. Eng. Chem._          Journal of Industrial and Engineering
                                Chemistry.

  _J. pr. Chem._                Journal für practische Chemie.

  _J. Russ. Phys. Chem. Soc._   Journal of the Physical and Chemical
                                Society of Russia.

  _J. Soc. Chem. Ind._          Journal of the Society of Chemical
                                Industry.

  _Met. Chem. Eng._             Metallurgical and Chemical Engineering.

  _Min. Mag._                   Mineralogical Magazine and Journal of
                                the Mineralogical Society.

  _Monats._                     Monatshefte für Chemie und verwandte
                                Theile anderer Wissenschaften.

  _Phil. Mag._                  Philosophical Magazine.

  _Phil. Trans._                Philosophical Transactions of the Royal
                                Society of London.

  _Pogg. Ann._                  Poggendorff’s Annalen der Physik und
                                Chemie.

  _Proc. Amer. Acad._           Proceedings of the American Academy.

  _Proc. Chem. Soc._            Proceedings of the Chemical Society.

  _Proc. Roy. Soc._             Proceedings of the Royal Society.

  _Prog. Age_                   Progressive Age (now Gas Age).

  _Publ. Astrophys. Observ.     Publikationen des Astrophysikalischen
  Potsdam_                      Observatoriums zu Potsdam.

  _Schweigg._                   J. Schweigger’s Journal für Chemie und
                                Physik.

  _Sitzungsber. kaiserl. Akad.  Sitzungsberichte der kaiserlich Akademie
  Wiss. Wien_                   der Wissenschaften zu Wien.

  _Sitzungsber. königl. Akad.   Sitzungsberichte der königlich Akademie
  Preussischen Wiss. Berlin_    der Wissenschaften zu Berlin.

  _Stahl Eisen_                 Stahl und Eisen.

  _Trans. Amer. El. chem. Soc._ Transactions of the American
                                Electrochemical Society.

  _Trans. Amer. Inst. Min.      Transactions of the American Institute
  Eng._                         of Mining Engineers.

  _Trans. Chem. Soc._           Transactions of the Chemical Society.

  _Tsch. Min. Mitt._            Tschermak’s Mineralogische
                                Mittheilungen.

  _U. S. Geol. Survey_          United States Geological Survey--Mineral
                                Resources of the United States.

  _U. S. P._                    United States Patent Specification.

  _Zeitsch. anal. Chem._        Zeitschrift für analytische Chemie.

  _Zeitsch. angew. Chem._       Zeitschrift für angewandte Chemie.

  _Zeitsch. anorg. Chem._       Zeitschrift für anorganische Chemie.

  _Zeitsch. Elektrochem._       Zeitschrift für Elektrochemie.

  _Zeitsch. Kryst. Min._        Zeitschrift für Krystallographie und
                                Mineralogie.

  _Zeitsch. physikal. Chem._    Zeitschrift für physikalische Chemie,
                                Stöchiometrie und Verwandtschaftslehre.

  _Zeitsch. pr. Geol._          Zeitschrift für praktische Geologie.

  _Zeitsch. wiss. Photochem._   Zeitschrift für wissenschaftliche
                                Photographie, Photophysik und
                                Photochemie.




INTRODUCTION

By SIR WILLIAM CROOKES, O.M., F.E.S.


The mysterious group of substances to which have been given the title of
“rare earths” has long been the subject of my special study, and no one
knows better the magnitude of the difficulties encountered in the
investigation, or realises more clearly the comparative insignificance
of the knowledge we have acquired. The rare earths constitute the most
striking example of the association of chemical substances with others
which are closely allied to themselves, and from which they are
separable only with extreme difficulty. They form a group to themselves,
sharply demarcated from the other elements, and it is my belief that by
following the study of them to the utmost limits, we may arrive at the
explanation of what the chemical elements really are and how they
originated, and discover the reasons for their properties and mutual
relations. When this knowledge has been wrested from Nature chemistry
will be established upon an entirely new basis. We shall be set free
from the need for experiment, knowing _a priori_ what the result of each
and every experiment must be; and our knowledge then will as much
transcend our present scientific systems as the knowledge of the skilled
mathematician of the present day exceeds that of primitive man, counting
upon his fingers. The great problem of the nature and genesis of the
elements is approaching solution, and when the consummation is reached
it will undoubtedly be found that the study of the rare earths has been
an important factor in bringing it about.

There has long been a need for a work in the English language dealing
historically and descriptively with these substances, and Mr. Levy’s
book is well fitted to fill the gap. The chapters on the technical
applications of the rare earths are particularly valuable, and the
chemical aspect of the incandescent lighting industry is admirably
treated. The author is to be congratulated upon having successfully
achieved an important and useful piece of work.

  WILLIAM CROOKES.

  _December 1914._




THE RARE EARTHS




PART I

OCCURRENCE OF THE RARE EARTHS


CHAPTER I

THE NATURE OF THE MINERALS AND THEIR MODE OF OCCURRENCE

The history of the rare earth minerals begins in the year 1751, when the
Swedish mineralogist Cronstedt described a new mineral, which he had
found intimately mixed with chalcopyrite[1] in the quarry of Bastnäs,
near Ryddarhyttan, in the province of Westmannland, Sweden. Cronstedt
gave the mineral the name Tung-sten (heavy stone); but as the name
Tenn-spat (heavy spar, or heavy mineral) had already been selected by
Wallerius (1747) for a new species from Bohemia, believed to contain
tin, the choice was not a happy one. More than fifty years after its
discovery, a new earth, now known as ceria, was isolated from
Cronstedt’s mineral, for which at the same time the name Cerite was
proposed.[2] Meanwhile, however, the Finnish chemist Johann Gadolin had
observed, in the year 1794, a new earth in a mineral discovered by
Arrhenius at Ytterby in Sweden in 1788; he called the new oxide
Ytterbia, and the mineral in which he observed it, Ytterbite. The
discovery was confirmed in 1797 by Ekeberg, who suggested the names
Yttria and Gadolinite for the oxide and mineral respectively; these
names were accepted by Klaproth, and soon came into general use.[3]
Whilst then Cerite was the first of the rare earth minerals to be
discovered, it was in Gadolinite that new elements were first
recognised, and the chemistry of the rare earths began in 1794 with
Gadolin’s observation.

  [1] Chalcopyrite, or Copper pyrites, is a mixed sulphide of iron and
  copper, of the approximate formula CuFeS₂.

  [2] For the history of the name Tungsten, see under the mineral
  Cerite, Ch. II.

  [3] The history of these names will be found somewhat more fully under
  Gadolinite, Ch. II.

During the nineteenth century a considerable number of rare earth
minerals was discovered and analysed; the quantities of the minerals
observed, however, were so small that the name ‘Rare earths,’ applied to
the new oxides found, was in every sense justified. Until the year 1885,
though by that time the scientific interest of the group had been fully
demonstrated by the discovery of several new elements, it was supposed
that the minerals were almost entirely confined to a few scattered
localities in Scandinavia and the Ural mountains. In that year Dr. Auer
von Welsbach announced his application of the rare earths to the
manufacture of incandescent mantles. Immediately there was a great
demand for raw material for the preparation of thoria and ceria. The
agents of the Welsbach Company visited all the important mining centres
of Europe and America, intent on a search which shortly made it clear
that the metals of the so-called ‘rare earths’ are really quite widely
distributed in nature. The chief commercial deposits are the monazite
sands of the Carolinas, the Idaho basin, and Brazil, the gem-gravels of
Ceylon, and the remarkable deposits of gadolinite and allied minerals at
Barringer Hill in Texas.

Whilst deposits of commercial importance are not very common, improved
scientific methods and more careful search have shown that in traces the
rare earths are of exceedingly wide distribution. Sir William Crookes
has shown that yttria earths are often present in calcite and in coral;
whilst Headden[4] noted that quite considerable amounts (up to 0·03 per
cent.) were present in a yellow phosphorescent variety of calcite from
Colorado. Similarly Humphreys[5] found that fluorspar usually contains
traces of yttrium, whilst one or two phosphorescent varieties contain
quantities varying up to 0·05 per cent. The presence of yttria elements
in phosphorescent varieties of calcite is interesting, and some
connection has been suggested; there is, however, no positive ground for
the belief in such a relation.

  [4] _Amer. J. Sci._, 1906, [iv.], ~21~, 301.

  [5] _Astrophys. J._, 1904, ~20~, 266.

More recently Eberhard[6] has found very considerable quantities of rare
earths in cassiterite (tin dioxide, SnO₂) and wolframite [an iron
manganese tungstate, (Fe,Mn)WO₄]. A specimen of wolframite from the
Erzgebirge was found to contain nearly 0·4 per cent. of rare earths,
over half of this quantity being scandium oxide. A process which is
readily susceptible of commercial application has been worked out by R.
J. Meyer,[7] for the extraction of scandia and the yttria earths from
the mixed oxides left after the treatment of wolframite for tungstic
acid.

  [6] _Sitzungsber. königl. Akad. Wiss. Berlin_, 1908, 851; 1910, 404.

  [7] Meyer, _Zeitsch. anorg. Chem._, 1908, ~60~, 134. Meyer und Winter,
  _ibid._, 1910, ~67~, 398.

Using the spectroscopic method, which is capable of detecting one part
of scandia in twenty thousand, Eberhard (_loc. cit._) has found that
minute quantities of scandia and yttria earths are present in almost all
the commoner rocks and minerals. The minerals richest in scandium were
beryl, cassiterite, wolfram, the zircon minerals, and the titanates and
columbates of the ceria and yttria oxides. These results are in
agreement with the observations of Sir William Crookes,[8] who has made
the study of scandium especially his own. From the fact that scandium
was often observed unaccompanied by any other member of the rare earth
group, Eberhard rather favours Urbain’s conclusion[9] that scandium may
not be a member of the rare earth family. Spectroscopic examination has
also shown the existence of some of the rare earth elements in the sun
and stars (see Europium, p. 189).

  [8] _Phil. Trans._ 1910, A, ~210~, 359.

  [9] See under Scandium in Pt. II.

In view of this extraordinarily wide distribution of the rare earths in
the mineral world, it is but natural that they should be found also in
the vegetable and animal kingdoms. Tschernik[10] found 10 per cent. of
rare earths in the ash of a coal from Kutais, in the Caucasus, and
smaller quantities have been found in the ashes of various plants;
members of the group have also been identified in the human body.

  [10] See Abstr. in _Zeitsch. Kryst. Min._, 1899, ~31~, 513.

Apart from the general occurrence in traces throughout the mineral
kingdom, the minerals in which the rare earths occur are not very
common; and though of fairly wide distribution, they are found usually
only in small quantities. The earliest known locality, and the most
fruitful in regard to number of species, has been the southern part of
the Scandinavian peninsula;[11] the minerals occur in the numerous
pegmatite veins traversing the granitic country-rock. The mining
district round Miask, in the Ural mountains, has also long been known as
a fruitful source. Other districts in Europe are the Harz and
Erzgebirge, the Laacher See in Prussia, Joachimsthal in Bohemia,
Dauphiné, Cornwall, etc. In the United States numerous localities are
known; the chief are in the Carolinas and Georgia, Idaho, Oregon,
California, Texas, Colorado, Virginia, Pennsylvania and Connecticut.
Many of the southern provinces of Brazil also furnish important sources;
the famous diamond fields of Minas Geraes, Matto-Grosso, Goyaz and the
surrounding provinces yield numerous species, whilst the sands along the
southern coasts of Bahia are rich in monazite, and form to-day the most
important source of the mineral. Monazite, as well as other rare earth
minerals, occurs also in South Africa. An interesting species,
plumboniobite (_q.v._), has recently been found in German East Africa.
From Australia numerous occurrences are reported, whilst in Canada only
a few districts are known to yield members of the group. In Asia
important localities are Ceylon--the famous gem-gravels being the most
accessible source--and one or two districts in Japan; monazite has been
reported recently in considerable quantities near Travancore, India.[12]
A more extended search will doubtless show that they occur in many other
places.

  [11] See Brögger, _Die Mineralien der Süd-Norwegische
  Granit-Pegmatitgänge_, Christiania, 1906.

  [12] _Bull. Imp. Inst._, 1911, vol. ~ix~; No. 2, p. 103.

For several reasons, the rare earth minerals[13] form a group of the
highest scientific interest. In the first place, they are generally of
very complex composition, more especially with regard to their rare
earth content. Thus, whilst it sometimes happens that one or other of
the two groups of oxides (the ceria and yttria groups) may predominate
to the complete exclusion of the second, it is no uncommon thing for a
species to contain almost all the elements of the rare earth family. On
the other hand, it is very uncommon for as much as 50 per cent. of the
rare earth content to consist of any one oxide. The usual case is that a
mineral contains chiefly yttria earths with some ceria earths, or _vice
versâ_, the two sub-groups being almost always complex mixtures of
several oxides, in which occasionally one may predominate. The
remarkable similarity in chemical behaviour of the rare earth elements,
and the difficulty of separating them, correspond to this peculiarity in
their occurrence.

  [13] The phrase ‘rare earth minerals’ will be used whenever it is
  desired to indicate collectively those minerals of which the yttria
  and ceria earths form an important constituent, as contrasted to those
  in which only traces of these oxides occur. Such minerals may often
  contain titanium, zirconium, or thorium, and, for convenience, the
  term may be taken to include the commoner zirconium and thorium
  minerals, but not the commoner titanium minerals.

A second point of even greater interest is that the rare earth minerals
are as a general rule strongly radio-active; further, it only
occasionally happens that any mineral in which the rare earths do not
form an important constituent has more than the feeblest activity; the
exceptions being, of course, those uranium minerals which do not contain
rare earths. The connection may be pushed even further; for whilst it
appears that hardly any rock or mineral possesses absolutely no
radio-activity, it is equally worthy of notice that traces of the rare
earths, if not quite universal in the mineral world, are yet normally
found in the majority of common minerals. As a natural consequence of
their activity, the rare earth minerals are also as a rule rich in
helium. These facts and the problems which they open up will be treated
more fully in a later chapter.

A point of further interest is that of the age of the rare earth
minerals. Except in a few cases where they are obviously of secondary
formation, these minerals are among the oldest known to us. They occur
usually in igneous rocks, particularly in granites which have been
considerably metamorphosed. Where erosion has occurred, they are found
in deposits of such a nature as to leave very little doubt that the
original rock was of plutonic formation and of very considerable age.
Whilst it is true, however, that the rare earth minerals are generally
of very great antiquity (none of the primary minerals being of more
recent date than the palæozoic age), Eberhard has pointed out that the
age and nature of common rocks seem to have absolutely no influence on
the traces of scandia and yttria oxides which they contain. The
geological evidence shows that the rare earth minerals are on the whole
exceedingly stable, and that they have been generally formed during the
pegmatitic alteration of granites. As early as the year 1840, Scheerer
drew attention to these facts, and to the extreme age of the rare earth
minerals; but so far his observation seems to have attracted little
attention, and no explanation has been put forward.

       *       *       *       *       *

In the following chapters no attempt is made to treat the rare earth
minerals fully. An alphabetical list of all the minerals of any
importance which contain rare earths, titanium, zirconium or thorium is
given, and of these several are selected for fuller treatment. The basis
of selection has been somewhat arbitrary. Those species which are of
mineralogical importance, as well as those to which any special
historical, scientific or commercial interest attaches, have of course
been singled out; in addition, the more recently discovered species have
occasionally been considered worthy of separate mention.[14]

  [14] A full list of the minerals containing rare earths known up to
  1904, with an account of their properties and very full references,
  will be found in the work of Dr. J. Schilling, _Das Vorkommen der
  Seltenen Erden im Mineralreiche_, 1904.

It is now being realised that some knowledge of crystallography is
essential to the chemist, and for this reason short accounts of the
crystallography of the selected types have been given. Apart from this,
every effort has been made to render the mineralogy intelligible to the
student of chemistry who has devoted no attention previously to this
subject, and also to stimulate an interest in the problems of mineral
chemistry, unfortunately too often ignored by our present-day teachers.
The rare earth minerals afford good examples of some phenomena of great
interest to the chemist, as, _e.g._ Isomorphism and Solid Solution,
Dimorphism, Isodimorphism, and Molecular Change, and in one or two cases
these are treated rather fully.

No special advantages are claimed for the system of classification,
which is merely one of convenience. The minerals are divided into five
groups:--

  (1) The Silicates, which are grouped into three sub-divisions.

  (2) The Titano-silicates and the Titanates.

  (3) The Tantalo-columbates, sub-divided into those free from titanium
  and those in which titanium is present.

  (4) The Oxides and Carbonates.

  (5) The Halides and Phosphates.

A separate chapter has been devoted to the monazite sands, and another
to the radio-active properties of the minerals.


  ALPHABETICAL LIST OF MINERALS CONTAINING TITANIUM, ZIRCONIUM, THORIUM,
  OR ELEMENTS OF THE CERIUM AND YTTRIUM GROUPS.

  The following list contains all but a few entirely unimportant members
  of these classes of minerals. The names of those species selected for
  fuller treatment are printed in heavy type, whilst names of those not
  so selected, which for convenience are included under the generic term
  ‘Rare earth mineral,’ _i.e._ roughly all those containing Thorium, or
  elements of the Cerium and Yttrium groups, and the commoner Zirconium
  minerals, as distinguished from minerals containing Titanium, are
  printed in italics. (See footnote on p. 4.) Their properties are given
  in the following order:--

  Chemical Composition and Rare Earth Content.
  Crystallographic Data.
  Physical Properties.
  Locality, etc.

  The following contractions are employed:

     E = any element or elements of the cerium or yttrium groups.
   Cer = oxides of the cerium metals.
  Yttr = oxides of the yttrium metals.
     G = Specific Gravity.
     H = Hardness.


Aenigmatite.

  A Titanosilicate of Fe´´ and Na, with small proportions of Fe´´´ and
  Al´´´. Closely allied to the amphiboles. TiO₂ = 7-8%.

  Anorthic. Habit prismatic.

  G = 3·80-3·86. H = 5¹⁄₂. Black; pleochroism strong.

  Greenland and S. Norway.


~Aeschynite~

  A Titanocolumbate of Cerium metals, with Th, Fe, Ca, Mn, aq. Cer =
  19·4-24·1; Yttr = 1·1-3·1; ThO₂ = 15·7-17·6; TiO₂ = 21-22%.

  Rhombic, holosymmetric. Habit prismatic or tabular.

  G = 4·9-5·7. H = 5·6. Black; opaque.

  Hitterö, Norway; Miask, Urals; also in Germany and Brazil.


~Allanite~ (Orthite).

  H₂O, 4R´´O, 3R´´´₂O₃, 6SiO₂, where R´´ = Ca, Fe´´, Be, and R´´´ = Al,
  Fe´´´, E. An epidote containing rare earths. Cer = 3·6-51 (usually
  10-25); Yttr = 0-8 (usually < 3); ThO₂ = 0-3·5%.

  Monoclinic; isomorphous with epidote.

  G = 3·5-4·2. H = 5¹⁄₂-6. Brown to black; opaque.

  Widely distributed in Greenland and Scandinavia.


_Alvite_ (Anderbergite).

  Silicate of Zr and E, with Ca, Mg, Be, Al, Cu, Zn, and aq. in small
  quantities. Cer → 3·98; Yttr → 22; ZrO₂ = 30·5-61·4%.

  Tetragonal; optically isotropic. Pseudomorphous after zircon.

  G = 3·3-4·3. H = 5-6. Yellowish brown; transparent.

  Ytterby, Sweden; Arendal, Norway; various localities in N. America.


~Anatase~ (Octahedrite).

  Titanium dioxide. TiO₂ = 97-100%.

  Tetragonal; habit octahedral.

  G = 3·82-3·95. H = 5¹⁄₂-6. Transparent to opaque; brown to black.

  Dauphiné; Bavaria; Cornwall; Norway; Brazil, etc.


_Ancylite._

  4Ce(OH)CO₃ + 3SrCO₃ + 3H₂O; with Fe, Mn, Ca, F, traces. Cer = 46·3%.

  Rhombic; prismatic.

  G = 3·95. H = 4¹⁄₂. Brown; translucent.

  Plain of Narsarsuk, Greenland.


_Annerödite._

  A parallel growth of Columbite on Samarskite, once believed to be a
  new species.

  Corresponding to Columbite.


Arfvedsonite.

  Metasilicate of Na, Ca, Fe´´, Zr; approximately
  4Na₂O,3CaO,14FeO,(Al,Fe)₂O₃,21SiO₂. ZrO₂ = 1-6%.

  Monoclinic--an amphibole.

  G = 3·44. H = 6. Black; pleochroism strong.

  S. Greenland and S. Norway.


Arizonite.

  Ferric metatitanate, Fe₂O₃,3TiO₂ or Fe₂(TiO₃)₃. TiO₂ = 36·7%.

  Uncertain; apparently monoclinic.

  G = 4·25. H = 6-7. Dark steel-grey; opaque.

  Hackberry, Arizona.


_Arrhenite._

  Silico-tantalate of Yttrium metals, with Ce, Al, Fe, Ca, Be, aq. Yttr
  = 33·2; Cer = 2·6; ZrO₂ = 3·4%.

  Amorphous.

  G = 3·68. Red; translucent to opaque.

  Ytterby, Sweden.


Astrophyllite.

  Titano-silicate of Fe, Al, Mn, Zr, K, Na, with aq. ZrO₂ = 1·2-4·5;
  TiO₂ = 7-14%.

  Rhombic. Cleavage (010) perfect.

  G = 3·2-3·4. H = 3. Golden to bronze yellow; strongly pleochroic.

  Brevik, Norway; El Caso Co., Colorado; Greenland.


_Auerbachite._

  An impure hydrated form of Zircon, ZrSiO₄. ZrO₂ = 55·2%.

  Tetragonal; isotropic. Pseudomorphous after zircon.

  G = 4·06. H = 6. Brownish-grey; translucent to opaque.

  Alexandrovsk, Russia.


_Auerlite._

  3ThO₂,[3SiO₂,P₂O₅]6H₂O; traces of Fe, Ca, Mg, Al, CO₂, etc. SiO₂
  replaced by P₂O₅/3? ThO₂ = 69·2-72·2%.

  Tetragonal; probably a pseudomorph after Thorite.

  G = 4·4-4·8. H = 2-3. Yellowish to orange-red.

  Henderson Co., N. Carolina.


~Baddeleyite.~

  ZrO₂, with small amounts of SiO₂, Fe₂O₃, Al₂O₃, CaO, etc. ZrO₂ =
  96·5%.

  Monoclinic.

  G = 4·4-6·0. H = 6¹⁄₂. Brown; pleochroic.

  São Paulo, Brazil; Rakwana, Ceylon.


_Bagrationite._

  A variety of Allanite (orthite) with no important chemical difference.

  Monoclinic; habit prismatic.

  G = 3·84. H = 6¹⁄₂. Black; translucent to opaque.

  Achmatovsk, Urals.


_Bastnäsite_ (Harmatite).

  Hydrated fluocarbonate of Cerium metals, E(F)CO₃. Cer = 64-93·5; ThO₂
  = 0-10%.

  Hexagonal prisms, pseudomorphous after Tysonite (_q.v._); or massive.

  G = 4·9-5·2. H = 4-4¹⁄₂. Yellow to brown; transparent.

  Bastnäs, Sweden; Pike’s Peak, Colorado.


~Beckelite.~

  Zirconosilicate of rare earths and lime, Ca₃E₄(Si,Zr)₃O₁₅. Cer = 59·7;
  Yttr = 2·8; ZrO₂ = 2·5%.

  Cubic, in octahedra and dodecahedra. Cubic cleavage.

  G = 4·15. Brown; transparent.

  Near Sea of Azov, Russia.


Benitoite.

  A Titano-silicate of barium, BaTiSi₃O₉. TiO₂ = 20·1%.

  Rhombohedral.

  H = 6¹⁄₂-7. Colourless to blue; transparent; pleochroism strong.

  Source of San Benito River, California.


~Blomstrandine.~

  Dimorphous with Polycrase (_q.v._), and of same composition.

  Orthorhombic; isomorphous with priorite (_q.v._).

  G = 4·5-5·0; H = 6¹⁄₂. Bright black; translucent.

  Hitterö and Arendal, Norway.


Blomstrandite.

  Hydrated titano-columbate of U, with some Fe and Ca. TiO₂ = 10·7%.

  Massive.

  G = 4·17-4·25. H = 5¹⁄₂. Black; opaque.

  Nohl, Sweden.


_Bodenite._

  A variety of Allanite (_q.v._), rich in Al and Ca, with no Be. Yttr =
  17; Cer = 18%.

  Monoclinic.

  As Allanite.

  Boden, near Marienburg.


_Britholite._

  A basic phosphosilicate of cerium metals, with Fe, Ca, Mg, Na, F. Cer
  = 60·5-60·9%.

  Hexagonal; habit prismatic.

  G = 4·446. H = 5¹⁄₂. Brown; transparent.

  Naujakasik, Greenland.


_Bröggerite._

  A variety of Uraninite (_q.v._), with rare earths, Th, Pb, Fe, Ca, Si,
  aq., etc. Cer = 0·4; Yttr = 1·4-4·3; ThO₂ = 4·7-6·1%. Traces of ZrO₂.

  Cubic, in octahedra and dodecahedra.

  G = 8·7-9·0. H = 5-6. Black; translucent to opaque.

  Anneröd, near Moos, Norway.


~Brookite.~

  Titanium dioxide, TiO₂ = 99-100%; trimorphous with Anatase and Rutile.

  Orthorhombic.

  G = 3·87-4·01. H = 5¹⁄₂-6. Brown; opaque.

  Dauphiné; Urals; Switzerland; Magnet Cove, Arkansas.


_Calciothorite._

  A variety of Thorite containing lime--5ThSiO₄,2Ca₂SiO₄ + 10H₂O. ThO₂ =
  59·3%.

  Completely amorphous.

  G = 4·114. H = 4¹⁄₂. Deep red; translucent.

  Islands of Läven and Arö, Langesund Fiord, Norway.


_Cappelenite._

  A borosilicate of rare earth metals and barium, with traces of Th, Ca,
  K, Na, aq. Approximately BaSiO₃, YBO₃. Cer = 4·2; Yttr = 52·5%.

  Hexagonal; habit prismatic.

  G = 4·407. H = 6-6¹⁄₂. Greenish brown; translucent.

  Island of Klein-Arö, Langesund Fiord, Norway.


_Caryocerite_ (Karyocerite).

  Complex fluosilicate of E, with Ta, Th, Ca; also CO₂, P₂O₅, B, Al, Fe,
  Mn, U, Mg, Na, aq., etc. Approaching Melanocerite, (_q.v._), but
  richer in Th. Very complex. Cer = 41·8; Yttr = 2·2; ThO₂ = 13·6; ZrO₂
  = 0·5%.

  Rhombohedral, but isotropic; apparently a pseudomorph after
  Melanocerite (_q.v._)

  G = 4·295. H = 5-6. Nut brown; translucent. Faces very brilliant, but
  striated. Lustre vitreous to resinous.

  Various rocks and shoals round Arö Island, Langesund Fiord, Norway.


_Castelnaudite._

  A variety of Xenotime (_q.v._) containing Zr. Yttr = 60·4; ZrO₂ =
  7·4%.

  Tetragonal.

  G = 4·5. H = 4-5. Greyish white to pale yellow.

  Diamond sands of Brazil.


_Cataplejite_ (Kataplejite).

  H₄(Na₂,Ca)ZrSi₃O₁₁. ZrO₂ = 29·6-40% (usually 30-33%).

  Monoclinic, pseudohexagonal. Becomes truly hexagonal at 140°C.

  G = 2·8. H = 6. Yellow to brown; transparent to opaque.

  A blue variety is known which contains no calcium.

  Islands of Langesund Fiord, Norway; Narsarsuk, Greenland.


~Cerite.~

  A basic silicate of Cerium metals, with Ca and Fe. Approximately
  H₃(Ca,Fe)Ce₃Si₃O₁₃. Cer = 50·7-71·8%. In a variety from Batoum,
  Tschermak reports Yttr = 7·6 and ZrO₂ = 11·7%.

  Orthorhombic; usually massive or granular.

  G = 4·9. H = 5-6. Brown to red; translucent to opaque.

  Ryddarhyttan, Sweden; Batoum, Caucasus?


_Chalcolamprite._

  A silico-columbate of E, Zr, Ca, Fe, Na, K; R₂Cb₂F₂SiO₉, where R
  represents various metals. E = 3·41; ZrO₂ = 5·7%.

  Cubic, in small octahedra.

  G = 3·77. H = 5¹⁄₂. Greenish brown; opaque. Metallic lustre (χαλκός =
  Copper, λαμπρός = lustre).

  Narsarsuk, S. Greenland.


_Churchite._

  Hydrous phosphate of Cerium metals and Ca; Cer = 51·87%.

  Monoclinic? Allegations only.

  G = 3·14. H = 3¹⁄₂. Greyish; transparent to translucent.

  Cornwall.


_Cleveite._

  A variety of Uraninite (_q.v._) rich in rare earths and helium. Cer =
  2·3-2·9; Yttr = 10·0-10·3; ThO₂ = 4·6-4·8%.

  Cubic; usually massive.

  G = 7·49. H = 5¹⁄₂. Black; opaque.

  Arendal, Norway.


~Cordylite.~

  Fluocarbonate of Cerium metals and Ba; E₂F₂Ba(CO₃)₃. Cer = 49·4%.

  Hexagonal; isomorphous with Parisite (_q.v._).

  G = 4·31. H = 4¹⁄₂. Yellow; transparent.

  Plain of Narsarsuk, Greenland.


Cossyrite.

  A variety of Aenigmatite (_q.v._) of very complex composition, TiO₂ =
  6-8%.

  Anorthic.

  G = 3·74. H = 5. Black; opaque.

  Island of Pantellaria (formerly Cossyra).


_Cyrtolite._

  A pseudomorph after zircon, allied to Alvite (_q.v._).

  Tetragonal.

  _See_ Alvite.

  Various localities in Scandinavia, and U.S.A.


_Davidite._

  A Titanate of Fe, U, V, Cr, and E--uncertain formula. TiO₂ > 50; E₂O₃
  = 5-10%.

  Cubic--in grains and rounded crystals.

  G = 4 about. Black, with brilliant lustre.

  Olary, S. Australia.


~Delorenzite.~

  2FeO,UO₂,2Y₂O₃,24TiO₂. Yttr = 14·63; TiO₂ = 55%.

  Rhombic; habit prismatic.

  G = 4·7. H = 5¹⁄₂-6. Black; translucent to opaque; lustrous.

  Craveggia, Piedmont, Italy.


Derbylite.

  FeO,Sb₂O₅ + 5FeO,TiO₂? TiO₂ = 35% about.

  Orthorhombic; habit prismatic.

  G = 4·53. H = 5. Pitch black; opaque; lustre resinous.

  Tripuhy, Minas Geraes, Brazil.


Dysanalyte (Perovskite).

  Approximately 6RTiO₃,R(Cb,Ta)₂O₆, where R = Ca, Fe´´. Believed by
  Hauser to be merely an impure Perovskite (_q.v._). Cer = 0-5·1; TiO₂ =
  41·5-59·3%.

  Cubic.

  G = 4·13. H = 5-6. Black; opaque.

  Vogtsburg, near Baden, Germany.


Elpidite.

  Na₂Zr(Si₂O₅)₃, 1¹⁄₂H₂O. ZrO₂ = 20·5%.

  Orthorhombic.

  G = 2·52-2·56. H = 7-8. Colourless to red; translucent.

  Various localities in Greenland.


_Endeiolite._

  R´´Cb₂O₆(OH)₂ + R´´SiO₃ (cf. Chalcolamprite). E₂O₃ = 4·43; ZrO₂ =
  3·78%.

  Cubic.

  G = 3·44. H = 4. Dark chocolate-brown; transparent.

  Narsarsuk, Greenland.


_Erdmannite_ (Michaelsonite).

  A silicate of E and Ca, with Zr, Be, Th, Al, Fe, aq., etc. An altered
  Homilite? Cer = 17·7-34·9; Yttr = 1·4-2·1; ThO₂ + ZrO₂ = 0-12%.

  Amorphous; isotropic.

  G = 3·01-3·39. H = 4¹⁄₂. Brown to leek-green.

  Near Brevig, Norway.


_Erikite._

  A phosphosilicate of E, Ca, Al, K and Na, with ThO₂, H₂O, etc. Cer =
  40·5; ThO₂ = 3·3%.

  Orthorhombic.

  G = 3·473. H = 5¹⁄₂-6. Brown; opaque.

  Julianehaab, Greenland.


~Eucolyte.~

  R´₄R´´₃Zr(SiO₃)₇, where R´ = K, Na, H, and R´´ = Ce(OH), Fe, Mn, Ca,
  and Zr(OCl) may replace SiO₂? A very complex mineral. ZrO₂ = 10·9-20;
  Cer = 0-5·2%.

  Rhombohedral.

  G = 3·0-3·1. H = 5-5¹⁄₂. Red to brown; translucent. Double Refraction
  strong, -ve.

  Various localities in Norway.


_Eucrasite._

  An altered Thorite (_q.v._) containing E, Ca, Fe, Mn, Na, Ti, H₂O,
  etc. Cer = 14; Yttr = 5·9; ThO₂ = 36·0; ZrO₂ = 0·6%.

  Rhombic (Paijkull). Amorphous, isotropic (Brögger).

  G = 4·39. H = 4¹⁄₂-5. Brownish black; opaque.

  Near Brevig, Norway.


~Eudialite.~

  A variety of Eucolyte (_q.v._) of the same composition.

  As Eucolyte.

  G = 2·92. Double Refraction strong, +ve. Otherwise as Eucolyte.

  Greenland; Lapland; Arkansas, U.S.A.


~Euxenite.~

  E(CbO₃)₃,E₂(TiO₃)₃,1¹⁄₂H₂O; with U and Zr. Cer = 2·3-8·4; Yttr =
  13·2-34·6; TiO₂ = 20-23%. ThO₂ + ZrO₂ usually in traces.

  Orthorhombic; usually massive.

  G = 4·6-5·0. H = 6¹⁄₂. Brownish-black; translucent to opaque.

  Hitterö, Brevig, Jolster, Arendal, Norway; Cooglegong, Australia; N.
  Carolina.


~Fergusonite.~

  Approximately E₂O₃, (Cb,Ta)₂O₅, with U, Fe, Ca. Cer = 0·5-13·9; Yttr =
  27·9-47·1; ThO₂ + ZrO₂ = 0-7%. [Berzelius found Cer = 36·3; Yttr = 0%
  in one specimen.]

  Tetragonal, polar.

  G = 5·84-4·3 when largely hydrated. H = 5·6. Brown to black.

  Norway; Australia; Texas, etc.


_Florencite._

  A silico-phosphate of E and Al. Cer = 28% approximately.

  G = 3·6. H = 5. Yellow to red. Resinous lustre.

  Minas Geraes and diamond localities in Brazil.


_Fluocerite._

  Basic fluoride of rare earth metals, E₂O₃,4EF₃. Cer = 81·4-82·6; Yttr
  = 1·1-4·3%.

  Massive. Original hexagonal mineral of Berzelius and Haidinger,
  probably Tysonite (_q.v._).

  G = 5·7-5·9. H = 4. Reddish yellow; opaque.

  Österby, Sweden.


_Freyalite._

  Silicate of E and Th, with Al, Fe, Mn, Na, aq., etc. Cer = 31·3; ThO₂
  = 28·4; ZrO₂ = 6·3%.

  Amorphous.

  G = 4·06-4·17. H = 6. Brown; opaque; lustre resinous.

  Brevig, Norway.


~Gadolinite.~

  FeO, 2BeO, Y₂O₃, 2SiO₂, where Y = yttrium metals. Cer = 3·4-51·5
  (usual 6-20); Yttr = 5-60 (usual 35-48)%.

  Monoclinic; habit prismatic. Often amorphous and isotropic.

  G = 4·0-4·5. H = 6¹⁄₂-7. Brown and green. Double Refraction strong,
  +ve.

  Ytterby and Fahlun, Sweden; Hitterö and Malö, Norway; Llano Co.,
  Texas; Colorado, etc.


Geikielite.

  (Mg,Fe´´)TiO₃. TiO₂ = 56·1-64·8%. Specimens rich in iron are called
  Picroilmenite.

  Massive.

  G = 4 about. H = 6. Purplish or brownish black.

  Ceylon.


Gorceixite.

  An alumino-phosphate of alkaline and ceria earths. Cer = 0-3%.

  Microcrystalline.

  G = 3. H = 6. White to brown. Translucent.

  Diamond sands of Brazil.


Guarinite.

  Formerly supposed to be dimorphous with Titanite (_q.v._); shown by
  Zambonini and Prior (1909) to be identical with Hiortdahlite (_q.v._).


Hainite.

  Tantalo-silicate and titanate of Zr, Ca, Na. ZrO₂ = 29-32%.

  Anorthic.

  G = 3·2. H = 5. Colourless to yellow; transparent.

  Bohemia.


~Hellandite.~

  3H₂0, 2R´´O, 3R´´´₂O₃, 4SiO₂, where R´´ = Ca, Mg, Th/2; R´´´ = E, Al,
  Fe, Mn. E₂O₃ = 40%.

  Monoclinic; habit prismatic.

  G = 3·70. H = 5¹⁄₂. Reddish-brown when fresh.

  Lindvikskollan and Kragerö, Norway.


Hiortdahlite.

  3CaSiO₃,Ca(F,OH)NaZrO₃. ZrO₂ = 21·5; TiO₂ = 1·5%.

  Anorthic; habit tabular.

  G = 3·27; H = 5-5¹⁄₂. Yellow, with weak pleochroism.

  Island of Läven, Langesund Fiord, Norway.


_Hjelmite_ (Hielmite).

  A stanno-tantalate of Ca, Mn, Fe, E, related to Yttrotantalite
  (_q.v._). E₂O₃ = 1-6%.

  Orthorhombic.

  G = 5·82. H = 5. Black; lustre metallic.

  Fahlun, Sweden.


Homilite.

  (Ca,Fe)₃(BO)₂(SiO₄)₂. Sometimes with ceria earths, 0-2·6%.

  Monoclinic--isomorphous with Gadolinite (? Brögger).

  G = 3·34-3·38. H = 4¹⁄₂-5. Black; pleochroic.

  Islands of Lanegsund Fiord, Norway.


_Hussakite_ (Xenotime).

  A prismatic form of Xenotime (_q.v._), erroneously supposed to contain
  > 6% SO₃.

  Diamond sands of Brazil.


Hydrotitanite.

  An altered Perovskite (_q.v._) with Fe´´´ and aq. TiO₂ = 82·8%.

  Amorphous.

  G = 3·68. H = 1-2. Yellowish grey.

  Magnet Cove, Arkansas.


~Ilmenite.~

  FeTiO₃; composition varies widely. TiO₂ = 3·5-52·3%.

  Rhombohedral.

  G = 4·5-5. H = 5-6. Black; opaque. Slightly magnetic.

  Norway; Dauphiné; Bohemia; Cornwall, etc.


Ilmenorutile.

  FeO,Nb₂O₅,5TiO₂? TiO₂ = 66-75%.

  Tetragonal, very near to Rutile (_q.v._).

  G = 4·3-5·0. H = 6-7. Brown to black; opaque.

  Ilmen Mountains, Russia.


_Johnstrupite._

  Silico-titanate of E, Al, Mg, Ca, Na, etc., with F and aq. Cer = 13·5;
  Yttr = 1·1; TiO₂ = 7-8; ThO₂ + ZrO₂ = 3·6%.

  Monoclinic, very close to Epidote.

  G = 3·19-3·29. H = 5. Brownish green; weakly pleochroic.

  Islands of the Langesund Fiord, Norway.


_Kainosite_ (Cenosite).

  CaY₂(SiO₃)₄,CaCO₃,2H₂O, where Y = Yttrium metals. Yttr = 30-37%.

  Uncertain; pseudo-hexagonal.

  G = 3·38-3·41. H = 5-6. Yellowish brown.

  Hitterö and province of Nordmark, Norway.


~Keilhauite~ (Yttrotitanite).

  An isomorphous mixture of Titanite (_q.v._) with (E,Al,Fe)SiO₅. E₂O₃ =
  5-12; TiO₂ = 26-30%.

  Monoclinic; isomorphous with Titanite.

  G = 3·52-3·77. H = 6¹⁄₂. Brown to black.

  Various localities in Norway.


_Kischtimite._

  A fluocarbonate of the Cerium metals, near Parisite (_q.v._). Cer =
  74·2%.

  Massive.

  G = 4·78. H = 4¹⁄₂. Yellowish brown; translucent.

  Barsovka River, Kyshtymsk, Urals.


_Knopite._

  A variety of Perovskite (_q.v._) containing E and Fe. Cer = 4-7; TiO₂
  = 55%.

  Pseudo-cubic.

  G = 4·2. H = 5¹⁄₂. Grey; opaque; lustre metallic.

  Alnö, Sweden.


_Kochelite._

  A columbate of E, Fe, Zr; with ThO₂, SiO₂, Ca, aq., etc. Allied to
  Fergusonite (_q.v._). Yttr = 17·22; ZrO₂ = 12·8; ThO₂ = 1·23%.

  Doubtful; may be tetragonal.

  G = 3·74. H = 3-3¹⁄₂. Brown to honey yellow; translucent.

  The Kochelweise, near Schreiberhau, Silesia.


_Koppite._

  Columbate of E, Ca, Fe, Th, K, Na, etc. Near Pyrochlore (_q.v._). Cer
  = 4-10; ZrO₂ = 0-5%.

  Cubic; in dodecahedra.

  G = 4·45-4·46. H = 5-6. Brown; transparent.

  Schelingen, Black Forest Mountains, Germany.


~Lanthanite.~

  Hydrated carbonate of Cerium metals, especially La; E₂(CO₃)₃,9aq. Cer
  = 54·9%.

  Orthorhombic; habit tabular.

  G = 2·6-2·7. H = 2. White; opaque.

  With Cerite (_q.v._) at Bastnäs, Sweden; Bethlehem, Pennsylvania,
  U.S.A.


Lavenite.

  (Mn,Ca,Fe)(ZrOF)Na(SiO₃)₂? ZrO₂ = 28·8-31·6%.

  Monoclinic; habit prismatic.

  G = 3·51-3·55. H = 6. Brown to yellow; translucent.

  Langesund Fiord, Norway; the Ardennes, France.


Leucosphenite.

  BaO,2Na₂O,2(Ti,Zr)O₂,10SiO₂. TiO₂ = 13·2; ZrO₂ = 3·5%.

  Monoclinic; wedge-shaped.

  G = 3·05. H = 6¹⁄₂. White; transparent.

  Narsarsuk, Greenland.


Lewisite.

  3R´´Sb₂O₆,2R´´TiO₃, where R = Ca, Fe´´ and Mn. TiO₂ = 11-12%.

  Cubic; in small octahedra.

  G = 4·95. H = 5¹⁄₂. Yellow to brown; translucent.

  Tripuhy, Minas Geraes, Brazil.


_Loranskite._

  Tantalate of E, Zr, Fe, etc. Yttr = 10; Cer = 3; ZrO₂ = 20%.

  Massive.

  G = 4·6. H = 5. Black; opaque. Metallic lustre.

  Finland.


Lorenzenite.

  Titano-silicate of Na and Zr; TiO₂ = 35; ZrO₂ = 12%.

  Orthorhombic; acicular.

  G = 3·4. H = 6. Colourless; transparent.

  South Greenland.


_Mackintoshite._

  Mixture of oxides, chiefly of Th and U; also Fe, Ca, Mg, Pb, Na, B,
  Ta, etc. Composition very complex. ThO₂ = 45·3; E₂O₃ = 1·9; ZrO₂ = 1%.

  Tetragonal, resembling thorite (_q.v._).

  G = 5·42. H = 5¹⁄₂. Black; opaque.

  Bluffton, Llano Co., Texas.


_Malacone._

  An altered Zircon (_q.v._), with E, Ca, Fe, H₂O, etc. ZrO₂ = 47-67%.

  Tetragonal; pseudomorphous.

  G = 3·9-4·1. H = 6. Brown, often dull white internally.

  Hitterö, Norway; Haute Loire, France; and in U.S.A.


Mauzeliite.

  Very similar to Lewisite (_q.v._), with Pb. TiO₂ = 8%.

  Cubic.

  G = 5·11. H = 5-6. Brown; translucent.

  Jakobsberg, Sweden.


_Melanocerite._

  Very complex fluosilicate of E and Ca, chiefly. Cer = 48; Yttr = 9·2;
  ThO₂ + ZrO₂ = 2%.

  Rhombohedral; habit tabular.

  G = 4·13. H = 5-6. Deep brown to black. Transparent.

  Langesund Fiord, Norway.


_Microlite._

  Complex columbate of Ca, E, Fe, etc., with F and H₂O. E₂O₃→ 8%.

  Cubic; habit octahedral.

  G = 5·48-5·56. H = 5-5¹⁄₂. Red to yellow.

  Stockholm, Sweden; Island of Elba; and in U.S.A.


Molengraafite.

  Titano-silicate of Ca, Na, Fe, Al, Mn, etc. TiO₂ = 28%.

  Monoclinic; in small prisms.

  Yellow. High refraction and birefringence.

  Pilandsberg, Transvaal.


~Monazite.~

  Phosphate of E, with Th and SiO₂. Cer = 49-74; Yttr = 1-4; ThO₂ =
  1-20%.

  Monoclinic.

  G = 4·9-5·3. H = 5-5¹⁄₂. Red to brown and yellow; translucent.

  The Carolinas; Idaho; Brazil; Scandinavia, etc.


_Mosandrite._

  In composition identical with Johnstrupite (_q.v._).

  Isomeric with Johnstrupite (_q.v._).

  G = 2·93-3·03. H = 4. Reddish brown; translucent.

  Langesund Fiord, Norway.


_Muromontite._

  A variety of Allanite (_q.v._), rich in yttria earths and Be, but poor
  in Al and ceria earths. Cer = 9·1; Yttr = 37·1%.

  _See_ Allanite.

  G = 4·263. H = 7. Black to greenish black.

  Mauersberg, Erzgebirge, Saxony.


~Naegite.~

  A silicate of Zr, ZrSiO₄, with E, Th, U, Cb, etc. ZrO₂ = 55·2; Yttr =
  9·12; ThO₂ = 5·01%.

  Tetragonal; in globular aggregates.

  Gr = 4·091. H = 7¹⁄₂. Dark green or brown; dull.

  Gravel-tin of Japan.


Narsarsukite.

  Na₆FeTi₂Si₁₂O₃₂F. TiO₂ = 14%.

  Tetragonal. Habit tabular.

  Gr = 2·75. H = 7-7¹⁄₂. Yellow to reddish-brown; pleochroic.

  Plain of Narsarsuk, Greenland.


Neptunite.

  (K,Na)₂(Fe,Mg,Ca,)₂(Ti,Si)₄O₁₂. TiO₂ = 18%.

  Monoclinic. Habit prismatic.

  G = 3·23. H = 5¹⁄₂. Black, red in flakes. Translucent to opaque.

  Narsarsuk, Greenland.


_Nivenite._

  A variety of Cleveite (_q.v._), readily soluble in dilute acids.

  Cubic; crystallisation indistinct.

  G = 8·01. H = 5¹⁄₂. Velvet black; opaque.

  Bluffton, Llano Co., Texas.


_Nohlite._

  A variety of Samarskite (_q.v._) containing water (→ 4·6%).

  Massive, without cleavage.

  G = 5·04. H = 4¹⁄₂-5. Brownish black; opaque.

  Nohl, near Kongelf, Sweden.


_Oerstedite._

  A variety of Zircon (_q.v._), poor in SiO₂. ZrO₂ = 69%.

  Tetragonal; angles exactly those of Zircon.

  G = 3·629. H = 5¹⁄₂. Reddish-brown; adamantine lustre.

  Arendal, Norway.


~Orangite.~

  ThSiO₄, usually with Fe, Ca, H₂O in traces. ThO₂ = 71·2-73·8%.

  Tetragonal. Habit prismatic.

  G = 5·19-5·40. H = 4¹⁄₂-5. Orange yellow; lustrous.

  _See under_ Thorite.


~Parisite.~

  E₂CaF₂(CO₃)₃. Cer = 50·8-64·4; Yttr = 0-2·5%.

  Hexagonal. Habit pyramidal.

  G = 4·36. H = 4¹⁄₂. Yellow to red; transparent.

  Muso Valley, Columbia; Montana, U.S.A.; Greenland; Norway; the Urals,
  etc.


Perovskite.

  CaTiO₃, with traces of Fe´´. TiO₂ = 58·9%.

  Pseudo-cubic? Optically biaxial.

  G = 4·017. H = 5¹⁄₂. Yellow; transparent to opaque.

  The Urals; Switzerland; Tyrol, etc.


_Pilbarite._

  PbO,UO₃,ThO₂,2SiO₂,2H₂0 + 2aq. ThO₂ = 31·3%. Cer and Yttr--traces.

  Amorphous.

  G = 4·4-4·7. H = 2¹⁄₂-3. Bright yellow; opaque.

  Pilbara goldfields, West Australia.


_Pitchblende._

  A mixture of oxides, chiefly UO₂ and UO₃, but without E₂O₃ or ThO₂.

  Amorphous.

  G = 5-6·5. H = 3-4. Black; resinous lustre.

  Bohemia; Cornwall; Carolina; Norway, etc.


_Plumboniobite._

  A variety of Samarskite (_q.v._) containing Pb; R´´₂Cb₂O₇,
  R´´´₄(Cb₂O₇)₃, where R´´ = Fe, Pb, Ca, UO, R´´´ = E, Al. Yttr = 14·3%.

  Massive, isotropic.

  G = 4·80-4·81. H = 5-5¹⁄₂. Dark brown to black.

  Morogoro, Uluguru Mountains, German E. Africa.


~Polycrase.~

  A titano-columbate of E and U; Yttr = 19·5-32·5; TiO₂ = 25-33%. Cer
  and ThO₂ traces. Isomorphous with Euxenite.

  Orthorhombic.

  G = 4·0-4·8. H = 6. Black; vitreous lustre.

  Norway.


~Priorite.~

  Dimorphous with Euxenite (_q.v._).

  Orthorhombic; isomorphous with Blomstrandine.

  G = 4·6-5·0. H = 6. Black; transparent in flakes.

  Swaziland, S. Africa.


Pseudobrookite.

  Fe₄(TiO₄)₃, ferric orthotitanate. TiO₂ = 44-53%.

  Orthorhombic.

  G = 4·39-4·98. H = 6. Dark brown to black.

  Norway; France.


_Pyrochlore._

  A columbate of Ca and E, with Th, Fe, Ti, F, etc. E₂O₃ → 18; TiO₂ =
  5-14%.

  Cubic.

  G = 4·2-4·36. H = 5-5¹⁄₂. Dark brown.

  Scandinavia; the Urals; Tasmania, etc.


Pyrophanite.

  MnTiO₃, with traces of SiO₂. TiO₂ = 50-53%.

  Rhombohedral; isomorphous with Ilmenite.

  G = 4·537. H = 5. Deep blood-red; translucent; lustrous.

  Pajsberg, Sweden.


_Retzian._

  Hydrated arsenate of Mn´´, Ca, E. Cer + Yttr = 8-11%.

  Orthorhombic, usually in prisms.

  G = 4·15. H = 4. Brown; pleochroic; transparent.

  Province of Nordmarken, Sweden.


_Rhabdophane_ (Scovillite).

  Hydrated phosphate of E, Al, Fe, Mg, etc., with SiO₂. Cer = 53·8-57;
  Yttr = 2·1-10·0%.

  Massive.

  G = 3·94-4·01. H = 3¹⁄₂. Brown to yellow; translucent.

  Cornwall; Scoville, Connecticut, U.S.A.


Rhönite.

  (Na,K,H)₃Ca₃(Fe´´,Mg)₁₅(Al,Fe´´´)₁₆(Si,Ti)₂₁O₉₀. TiO₂ = 9·5%.

  Anorthic, isomorphous with Aenigmatite.

  G = 3·5-4·3. Brown, with strong pleochroism.

  Rhön Mountains, Saxony.


_Rinkite._

  A titanosilicate closely allied to Mosandrite and Johnstrupite
  (_q.v._)--Na₉Ca₁₁Ce₃(Ti,Th)₄₁Si₁₂O₄₆? Cer = 21; Yttr = 0·4-1·4; TiO₂ =
  13-14%.

  Monoclinic, very close to Johnstrupite.

  G = 3·46. H = 5. Yellow, pleochroic; translucent.

  Kangerdluarsuk, Greenland.


~Risörite.~

  An yttria columbate, near Fergusonite, but with no U and considerable
  TiO₂; Yttr = 37; Cer = 2·9-4·0; TiO₂ = 6·5%.

  No data yet determined. Isotropic.

  G = 4·179. H = 5¹⁄₂. Yellowish brown.

  Norway.


_Rogersite._

  Hydrated yttria columbate. Yttr = 60·12%. A weathered Samarskite?

  Amorphous, mamillary.

  G = 3·313. H = 3¹⁄₂. White.

  Mitchell Co., N. Carolina.


_Rosenbuschite._

  Titanosilicate of Ca, Zr, Na, E, Fe, Mn, with F. ZrO₂ = 18·7-20; Cer =
  0·3-2·4%.

  Monoclinic, in spherical aggregates.

  G = 3·30-3·31. H = 5-6. Orange-grey.

  Near Brevik, Sweden.


_Rowlandite._

  Silicate of E, with Th, Ti, Fe, etc.--2Y₂O₃, 3SiO₂. Cer = 14·4; Yttr =
  47·7; ThO₂ = 0·6%.

  Massive.

  G = 4·515. H = 6. Pale dull green.

  Llano Co., Texas.


~Rutile.~

  Titanium dioxide; TiO₂ = 98-100%

  Tetragonal; habit prismatic.

  G = 4·18-4·25. H = 6-6¹⁄₂. Reddish-brown to black.

  Very widely in Europe and America.


~Samarskite.~

  R´´₃R´´´₂(Cb,Ta)₆O₂₁, where R´´ = Fe, Ca, UO₂; R´´´ = E. Cer =
  1·2-6·4; Yttr = 4·72-21·2; ThO₂ + ZrO₂ → 7%.

  Orthorhombic; usually massive.

  G = 5·6-5·8. H = 5-6. Deep velvet black; opaque.

  Miask; Urals; Mitchell Co., N. Carolina.


Schorlomite.

  A titaniferous Garnet--3CaO,(Fe,Ti)₂O₃,3(Si,Ti)O₂. TiO₂ = 12·5-22%.

  Cubic; usually massive.

  G = 3·81-3·88. H = 7-7¹⁄₂. Black; transparent in flakes.

  Magnet Cove, Arkansas.


Senaite.

  (Fe,Mn,Pb)O,TiO₂, cf. Ilmenite. TiO₂ = 49-52%.

  Rhombohedral; isomorphous with Ilmenite, Geikielite, etc.

  G = 5·3 (to 4·2 when weathered). H = 6¹⁄₂. Black.

  Diamantina, Minas Geraes, Brazil.


~Sipylite.~

  Columbate of E, Zr, Fe, U, Sn, etc.; near Fergusonite (_q.v._).

  Cubic, in octahedra. Usually granular.

  G = 4·89. H = 6. Brownish-black; translucent.

  Amhurst Co., Virginia.


_Steenstrupine._

  A silicate of E, Fe, Na, Th, Mn, Al, Ti, H₂O, etc.; near Melanocerite.
  Cer = 14·4-32·5; Yttr = 0-15·9; ThO₂ = 2·1-7·1%.

  Rhombohedral.

  G = 3·38. H = 4. Brown; faces dull.

  Kangerdluarsuk, Greenland.


Strüverite.

  FeO,(Nb,Ta)₂O₅,4TiO₂. TiO₂ = 69-71%.

  Tetragonal; angles very close to those of rutile.

  G = 5·0. H = 6-7. Black; opaque.

  Craveggia, Piedmont, Italy; and in Madagascar.


_Tachyaphaltite._

  An altered zircon, containing H₂O. ZrO₂ = 40-50%.

  Tetragonal; very close to Zircon.

  G = 3·6. H = 5¹⁄₂. Dark brown.

  Kragerö, Norway.


_Tengerite._

  Hydrated carbonate of E, Be, Ca, etc.; a weathered Gadolinite
  (_q.v._). E₂O₃ = 39·2-47·8%.

  Amorphous.

  White; opaque; very soft.

  Llano Co., Texas.


~Thalenite.~

  H₂E₄Si₄O₁₅, with traces of Fe´´´ and Al. Yttr = 58·6-63·9%.

  Monoclinic.

  G = 4·23. H = 6¹⁄₂. Bright red and yellow,

  Österby, Sweden.


~Thorianite.~

  Mixed ThO₂ + UO₂, with E, Pb, Zr, Si, Fe, etc. ThO₂ = 72-79; Cer =
  1-8%.

  Rhombohedral; pseudocubic.

  G = 8·0-9·7. H = 7. Jet black; bright resinous lustre.

  Gem-gravels of Ceylon.


~Thorite.~

  ThSiO₄, with H₂O, U, Fe, E, Ca, Al, etc. ThO₂ = 41·4-57·9; E₂O₃ =
  0-6%.

  Tetragonal; habit prismatic.

  G = 4·4-4·8; H = 4¹⁄₂-5. Brown to black.

  Various localities in Scandinavia.


_Thorogummite._

  UO₃,3ThO₂,3SiO₂,6H₂O? An altered Mackintoshite (_q.v._)? ThO₂ = 41·4;
  E₂O₃ = 6·7%.

  Usually massive; sometimes in crystals resembling Zircon.

  G = 4·43-4·54. H = 4-4¹⁄₂. Dull brown; opaque.

  Llano Co., Texas.


~Thortveitite.~

  E₂O₃,2SiO₂, with Fe´´´, Al, Mn´´´ traces; E = chiefly Sc. Yttr =
  54·5%.

  Orthorhombic, in radial aggregates.

  G = 3·571. H = 6-7. Greyish green; translucent.

  Iveland, Sätersdalen, Norway.


~Titanite~ (Sphene, Grothite).

  CaSiTiO₅, with Fe´´, Mn´´. TiO₂ = 34-45% (usually 41%).

  Monoclinic; wedge-shaped.

  G = 3·40-3·56. H = 5-5¹⁄₂. Yellow, green, or brown; pleochroism
  strong; lustre resinous.

  Widely distributed in Europe and N. America.


Titanium Olivine.

  (H₂,Fe´´,Mg)₂(Si,Ti)O₄; Mn and F in traces. TiO₂ = 3-12%.

  Orthorhombic.

  G = 3·25-3·27. H = 6¹⁄₂-7. Deep red to yellow; pleochroic.

  Pfunders, Tyrol; Zermatt, Switzerland.


_Tritomite._

  A fluo-borosilicate of E, Th, Ca, with Zr, Na, H₂O, etc. Cer =
  44·2-59·2; Yttr = 0·4-4·6; ThO₂ + ZrO₂ = 0-10·6%.

  Rhombohedral; in crystals resembling regular tetrahedra.

  G = 4·15-4·25. H = 5¹⁄₂. Dark brown; transparent to opaque.

  Langesund Fiord, Norway.


_Tscheffkinite._

  Titano-silicate of E, Th, Fe, Ca, etc. Cer = 23-47; Yttr = 0-3·4; ThO₂
  + ZrO₂ = 0-20; TiO₂ = 16-21%.

  Massive, amorphous.

  G = 4·26-4·55. H = 5-5¹⁄₂. Velvet black.

  Ilmen Mountains; Nelson Co. and Bedford Co., Virginia, U.S.A.


_Tysonite._

  Fluoride of E, with Th, H₂O, CO₂, etc. Cer = 69·2-70·6; ThO₂ = 0-31%.

  Hexagonal; in thick prisms.

  G = 6·12-6·14. H = 4¹⁄₂-5. Wax yellow; transparent to translucent.

  Fahlun and Österby, Sweden; Pike’s Peak, Colorado.


_Uhligite._

  Titanate of Zr, Ca, Al; Ca(Zr,Ti)O₃ + Al(Ti,Al)O₃? TiO₂ = 48; ZrO₂ =
  22%.

  Cubic; near to Perovskite (_q.v._).

  H = 5-6. Black. Transparent in flakes.

  Lake Magad, E. Africa.


~Uraninite.~

  Oxides of U (60-75%), with PbO₂, ThO₂, ZrO₂, E₂O₃, Fe₂O₃, etc. Cer. =
  0-2·7; Yttr = 0-10·2; ThO₂ = 1·6-11·1; ZrO₂ = 0-8·1%.

  Cubic, usually massive; alters to amorphous pitchblende.

  G = → 6·4 (massive); → 9·7 (crystalline). H = 5¹⁄₂. Black; transparent
  in splinters.

  Norway; Bohemia; Saxony; Cornwall; Carolina, etc.


_Vietinghofite._

  A hydrated ferruginous samarskite (_q.v._). E₂O₃ = 8·2; ZrO₂ = 1·0%.

  Amorphous.

  G = 5·53. H = 5¹⁄₂-6. Dull black; opaque.

  Lake Baikal, Siberia.


Warwickite.

  6MgO,FeO,2TiO₂,3B₂O₃? TiO₂ = 23·5%.

  Orthorhombic; habit prismatic, elongated.

  G = 3·35-3·36. H = 3-4. Dark brown to black; pleochroic. Double
  refraction strong, +ve.

  Edenville, New York State.


_Weibyite._

  Carbonate of E, with Ca, Sr, F, and H₂O; allied to Bastnäsite
  (_q.v._). Cer = 66·96%?

  Orthorhombic; in pyramids resembling those of Zircon.

  Crystals are small, and covered with a thin yellow crust; they are
  intergrown with Parisite (_q.v._)

  Langesund Fiord, Norway.


~Wiikite.~

  Titano-tantalo-silicate of Zr, Th, E, Fe, U, with Cb₂O₅, H₂O, etc. Cer
  = 2·5; Yttr = 7·6; Sc₂O₃ = 1·2; ThO₂ = 5·5; ZrO₂ + TiO₂ = 23·4%.

  Perfectly amorphous.

  G = 4·85. H = 6. Black; opaque; infusible.

  Impilaks, Lake Ladoga, Finland.


_Wöhlerite._

  Silicate and columbate of Ca, Zr, Na; Si₁₀Zr₃Cb₂O₄₂F₃Ca₁₀Na₅? ZrO₂ =
  15·2-22·7%. Cer, traces.

  Monoclinic; prismatic or tabular habit.

  G = 3·41-3·44. H = = 5¹⁄₂-6. Light yellow; pleochroic.

  Langesund Fiord.


~Xenotime.~

  Phosphate of E, with ThO₂, SiO₂, Zr, etc. Cer = 0-11; Yttr =
  54·1-64·7; ThO₂ = 1-5%.

  Tetragonal; isomorphous with Zircon?

  G = 4·45-4·56. H = 4-5. Brown to yellow; opaque.

  Diamond sands of Brazil; Norway.


_Yttrialite_ (Green Gadolinite).

  A weathered gadolinite (_q.v._)--E₂O₃,2SiO₂. Cer = 6·6-8·2; Yttr =
  43·4-46·5; ThO₂ = 10·8-12·8%.

  Amorphous, massive.

  G = 4·6. H = 5¹⁄₂. Green to brown; translucent.

  Bluffton, Llano Co., Texas.


~Yttrocerite.~

  Ca₃E₂F₁₂, 1¹⁄₂H₂O. Cer = 9·3-18·2; Yttr = 8·1-29·4%.

  Massive, granular.

  G = 3·45. H = 4¹⁄₂. White to violet blue or brown.

  Various localities in Scandinavia.


_Yttrocrasite._

  (Ca,Pb)O,(Th,U)O₂,3E₂O₃,16TiO₂,6H₂O. Yttr = 25·7; Cer = 2·9; ThO₂ =
  8·7; TiO₂ = 49·7%.

  Orthorhombic; axial ratios unknown.

  G = 4·80. H = 5¹⁄₂-6. Black; lustrous.

  Burnet Co. Texas.


~Yttrofluorite.~

  _n_CaF₂ + _m_YF₃ in isomorphous mixture? Yttr = 20-25; Cer = 1-2%.

  Cubic.

  G = 3·54-3·56. H = 4¹⁄₂. Closely resembles fluorspar, except in
  badness of cleavage.

  Northern Norway.


_Yttrogarnet._

  A variety of garnet with E and Zr. Yttr = 1-6·7; ZrO₂ = 0-3%.

  Cubic (cf. Garnet).

  Dark reddish brown (cf. Garnet).

  Stockö, Norway; Schreiberhau, Germany.


_Yttrogummite._

  UO₃, 3ThO₂, 3SiO₂, 6H₂O? E₂O₃ = 6·7; ThO₂ = 41·4%.

  Tetragonal; angles near Zircon. Usually massive.

  G = 4·43-4·54. H = 4-4¹⁄₂. Yellowish brown.

  Llano Co., Texas.


_Yttrotantalite._

  R´´R´´´₂(Cb,Ta)₄O₁₄ + 4H₂O; R´´ = Fe´´, Ca; R´´´ = E; Cer = 0-2·4;
  Yttr = 17·2-38·3%.

  Orthorhombic; isomorphous with Samarskite (_q.v._).

  G = 5·5-5·8. H = 5-6. Yellow to black.

  Ytterby, Sweden; South Norway.


~Zircon.~

  ZrSiO₄, with Fe, Th, etc., in traces. ZrO₂ = 61·0-70·0%.

  Tetragonal; habit prismatic.

  G = 4·68-4·70. varying considerably. H = 7¹⁄₂. Colour very variable.

  Widely distributed as a rock mineral, in sands, etc.


_Zirkelite._

  (Ca,Fe)(Zr,Ti,Th)₂O₅, with E, U, Mg, etc. ZrO₂ = 48·9-52·9; ThO₂ =
  0-7·3; TiO₂ = 14-15; E₂O₃ = 0-3%.

  Cubic; in twinned octahedra.

  G = 4·7. H = 5. Black; transparent in thin flakes.

  Jacupiranga, São Paulo, Brazil.


CHAPTER II

THE SILICATES


(_a_) SILICATES OF THE YTTRIUM AND CERIUM METALS


~Cerite.~--Cerite is a silicate of the cerium metals, with small amounts
of lime, ferrous oxide and water. Hintze gives the formula
H₃(Ca,Fe)Ce₃Si₃O₁₃,[15] which Groth interprets as a basic metasilicate
(Ca,Fe)[CeO]Ce₂(OH)₃(SiO₃)₃, _i.e._ a basic salt of the acid H₆Si₃O₉, a
polymer of metasilicic acid, H₂SiO₃.

  [15] The symbol (Ca,Fe) here indicates that the iron and calcium occur
  in variable proportions, the variation however occurring in such a way
  that the equivalent of the two taken together is always the same,
  _i.e._ the iron can replace the calcium, or _vice versa_, atom by
  atom. The recognition of this possibility of ‘Vicarious Replacement’
  between similar elements first brought order into the confused field
  of mineral chemistry, and allowed a systematic classification of
  minerals according to chemical composition to be made. Iron and
  calcium, or, according to the more convenient nomenclature of the
  mineralogists, lime and ferrous oxide, are here vicarious
  constituents.

  The symbol Ce here stands for elements of the cerium group, which are
  never found singly.

Crystals are not very common, the mineral usually occurring granular or
massive.

  Crystals, orthorhombic, holosymmetric; _a_ : _b_ : _c_ = 0·9988 : 1 :
  0·8127. Usual forms--the Pinakoids _a_, _b_, and _c_ {100}, {010} and
  {001}, prisms _m_ {110} and _q_ {130}, domes _u_ {101}, _t_ {301} and
  _n_ {011}, and some pyramids {hkl}.

  Angles, _a_ ∧ _m_ = 44° 58´, _u_ ∧ _c_ = 39° 8´, _n_ ∧ _c_ = 39° 6´.

  The crystals usually occur as short prisms. No cleavage. Optical
  constants unknown. In flakes the absorption spectrum of didymium can
  be observed.

  The mineral is brittle; hardness 5 to 6 on Mohs’ scale; sp. gr. varies
  a little about 4·9. Fracture splintery; lustre dull, resinous. Colour
  brown to red and greyish-red, streak greyish-white. The mineral is
  almost opaque.

Cerite is infusible before the blowpipe. It is attacked readily by
sulphuric acid, less easily by hydrochloric acid, with which it gives a
gelatinous mass. Rammelsberg[16] found that the silica left behind on
treatment of the powdered granular variety with the latter acid
contained a variable proportion of bases, which he obtained and
estimated after fusing the siliceous residue with sodium carbonate. From
the different proportions of the earths in the part attacked by the acid
and that left in the silica, he remarks, ‘It would almost appear that
Cerite is a mixture of silicates which are not all attacked with the
same ease by hydrochloric acid.’ Apparently without previous knowledge
of this observation, Welsbach[17] noticed the same thing in 1884. He
concluded that ordinary granular ‘cerite’ is a mixture of several
minerals, among which there are at least two which contain rare earths.
Of these, one, the chief constituent of the aggregate, is probably
identical with the crystallised mineral, and is characterised by the
readiness and completeness with which it is attacked by hydrochloric
acid. The other does not react, with hydrochloric, but is readily
attacked by sulphuric acid; it contains yttria earths, in addition to
the ceria earths. In the extraction of ceria earths from the mineral
aggregate, Welsbach used hydrochloric acid, so leaving this second
mineral unchanged; but to avoid loss of the rare earths, sulphuric acid
is more commonly employed for the decomposition.

  [16] _Pogg. Ann._, 1859, ~107~, 631.

  [17] _Monats._, 1884, ~5~, 512.

Though of great historical interest, cerite is of very small importance
for the extraction of rare earths at the present time, on account of its
very rare occurrence. The mineral seems to be almost entirely confined
to the Bastnäs quarry near Ryddarhyttan, Sweden, where it is found with
the rare earth silicate allanite (_q.v._), biotite, hornblende, bismuth
glance, chalcopyrite, etc. Here it was observed in 1751 by Cronstedt,
who called it Tungsten (_vide supra_, p. 1). In 1781 Scheele examined a
specimen of Wallerius’s ‘Tenn-spat’ from Bipsberg, Dalecarlia, and found
Tungstic Oxide (Acid), WO₃, in it.[18] After Scheele’s work, the
Ryddarhyttan mineral was known as Red Tungsten, until Bergmann (1780)
and d’Elhuyar (1784) showed that the two minerals were chemically
distinct. They considered the red variety to be a silicate of iron and
calcium, the rare earths being mistaken for lime. In 1804 Klaproth
examined it, and found a new earth; he called the mineral ‘Ochroite,’
from its colour. In the same year, but independently of Klaproth,
Berzelius and Hisinger made the same discovery; they called the mineral
Cerite and the new metal Cerium, in honour of the discovery of the minor
planet Ceres by Piazzi in 1801.

  [18] This mineral, which Scheele knew as Tungstein, is now called
  Scheelite.

The analyses of cerite made in the earlier part of the nineteenth
century resulted in some confusion. Klaproth in 1807 found 34·5 per
cent. SiO₂ in a specimen (his Ochroite); Vauquelin in 1805, and Hisinger
in 1810, found 17·0 and 18·0 per cent. respectively.[19] Hermann[20]
called attention to this discrepancy in 1843 (and again in 1861), and
declared that the two could not be the same. For Klaproth’s mineral he
proposed to revive the name Ochroite, whilst from his own analyses he
proposed for the cerite of Berzelius the name Lanthanocerite, having
found carbon dioxide and lanthanum, with much less cerium, in the
latter.[21] In 1861 Kenngott partly explained these results by showing
that the sample of cerite which Hermann had analysed contained
Lanthanite[22]; but the extraordinarily high percentage of silica
obtained by Klaproth remained unexplained. It may have been due to
impurities of high silica content in the specimen he examined.

  [19] _Vide_ Hintze, _Handbuch der Mineralogie_, Leipzig, 1897, ~ii.~,
  1329.

  [20] Hermann, _J. pr. Chem._ 1843, ~30~, 194, and 1861, ~82~, 406.

  [21] The announcement of the discovery of Lanthanum by Mosander was
  made in 1839.

  [22] Lanthanite (see list) is an hydrated carbonate, R₂O₃,3CO₂,9H₂O,
  where R = cerium metals, chiefly Lanthanum.

Cerite contains from 59·4 to 71·8 per cent. of rare earths (oxides), the
amount and nature of which vary with the precise locality. The oxides
consist chiefly of ceria, lanthana, and didymia (praseodymia and
neodymia), the complexity of the so-called ceria having been shown by
Mosander in the case of ceria separated from gadolinite as well as from
cerite; but yttria earths are also found to a small extent in the
mineral.

It is remarkable that neither thorium nor uranium has been found in
cerite, which is thus practically unique among the rare earth minerals.

This anomaly becomes even more marked in view of the very high
percentage of inert gases found by Tschernik[23] in a related mineral
from Batoum. This is a very complex mineral in which the basic part is
represented by rare earths, chiefly ceria earths (50·8 per cent.) with
water (3·4 per cent.), and oxides of iron, calcium and copper (6·8 per
cent.); the acidic oxides being silica (6·6 per cent.), zirconia (11·6
per cent.), and titanium dioxide (14·7 per cent.), with phosphorus
pentoxide (3·2 per cent.), and sulphuric anhydride (1·7 per cent.).
Traces of thoria are present, but no uranium; very considerable
quantities (up to 1 per cent.?) of helium were found.

  [23] G. Tschernik, _J. Russ. Phys. Chem. Soc._ 1896, ~28~, 345; 1897,
  ~29~, 291. Abstracts in _Zeitsch. Kryst. Min._ 1899, ~31~, 513 and
  514.

It is somewhat heavier than cerite (sp. gr. 5·08), but otherwise
resembles it closely.


~Gadolinite~ (Ytterbite).--Gadolinite is a silicate of iron, beryllium,
and the yttria earths, of the formula 2BeO,FeO,Y₂O₃,2SiO₂, which may be
written FeBe₂Y₂Si₂O₁₀. According to Groth, it is a basic orthosilicate,
Be₂Fe(YO)₂(SiO₄)₂, derived from the acid H₈Si₂O₈. The beryllium content
varies considerably, and some authors recognise two varieties of the
mineral, one rich, and one poor in beryllium; but Scheerer pointed out
in 1840 that iron and beryllium are probably vicarious constituents.

Analysis gives silica 21·8 to 25·3 per cent.; yttria earths 22 to 47 per
cent.; ceria earths 5 to 31 per cent. In a variety from Ytterby, the
rare earth Scandia was first found, forming up to 0·02 per cent. of the
mineral. Small quantities of thoria, ThO₂ may be present, and traces of
helium were found by Ramsay, Collie, and Travers. According to Strutt it
contains also uranium and radium. Like cerite, it does not often occur
crystalline, being usually found in amorphous masses.

  The crystals are monoclinic; _a_ : _b_ : _c_ = 0·6273 : 1 : 1·3215; β
  = 89° 26¹⁄₂´.

  Common forms are--Ortho-, clino-, and basal pinakoids, _a_ {100}, _b_
  {010}, and _c_ {001}, hemi-prisms _m_ {110}, _v_ {120}, clino-prisms
  _w_ {012}, _q_ {011}, and many others; and various hemi-pyramids {hkl}
  and {h̅kl}.

  Angles _a_ ∧ _m_ = 32° 6´, _c_ ∧ _q_ = 52° 53´, _c_ ∧ (101) = 64° 9´.

  Crystals commonly prismatic, terminated by _c_. Faces rough and
  coarse; lustre vitreous to greasy, seen only on freshly-broken
  surfaces. Brittle. No cleavage. Fracture conchoidal to splintery.
  Hardness 6¹⁄₂-7; sp. gr. 4·0-4·5.

  Colour black, greenish- and brownish-black; green and transparent in
  flakes. The crystalline variety has strong positive birefringence,
  with the plane of the optic axes parallel to (_b_), the plane of
  symmetry; the amorphous variety is of course isotropic. The brown
  variety shows very distinct pleochroism, _i.e._ the colour as seen by
  transmitted light varies with the direction in which the light
  traverses the crystal; the green kinds have much weaker pleochroism.

Gadolinite is of common occurrence in the pegmatite veins of the
Scandinavian granite. It was first found in a felspar quarry on the
island of Ytterby, near Stockholm, by a Lieutenant Arrhenius[24]; it is
also found, together with a large number of other rare earth minerals,
at Fahlun. It occurs in Norway on the islands of Hitterö and Malö, and
in Germany in the Riesengebirge and the Harz. Probably the largest
deposit is that in Texas, at Barringer Hill, near Bluffton, on the west
bank of the Colorado River, Llano County, now owned and worked by the
Nernst Light Company of Pittsburg; in 1904 a mass of very pure
gadolinite weighing 200 lb. was found here.[25]

  [24] _Vide_ Geijer, _Crell’s Chemische Annalen_, 1788, ~1~, 229.

  [25] See _U.S. Geol. Survey_ (_Minerals_), 1904, 1213.

In the same place a decomposition product of gadolinite was discovered
by Hidden and Mackintosh in 1889. They named it Yttrialite or Green
Gadolinite. It contains no beryllium, and twice as much silica as the
parent mineral, and approximates to the formula R₂O₃,2SiO₂, where R₂O₃
is chiefly yttria oxides; it is thus similar in composition to the newly
found scandium silicate, Thortveitite (_q.v._). It is amorphous and
massive; and is often found in continuous growth with gadolinite. Pieces
up to 10 lb. in weight have been obtained.

As stated above, Gadolinite was discovered by Arrhenius in 1788. Geijer
examined it in the same year, and described it as a black zeolite. In
1794 it was analysed by Gadolin, who declared it to be a silicate of
iron, aluminium, and a new element which he called Ytterbium. In 1797
Ekeberg examined it, and confirmed the discovery. He proposed the name
Gadolinite for the mineral, and Yttria for the new earth; these names
were accepted by Klaproth, who examined it with Vauquelin in 1800, and
by the French crystallographer Haüy. In 1802 Ekeberg showed that the
oxide originally taken for alumina was in reality beryllia; in 1816
Berzelius showed that ceria was present with the yttria.[26] About 1838
Mosander began his classical work on the earths in gadolinite. In that
year he announced the separation of Lanthana,[27] and in 1842 that of
Didymia, which he had actually discovered eighteen months earlier. In
the latter year he announced[28] the separation of erbia and terbia. In
1842 also Scheerer[29] declared that the yttria from gadolinite was a
mixture of earths, from its different behaviour on heating in closed and
open vessels; but when Mosander announced the discovery of didymia (the
announcement appears to have been hastened indeed by Scheerer’s
observation) it was agreed that the colouration observed was probably
due to that earth. The further history of these earths must be continued
elsewhere (_vide_ p. 111).

  [26] _Schweigg. J._, 1816, ~16~, 405.

  [27] Berzelius (a letter to Pelouze), _Pogg. Ann._, 1839, ~46~, 648.

  [28] _Berz. Jahres._, ~23~, 145; ~24~, 105.

  [29] _Pogg. Ann._, 1842, ~56~, 483.

The behaviour of gadolinite on heating is of great interest. When heated
uniformly, in closed or open vessels, the mineral suddenly glows very
strongly at a definite temperature (according to Hofmann and Zerban[30]
at 430°C.), with considerable alteration in properties. The amorphous
variety exhibits the phenomenon much more markedly than the crystalline
form. The change in the two cases is entirely distinct, the only effect
in common being that both varieties are rendered insoluble in acids
after the glowing. The amorphous variety, in the act of glowing, changes
to the crystalline form.

  [30] _Ber._, 1903, ~36~, 3095.

This phenomenon of phosphorescence, or glowing, on heating, with a
change in properties, was first observed by Berzelius in 1816. He found
that the oxides of many metals, _e.g._ chromium, tantalum, and rhodium,
became denser and insoluble in acids after being heated. Later in the
same year he observed the glowing, with a similar change in properties,
in the case of a gadolinite from Fahlun.[31] Apparently without
knowledge of this observation, Wollaston published a similar account of
the glowing of a gadolinite in 1825. In 1840 Scheerer noted an almost
identical change in the case of the mineral allanite (_q.v._). Scheerer
made a careful study of the phenomena in the cases of allanite and
gadolinite.[32] In each case he found that the variety of lower specific
gravity showed, on heating, a very strong phosphorescence, accompanied
by change of colour and optical properties, and a marked increase of
specific gravity. Gadolinite suffered no appreciable loss of weight, but
allanite had lost a little water after the change. Careful measurement
of the specific gravity before and after the change showed, in the case
of two varieties of gadolinite and one of allanite, that the volume had
decreased in the ratio 1 : 0·94. Scheerer assumed that this ratio was
constant for all such cases, and advanced a general explanation. We know
now that numerous cases of similar phenomena occur, in which the change
of volume is quite different; but Scheerer’s explanation is so
ingenious, and so foreshadows some modern theories, that it is given
here in full.

  [31] _Schweigg. J._, 1816, ~16~, 405.

  [32] _Pogg. Ann._, 1840, ~51~, 493.

He ascribes the alteration to ‘interatomic change, involving change of
relative position of atoms and decrease of interatomic distances.’
(Scheerer and the chemists of that period understood by atoms the
ultimate particles of a body, making no distinction between elements and
compounds; in this case he meant by atoms what we mean by molecules, and
the word ‘molecule’ has therefore been substituted for ‘atom’ in what
follows.) The change is simply one of closer packing of the molecules,
which take up a more stable position with liberation of energy as heat
and light. He imagines his molecules as uniform spheres arranged in
horizontal layers, as shown in Fig. 1. In placing one layer vertically
over another there are three possible arrangements, of which only two
concern us. In the arrangement for closest packing, B, say, a molecule
of any one layer touches three molecules in each of the layers above and
below, which with the six it touches in its own layer make twelve
altogether. In the next closest arrangement, A, say, a molecule of any
one layer touches only two molecules in each of the layers above and
below it, so that one molecule is in contact with ten others altogether.

[Illustration: FIG. 1]

Now it can be shown that the volumes of equal numbers of molecules in
the arrangements A and B will be to one another as the height, H, of an
equilateral triangle, to the height, h, of a regular tetrahedron whose
edges are equal to the sides of the triangle, a length R (which will be
equal to the diameter of a molecule).

  Then H = ¹⁄₂R√3, _h_ = R√²⁄₃.

  Then vol. in arrangement A : vol. in arr. B ∷ H : _h_

    _i.e._                                    ∷ √3/2 : √²⁄₃

                                              ∷ 1 : 0·943.

That is, the volume changes in the ratio 1 to 0·943, the amorphous
variety of gadolinite consisting of molecules in arrangement A, which go
over to the closer packed arrangement B in the change to the crystalline
form.

More extended work has shown that this ingenious and interesting
explanation is not of general application. Thus H. Rose[33] found that
samarskite (_q.v._) exhibited the phenomenon of glowing, but that the
specific gravity was actually less after the change than it was before,
_i.e._ there was an increase of volume. Damour observed glowing in the
case of zircon from Ceylon (_q.v._) with increase of density, the volume
change being from 1 to 0·922, _i.e._ even greater than for gadolinite.
Again, Hauser[34] observed in the case of his new rare earth mineral
risörite a sudden change at a red heat, the mineral losing water,
becoming very brittle, and increasing very considerably in specific
gravity (the volume changing from 1 to 0·90 approximately), but without
glowing. Ramsay and Travers[35] found that fergusonite (_q.v._) glowed
strongly when heated to 500°-600°, with decrease of specific gravity
(5·62 before to 5·37 after), evolution of all its helium, and very
considerable evolution of heat; they suggested that helium was present
in combination, in an endothermic compound decomposed by heat, but in
view of the properties of helium, this hypothesis seems hardly tenable.

  [33] _J. pr. Chem._ 1858, ~73~, 391.

  [34] _Ber._ 1907, ~40~, 3118.

  [35] _Zeitsch. physikal. Chem._ 1898, ~25~, 568.

It appears unlikely that any one explanation can cover all these
interesting facts; there are in each case peculiar factors to be taken
into account. In 1841, Regnault,[36] considering the case of the oxides
observed by Berzelius, inferred that the development of light and heat
denoted that the bodies possessed a lower specific heat after the change
than before. The experimental difficulties encountered in attempting to
dry the oxides prevented him from confirming this view. He measured the
specific heats of the minerals calcite and aragonite (CaCO₃), and of the
two allotropic modifications of phosphorus, but could observe no
appreciable differences. H. Rose (_vide supra_) showed by experiment
that considerable heat was evolved on the glowing of gadolinite, with a
decrease of about one-fourteenth in the specific heat. In the case of
samarskite there was, however, no appreciable evolution of heat, nor
could he determine any difference in the specific heats before and after
glowing.

  [36] _Pogg. Ann._ 1841, ~53~, 249.

Probably the only inference that can be safely drawn is that in most
cases the change is due to some molecular re-arrangement. The evolution
of water, helium, etc., in some cases, may possibly be due to
intramolecular change, but on the one hand the current view at present
is that the helium is mechanically held in radio-active minerals, and on
the other hand it is not known that the water evolved is water of
constitution; in an intermolecular change at fairly high temperature,
these might be evolved without disruption of the true mineral molecules.
The question of the energy involved, and consequently of the specific
heats, appears to depend on factors peculiar to each case, of which at
present no accurate conception can be formed; and the change in specific
gravity is probably bound up with these. The loss of solubility in acids
is a factor not always connected with glowing, as it is frequently
observed in the laboratory after ignition of compounds, but here again
no adequate explanation is forthcoming.

The possibility of chemical change in one or two cases, however, must
not be ignored. Thus ammonium magnesium phosphate, NH₄MgPO₄, on heating
glows, and is converted to magnesium pyrophosphate, according to the
equation:

  2NH₄MgPO₄ = Mg₂P₂O₇ + H₂O + 2NH₃

A case possibly analogous to this is that of the mineral sipylite
(_q.v._), R´´´₂Cb₂O₈, with ‘basic water’ (_i.e._ R´´´ partially replaced
by H). Before the blowpipe this decrepitates with loss of water, and
glows brilliantly. The specific gravity after the change does not appear
to have been determined. Mallet explains the glow as due to a change to
the pyrocolumbate.

Similar explanations may possibly hold in the cases of allanite and
risörite, but it must be remembered that we are really ignorant of the
part played by the water in these minerals.


~Allanite.~--Allanite, or Orthite, as it is often called, is a mineral
of the epidote family, containing rare earths. The general formula for
Epidote is H₂O,4R´´O,3R´´´´₂O₃,6SiO₂, where R´´ is a divalent and R´´´ a
trivalent metal, or vicarious series of metals. In the case of Allanite,
R´´ = (Fe´´,Ca), R´´´ = (Al,Fe´´´,E), where E stands for metals of the
cerium and yttrium groups (Engström’s formula). Groth formulates it as
a basic salt, R´´´₃(OH)R´´₂Si₃O₁₂, of the acid H₁₂Si₃O₁₂ (= 3H₄SiO₄).

Crystals are fairly common, but the mineral usually occurs massive or in
rounded grains.

  Crystals--Monoclinic, holosymmetric; _a_ : _b_ : _c_ = 1·5509 : 1 :
  1·7691, β = 64° 59´.

  Common forms--Ortho- and basal pinakoids _a_ {100} and _c_ {001}; _m_
  {110} and other prisms, _e_ {101} and other hemi-ortho-prisms, _o_
  {011}, _d_ {111} and other hemi-pyramids.

  Angles, (100) ∧ (110) = 54° 34´; (001) ∧ (101) = 63° 24´; (001) ∧
  (011) = 58° 3´.

  Tabular, parallel to _a_, or long and slender by elongation parallel
  to axis _b_.

  Birefringence weak, variable. Refraction strong. Colour brown to
  brownish-black; almost opaque. In flakes very strongly pleochroic, the
  colours for light parallel to the three vibration directions ~c~, ~b~
  and ~a~ being brownish-yellow, reddish-brown, and greenish-brown
  respectively.

  Brittle. Hardness 5¹⁄₂-6; sp. gr. 3·5-4·2.

On heating, allanite becomes amorphous and isotropic with increase of
specific gravity (cf. Gadolinite). Before the blowpipe it loses water,
and melts to a black magnetic glass, many varieties phosphorescing
strongly (_vide supra_). With hydrochloric acid it gelatinises, unless
previously heated strongly, in which case it is not attacked.

Analyses show that the rare earth content varies considerably
(vicariously as regards ferric iron and aluminium), ceria earths varying
from 3·6 to 51·1 per cent. and yttria earths from traces up to 4·7 per
cent.[37] Thoria is usually present, 0 to 3·5 per cent. In 1909
Fromme[38] found small quantities of beryllia in the mineral, and in
1911 Meyer[39] found amounts of scandium oxide up to 1 per cent. It
contains traces of uranium, and is weakly radioactive. Ramsay, Collie
and Travers found no helium (1895), but in 1905 Strutt found radium in
it, so that the presence of helium seems _a priori_ probable.

  [37] _Vide_ Schilling, pp. 70-75 for analyses of this mineral.

  [38] Fromme, _Tsch. Min. Mitt._ 1909, ~28~.

  [39] Meyer, _Sitzungsber. königl. Akad. Wiss. Berlin_, 1911, 379.

Many varieties of the mineral are known, differing in habit, colour,
water content, specific gravity, etc., and the percentage composition
varies very much by reason of vicarious replacement of the bases.
Goldschmidt[40] has found ‘Epidote-orthites’ which are isomorphous
mixtures of orthite with an iron epidote; he concludes that most
orthites are probably similar solid solutions, and in this way accounts
to a large extent for the varying composition.

  [40] _Centr. Min._ 1911, 4.

Allanite is of very wide distribution, though it is not often found in
large quantities. The usual occurrence in pegmatitic veins in granites,
syenites and other acid plutonic rocks has been often noted, _e.g._ in
many parts of Sweden and Norway. It is found also in the extinct crater
now forming the Laacher See, near Coblenz, Germany, and at Impilaks,
near Lake Ladoga, on the border of Finland; a mass of the pure mineral
weighing 300 lb. was recently discovered at Barringer Hill, (cf. under
Gadolinite), and it occurs in large quantities in Amherst Co., Virginia.
It is an accessory constituent of many acid volcanic and hypabyssal
rocks, and has been found also in limestone, and in magnetic iron ores.
On account of its exceedingly wide distribution, and the variations in
appearance and composition, it has been repeatedly described under
various names, varieties being constantly mistaken for new mineral
species.

Its history is rather curious.[41] In 1806 the Danish mineralogist
Giesecke made a protracted voyage to Greenland, collecting minerals and
rocks; he remained there until 1813. In 1808 he sent off his first
collection by ship to Copenhagen; on the voyage the ship was taken by an
English privateer, and the cargo landed and sold at Leith. The minerals
were bought by Allan, a Scotch mineralogist, who recognised, that they
were from Greenland by the presence of cryolite, at that time only known
to occur in Greenland. He mistook the mineral subsequently named after
him for gadolinite, and sent it to Thomson for analysis.[42] Thomson
recognised it as a new mineral, and named it Allanite (1810). In 1815
Hisinger described a mineral from Ryddarhyttan, Sweden, which he called
Cerin; Leonhard (1821) and Hauy (1822) showed that this was identical
with Allanite. In 1818 Berzelius described two varieties of a mineral
from Finbo, near Fahlun, Sweden, which he called Orthite, and
Pyrorthite; these were eventually shown by Scheerer (1844) to be
varieties of Allanite. In 1824 the French mineralogist Lévy described a
mineral from Arendal, Norway, which he named Bucklandite, in honour of
the English naturalist; in 1825 this was identified with a ‘black
zeolite’ from the Laacher See by G. Rose, and in 1828 both were shown by
Hermann to have the same composition as orthite or allanite. The list
might be extended at will; the Tautolite of Kokscharow (1847), the
Bodenite of Breithaupt (1844), the Muromontite of Kemdt (1848), and the
Vasite of Bahr (1863) have all been shown to be varieties of the same
bewildering mineral.

  [41] _Vide_ Schilling, pp. 75-76, where full references are given.

  [42] See Kobell’s _Geschichte der Mineralogie_, 1864, p. 679.


~Hellandite.~--Hellandite[43] is a mixed silicate of rare earths
with lime, magnesia, alumina, ferric and manganic oxides, with
considerable quantities of water. The formula approximates to
3H₂O,2R´´O,3R´´´´₂O₃,4SiO₂, where R´´ = (Ca,Mg,Th/2)--Thorium being able
to replace two atoms of calcium or magnesium--and R´´´ = (Al,Fe´´´,Mn´´´
and rare earth metals). This may be written as a basic orthosilicate,
R´´₂[R´´´´(OH)]₆(SiO₄)₄, a basic salt of the acid H₁₆Si₄O₁₆ (= 4H₄SiO₄).
This composition puts it in the class containing topaz and some rarer
silicates.

  [43] Brögger, _Zeitsch. Kryst. Min._ 1906, ~42~, 417.

The mineral is crystalline, the crystals being well developed, but often
dull and opaque by alteration (hydration).

  Crystal system--Monoclinic, holosymmetric, _a_ : _b_ : _c_ = 2·0646 :
  1 : 2·507. β = 109° 45´. Habit usually prismatic, with {100}, {010},
  and several prisms {_hko_}, terminated by various pyramid forms.

  Angles (100) ∧ (001) = 70° 32´; (100) ∧ (110) = 62° 22´; (010) ∧ (110)
  = 27° 14´; (110) ∧ (11̅0) = 125° 0´.

  Twinned on (001), twin plane (001), forming knee-shaped twins.
  Hardness varies from 5¹⁄₂ in the least altered to 1 in the most
  altered specimens; sp. gr. 3·70 in least altered specimens, decreasing
  with hydration. Colour of fresh crystals, reddish-brown; on alteration
  they become brownish-black, yellow, or even white.

The mineral dissolves easily in hydrochloric acid, with evolution of
chlorine; it is less soluble in nitric and sulphuric acids. It readily
fuses to a yellow mass.

It was first discovered by Brögger at Lindvikskollan, in 1903, and
later, in larger quantities, at Kragerö in Norway. It occurs in
pegmatite veins in granite.


~Thalénite.~[44]--A silicate of yttria earths with water and small
quantities of alumina, ferric oxide, carbon dioxide and alkalies. The
ratio of rare earths to silica gives the formula R₂O₃,2SiO₂, or R₂Si₂O₇;
if the water be included, the formula becomes H₂R₄Si₄O₁₅. The presence
of both water and carbon dioxide indicates, however, that the mineral
has been somewhat altered, and the simpler formula R₂Si₂O₇, (cf.
Thortveitite, below) probably expresses the composition of the original
mineral. It contains considerable quantities of nitrogen and helium,
though uranium and thorium appear to be absent.

  [44] Benedicts, Abstract in _Zeitsch. Kryst. Min._ 1900, ~32~, 614.

  Monoclinic; _a_ : _b_ : _c_ = 1·154 : 1 : 0·602. β = 80° 12´.

  Common forms are the pinakoids {100} and {010}, hemi-prism {110},
  hemi-pyramids {111} and {111̅}, and others, and the hemi-dome {021}.

  Angles, (100) ∧ (010) = 91° 0´; (100) ∧ (110) = 48° 9´; (100) : (111)
  = 59° 4´.

  Double refraction weak. No cleavage. Brittle. Hardness 6¹⁄₂. Colour,
  bright flesh-red; translucent, with greasy lustre; sp. gr. 4·227,
  increasing to 4·29 after ignition. A yellow variety has sp. gr.
  4·11-4·16, and is transparent.

The ‘average atomic weight’ of the rare earth metals is 99, from which
it appears that these consist chiefly of yttrium, with a smaller
quantity of the metals of higher atomic weight.

It was discovered in 1898 by Benedicts, accompanying fluocerite (_q.v._)
in a quartz quarry at Oesterby in Dalekarlia.


~Thortveitite.~[45]--A silicate of yttria earths, chiefly scandia, of
the formula R₂O₃,2SiO₂. Scandia forms about 37 per cent. of the whole
(R. J. Meyer); yttria with small quantities of the other yttria earths
forms the bulk of the remainder of the bases, the ceria group being
almost completely absent. Ferric oxide (with traces of manganic oxide
and alumina) forms about 3 per cent. Thorium is present only in traces,
and radioactivity is barely perceptible.

  [45] J. Schetelig, _Centr. Min._ 1911, 721.

Thortveitite is the first mineral to be discovered in which the content
of scandia is greater than 2 per cent.; in 1908 Crookes[46] examined a
very large number of yttria minerals for scandia, and finally chose for
extraction of the earth Wiikite (_q.v._) which has a scandia content of
1·2 per cent.[47]

  [46] _Phil. Trans._ 1908, A, ~209~, 15.

  [47] According to Eberhard, some varieties of Wiikite have a much
  lower scandia content.

  Thortveitite is orthorhombic; _a_ : _b_ : _c_ = 0·7456 : 1 : 1·4912;
  commonly combinations of pyramids _o_ {111} and _s_ {211} with prism
  _m_ {110}, in radial aggregates of crystals elongated parallel to the
  _c_ axis. Cleavage parallel to _m_, fair. Twin plane _m_ (110),
  twinning very common.

  Refraction strong; birefringence strong, negative. Acute bisectrix
  perpendicular to (001), plane of the optic axes (010). Hardness, 6-7;
  sp. gr. 3·571. Extremely brittle; lustre brilliant, vitreous to
  adamantine. Colour, greyish-green, white to reddish-grey on
  alteration; in transmitted light yellowish-green, after ignition,
  reddish; the change being probably due to presence of oxides of iron.

It is fusible with difficulty, and only partially attacked by
hydrochloric acid. It was found by Thortveit, in 1910, in a pegmatite
vein in granite, at Iveland, Sätersdalen, S. Norway, accompanied by
euxenite, monazite, beryl, and the usual vein-materials (quartz,
felspar, etc.). It was analysed and recognised as a new mineral by
Schetelig (_loc. cit._).

       *       *       *       *       *

The following minerals, of which particulars will be found in the
alphabetical list, also belong to this class:

_Bagrationite_, _Bodenite_, and _Muromontite_, varieties of allanite
with differences in composition and physical properties.

_Yttrialite_, a weathered variety of gadolinite.

_Elpidite_, _Erdmannite_ and _Cainosite_, more complex silicates.

_Rowlandite_, a comparatively simple silicate of the yttrium metals.

_Yttrogarnet_, a variety of garnet containing yttrium metals.


(_b_) SILICATES OF THORIUM AND ZIRCONIUM


~Thorite.~--Thorite and its variety Orangite are somewhat altered forms
of a pure silicate of thorium, ThSiO₄, containing also small quantities
of water, usually uranium, and often rare earths, with iron, lead,
calcium, and aluminium. Orangite differs from thorite in its beautiful
orange colour and greater specific gravity. Both varieties are
radio-active.

When unaltered, the crystals are tetragonal and uniaxial, the pure
mineral ThSiO₄ being isomorphous with zircon, ZrSiO₄ (_q.v._). By
alteration they become isotropic.

  Crystals are tetragonal, holosymmetric; _c_ = 0·6402; _p_ ∧ _p_´ = 56°
  40´.

  Common forms are the prism _m_ {110} with the pyramids _p_ {111} and
  _z_ {311}.

  Hardness 4¹⁄₂-5; sp. gr. 4·4 to 4·8 for thorite, 5·2 to 5·4 for
  orangite.

Thorite contains from 1·4 to 3·1 per cent. of rare earths. According to
Nilson and Blomstrand, the uranium is present as uranium dioxide, UO₂
replacing thoria, ThO₂, but Dunstan and Blake state that the two oxides
are isomorphous (see under Thorianite, p. 74), and so they might be
expected to be vicarious. Thorite was discovered by Esmark in 1828, and
first analysed by Berzelius,[48] who announced the discovery of a new
earth in it in 1829. The name Thorite is from Thor, the god of
Scandinavian mythology.

  [48] _Pogg. Ann._, 1829, ~16~, 385.

Thorite is a member of a peculiarly interesting series of isomorphous
minerals, which includes Cassiterite (SnO₂), Rutile (TiO₂), Zircon
(ZrSiO₄), and most probably the allied silicate Naegite, and the rare
earth phosphate Xenotime (_q.v._), which are very similar in forms and
angles. The oxide TiO₂ is itself trimorphous, being known in the three
crystallographically different forms, Rutile, Anatase, and Brookite
(_q.v._). On account of the isomorphism of cassiterite and rutile with
the two silicates, it has been suggested that the oxide formulæ be
doubled and written Sn(SnO₄) and Ti(TiO₄) respectively,[49] to show the
analogy with Th(SiO₄) and Zr(SiO₄). Consideration of the molecular
volumes (obtained by dividing molecular weight by specific gravity,
_i.e._ multiplying by specific volume) lends a certain amount of support
to this view. It has often been observed that isomorphous compounds, and
many compounds which occur in parallel growth to one another, have
nearly equal molecular volumes; there are, however, many exceptions.
Taking molecular volumes for the series under consideration, we have,
using approximate numbers only--

                         Mol. Wt.   Sp. Gr.           Mol. Vol.

  Cassiterite, SnO₂        151        6·9                22
  Rutile, TiO₂              80        4·2                19
  Zircon, ZrSiO₄           182        4·7                39
  Thorite, ThSiO₄          325        5·4 (Orangite)     60
  Xenotime, XPO₄           184        4·5                41

  [49] This isomorphous series has recently been extended by Zambonini,
  and also by Schaller, by the inclusion of minerals containing
  Columbium and Tantalum; see under Ilmenorutile and Strüverite, end of
  Ch. IV., p. 71.

It will be seen that if the numbers for cassiterite and rutile be
doubled, four out of the five show very fair approximation to the
constant value 40. The number 60 for thorite is quite irreconcilable
with the values obtained from the other members; of course pure silicate
of thorium, ThSiO₄, is not known as a mineral, but it is most unlikely
that the relatively small amount of impurity in the densest specimens of
orangite should have depressed the specific gravity by over two units,
as would be required if the molecular volume of thorite were to show
even the most approximate semblance of agreement with the others. It
cannot be too often remarked, however, that very little indeed is known
of the molecular formulas of minerals, and that very little reliance can
be placed on such figures as the above. On the contrary, it is hardly
conceivable that amphoteric oxides like those of tin and titanium,
occurring in the form of heavy crystalline minerals, should have
molecular formulæ only double the empirical formulæ. Where agreements of
the kind do occur, they must be taken as indicating approximately equal
degrees of molecular complexity in the minerals concerned, rather than
as affording any real insight into the molecular condition.


~Zircon.~--Zircon is a silicate of zirconium, ZrSiO₄, with small
quantities of other elements. Most varieties contain ferric oxide and
thoria; more rarely small proportions of the yttria earths may be
present. All varieties contain traces of a large number of the common
metals. Traces of radium are usually present, with helium and neon,[50]
and the mineral is strongly radioactive.

  [50] Strutt, _Nature_, 1906, 102.

  System tetragonal, holosymmetric sub-class. _c_ = 0·6404; (001) ∧
  (101) = 32° 38´.

  Usual forms--Prisms _a_ {100} and _m_ {110}; pyramids _e_ {101}, _p_
  {111}, _u_ {221} and _x_ {311}, etc. The basal pinakoid _c_ {001} is
  rare. The usual combination is one or both of the prisms _a_, _m_,
  with one or two pyramids. Twinning is rare, the twin plane being _e_
  (101), giving knee-shaped twins similar to those so characteristic of
  cassiterite and rutile. Cleavage ∥ _m_ imperfect, ∥ _p_ bad.

  Brittle; conchoidal fracture. Hardness 7¹⁄₂; sp. gr. usually
  4·68-4·70, but varying from 4·2 to 4·86. Adamantine lustre. Clear and
  colourless to yellow-, red- or greenish-brown. Transparent to opaque.
  Refraction and double refraction strong, double refraction positive (ω
  = 1·924, ε = 1·968, for sodium light); on heating it becomes biaxial,
  and occasionally is found biaxial in nature. By alteration it becomes
  isotropic.

It is infusible before the blowpipe, but loses its colour; some
varieties glow and increase in density (see p. 38). In some varieties
also the colour changes or disappears rapidly on exposure to sunlight,
and is often restored on keeping in the dark. These phenomena of colour
change have been attributed variously to alteration in the state of
oxidation of the iron present, and to the presence of organic matter. It
seems probable that either cause or even both may be at the root of the
change in particular cases.

On account of the hardness, unalterability, and strong refraction and
double refraction, good crystals of zircon are used as gems. The two gem
varieties, Hyacinth and Jargon, are found chiefly in the gem gravels of
Ceylon. It was in a zircon from Ceylon that Klaproth discovered the new
earth, Zirconia, in 1789.[51] In 1795 he found the same earth in
hyacinth, and so showed the two to be identical.

  [51] _Schriften der Gesellschaft naturforschender Freunde in Berlin_,
  1789, vol. 9.

Artificial crystals of zircon have been obtained by the action of
silicon tetrachloride and silicon tetrafluoride on zirconia, and by the
action of zirconium tetrafluoride on silica at high temperatures.

Zircon is one of the most widely distributed minerals known, though
usually it occurs in very small quantities. Good crystals have been
found in New Zealand, in Ceylon, at Miask in the Urals, and in North
Carolina. This last deposit has been worked commercially for the
extraction of zirconia for Nernst lamps (_vide_ p. 320). It occurs in a
decomposed felspar in a pegmatite dyke in the Archæan gneiss near
Zirconia, Henderson Co., and can be easily extracted by picking or
washing, after crushing if necessary. Should there ever be a
considerable demand for zirconia, it could doubtless be saved as a
by-product in the extraction of thoria from monazite sands (_q.v._),
zircon being very generally found in those sands (see below).

Zircon is common in crystalline rocks, limestones, schists, syenites,
granites, etc. It is a constant accessory constituent in the acid
igneous rocks, especially in the more acid eruptive rocks. It is readily
detected under the microscope by the pleochroic haloes with which the
tiny crystals are surrounded; these have been shown by Joly to be due to
alteration of the surrounding rock by the radiations emitted by the
radio-active constituents of the zircon. It also occurs as a constituent
of those sands which are formed by the erosion of the igneous rocks in
which it is enclosed, and hence it almost invariably accompanies
monazite in the so-called monazite sands.

Zircon is one of the least easily altered minerals; by the prolonged
action of chalybeate and other waters, during many geological ages,
however, it gradually changes, losing silica and gaining lime, oxides of
iron, and water. Some of these altered varieties have received special
names, as, _e.g._ Auerbachite, Malacone, Cyrtolite, and Alvite; but none
of them is of special interest.


~Naegite.~[52]--This rare mineral is a silicate closely related to
zircon, but of rather more complex composition. It may be represented as
silicate of zirconium, ZrSiO₄ (zirconia = 55·3, silica = 20·6 per
cent.), with rare earths (chiefly yttria, 9·1 per cent.), uranium (UO₃ =
3 per cent.), and thorium (ThO₂ = 5·0 per cent.), partly as silicates,
partly as columbates and tantalates ((Cb,Ta)₂O₅ = 7·7 per cent.).[53]

  [52] _Beiträge zur Mineralogie von Japan_, 1906, ~2~, 23.

  [53] An earlier analysis (_Abstr. Chem. Soc._ 1905, ~88~, [ii.], 177)
  gave over 20 per cent. of uranous oxide, UO₂; the greater part of this
  appears to have been zirconia, ZiO₂.

It is tetragonal, usually occurring in globular aggregates of crystals.
The measurable angles are extremely close to those of zircon, and it is
probable that naegite is isomorphous with the series mentioned above
under Thorite.

  The hardness is 7¹⁄₂, the sp. gr. 4·091. The colour is dark green or
  brown, becoming dull by weathering. The double refraction is extremely
  weak.

So far it has only been found in the ‘placer’ tin deposits or ‘gravel
tin’ of Japan.

       *       *       *       *       *

The following minerals (see list) are also to be included in this
sub-class:

_Alvite_ (Anderbergite or Cyrtolite), _Auerbachite_, _Malacone_,
_Oerstedite_ and _Tachyaphaltite_, altered varieties of zircon.

_Calciothorite_, _Eucrasite_ and _Freyalite_, altered varieties of
Thorite.

_Pilbarite_, _Thorogummite_ and _Yttrogummite_, hydrated silicates of
thorium with uranium and other metals.


(_c_) COMPLEX SILICATES


~Eudialyte~ (Eucolyte).--This is a complex silicate of alkalies, lime,
ferrous oxide, rare earths, etc., containing chlorine and a high
proportion (up to 17 per cent.) of zirconia. The empirical formula is
given by Dana as Na₁₃(Ca,Fe)₆Cl(Si,Zr)₂₀O₅₂. Brögger gives the simpler
metasilicate formula R´₄R´´₃Zr(SiO₃)₇, where R = (Na,K,H), R´´ =
(Ca,Fe,Mn,CeOH), and Zr(OCl) may partly function as an acid in place of
SiO₂. The true formula, however, is quite uncertain, as the zirconia may
function either as an acidic or basic oxide. The fact that a mineral of
such exceedingly complex composition occurs in perfectly well-defined
crystals indicates the intricate nature of the problems to be solved in
mineral chemistry.

  The crystals are rhombohedral, _a_ : _c_ = 1 : 2·1116.

  Common forms are--the pinakoid _c_ {111}, prisms _a_ {101}, and _m_
  {211}, and pyramids _r_ {100} and _e_ {110}. _c_ ∧ _r_ = 31° 22´.
  Habit tabular parallel to _c_, rhombohedral with _e_ prominent, or
  prismatic with _a_ prominent.

  Cleavage ∥ _c_ very good, ∥ _a_ difficult.

  The colour is brown or red to brownish- or bluish-red. Brittle.
  Hardness 5 to 5¹⁄₂; sp. gr. 2·92 for eudialyte, 3·0 to 3·1 for
  eucolyte.

The double refraction is strong, being positive for eudialyte, negative
for the Norwegian variety, eucolyte. From careful microscopic
examination, Ramsay has found that zones of positive and negative
birefringence, as well as isotropic (singly-refracting) zones can occur
on the same crystal, and he suggests that the mineral is really composed
of two isomorphous compounds forming mixtures. In view of the continuous
variation of optical properties in an isomorphous series like the
felspars, such an explanation seems doubtful. The optical behaviour of
minerals is very often anomalous, and the phenomena in this case are
probably due to repeated twinning, with some alteration in the double
refraction, or to the lamellar intergrowth of two varieties having
slightly different optical properties.

On heating, the mineral evolves moisture and readily fuses. It is easily
attacked even by dilute acids, being named by Strohmeyer (1819) on
account of this property. The dilute hydrochloric acid solution reddens
turmeric paper--a test for the presence of zirconium.

It is found in Greenland, usually embedded in felspar, in Norway, in
Lapland and in Arkansas, being generally associated with minerals rich
in alkalies, _e.g._ ægirine, ælæolite, nepheline, sodalite,
arfvedsonite, etc.


~Beckelite.~--This is a mineral similar in composition to eudialyte,
though not so complex, and of more recent discovery.[54] It is a
silicate of ceria earths and lime, in which zirconia replaces silica;
the oxygen ratio (_i.e._ ratio of oxygen in basic oxides to oxygen in
acid oxides) is 3 : 1, and the formula Ca₃R´´´₄(Si,Zr)₃O₁₅, where R =
rare earth metals, chiefly of the cerium group. It is thus a salt of an
acid H₁₈Si₃O₁₅ [= 3H₆SiO₅ = 3(3H₂O,SiO₂)] with zirconium and silicon
vicarious.

  [54] _Abstr. Chem. Soc._ 1905, ~88~, ii, 177.

  The crystals appear to belong to the cubic system, occurring in cuboid
  grains, and in octahedra and dodecahedra. It is brown, and isotropic,
  with cubic cleavage. Sp. gr. = 4·15.

It is soluble in hot hydrochloric acid, even after ignition; the
solution gives the turmeric test for zirconium.

It was found in a dyke in an ælæolite syenite, near the Sea of Azov.

       *       *       *       *       *

The following minerals (see list) are also to be placed in the class of
mixed silicates:

_Arfvedsonite_ and _cataplejite_, complex zircono-silicates.

_Hiortdahlite_ (Guarinite) and _Lavenite_, zircono-silicates with
fluorine.

_Caryocerite_, _Melanocerite_ and _Steenstrupine_, complex
fluosilicates.

_Auerlite_, _Britholite_, _Erikite_ and _Florencite_, phospho-silicates.

_Cappelenite_, _Homilite_ and _Tritomite_, boro-silicates.


CHAPTER III

THE TITANO-SILICATES AND TITANATES


(_a_) THE TITANO-SILICATES


~Yttrotitanite or Keilhauite.~--A titano-silicate of calcium, aluminium,
iron and yttrium metals. The mineral is isomorphous with titanite,
CaO,TiO₂,SiO₂ (_q.v._), and is itself probably an isomorphous mixture of
titanite with the silicate (Y,Al,Fe)₂SiO₅, where Y = yttrium metals. Its
composition will then be represented by the formula _m_ (Y,Al,Fe)₂(SiO₅)
+ _n_ CaTi(SiO₅).

  It is monoclinic, with axial ratios and angles very close to those of
  titanite. Usual forms--pinakoids _a_ {100} and _c_ {001}, hemi-prism
  _m_ {110}, hemi-pyramids _n_ {111}, _e_ {1̅11} and _l_ {1̅12}.
  Cleavage ∥ _n_ distinct. Birefringence weak, +ve. Colour brown to
  brownish-black. Hardness 6¹⁄₂; sp. gr. 3·52 to 3·77.

The mineral is fusible before the blowpipe, and is decomposed by
hydrochloric acid.

It was named by Scheerer in 1844 from its composition, and by Ekeberg in
the same year in honour of the Norwegian geologist Keilhau.


~Titanite or Sphene.~--This species, important as an accessory mineral
of many rocks, is a titano-silicate of calcium, generally containing
small quantities of aluminium and iron. The approximate formula usually
given, CaTiSiO₅, is unsatisfactory; some specimens contain as much as 7
per cent. of ferric oxide, others up to 2 per cent. of manganese, whilst
the percentage of titanium oxide, TiO₂, varies very considerably (30 to
45 per cent.). Zambonini and Nickolan have independently analysed
specimens for which no satisfactory formulæ could be deduced. For
specimens containing trivalent metals, Groth considers the mineral to be
an isomorphous mixture of CaTiSiO₅ and R´´´₂SiO₅ (see under
Yttrotitanite, above); Blomstrand, however, advances the formula
2(R´´R´´´₂O₂,TiO)O,SiO₂, where TiO is basic, and the trivalent metals
occur in the divalent group R´´´₂O₂; this formula is also supported by
Zambonini.

More recently the problem of the constitution has been attacked by
Bruckmoser, using Tschermak’s method of determining the nature of the
salts present in silicates. In this method, the mineral is digested with
hydrochloric acid, at a temperature not greater than 60°, until
decomposition is complete; the silicic acid formed is washed by
decantation, and dried in air at a constant temperature; it is weighed
at regular intervals until the weight is constant. It is stated that if
a curve of times and weights be plotted, a break is observed at the
point where drying ceases (for the acid is of course wet) and
decomposition begins; the composition at this point, which is taken as
the composition of the acid required, can be determined from the weight
of the acid, and the weight of anhydrous silica present, which is
determined by ignition after the weight has become constant.

Employing this method in the case of titanite, Bruckmoser claims to have
obtained the acids H₂Si₂O₅ and H₂Ti₂O₅. He therefore concludes that the
constitution of the mineral is represented by the formula Si₂O₅,Ti₂O₅Ca,
which presumably may be written Ca(Ti,Si)₂O₅.

  Crystal system--monoclinic; _a_ : _b_ : _c_ = 0·7547 : 1 : 0·8543. β =
  60° 17´.

  Common forms (Des Cloizeaux’s orientation)--the pinakoids _a_ {100}
  and _c_ {001}, with _m_ {110}, _s_ {021}, _x_ {102}, _n_ {111}, and
  many others.

  (100) ∧ (110) = 38° 14¹⁄₂´; (001) ∧ (1̅01) = 65° 57´; (001) ∧ (011) =
  36° 34´.

  The habit is very varied, the commonest being the wedge form,
  elongated ∥ _c_. Twinning is fairly common, especially on the
  law--Twin plane ∥ _a_, which gives both contact and interpenetrant
  twins. Cleavage ∥ _m_, fairly distinct. Hardness 5 to 5¹⁄₂; sp. gr.
  3·40 to 3·56. Lustre adamantine to resinous. The colour varies very
  much, doubtless with the content of iron and manganese; it is commonly
  yellow, green, or brown. Pleochroism is very distinct. The refraction
  and dispersion are very high, giving the facetted stone a ‘fire’
  inferior only to that of diamond. Birefringence positive, strong; the
  axial angles vary very widely in different specimens.

It is fusible with difficulty before the blowpipe. Hot concentrated
hydrochloric acid decomposes it partially, with separation of silica;
boiling sulphuric acid, or, better, fused potassium hydrogen sulphate,
decomposes it completely.

On account of the high dispersion and refractive index, clear specimens
of sphene make very beautiful gems, but the stone is not sufficiently
hard to stand much wear.

The mineral was discovered in Chamouni by Pictet in 1787, and was named
Pictite by Delamètherie (1797). In 1795 Klaproth analysed a specimen
from Passau, and, observing the presence of titanium (which he had just
discovered in rutile), proposed the name Titanite. The mineral described
by de Saussure (1796) as ‘Schorl rayonnante,’ and afterwards by Hauy
(1801) as Sphene (σφήν = a wedge), was shown to be identical in
composition with titanite by Cordier, and also by Klaproth (1810); the
crystallographic identity was proved by G. Rose (1820).

On account of the difference in colour and composition, a large number
of varieties are distinguished. The ordinary yellow and brown varieties
are known indifferently as sphene or titanite. _Ligurite_ has an
apple-green colour; _Semeline_ is a greenish form named from a fancied
resemblance to flax seed. _Lederite_ is a brown variety of tabular
habit; _Greenovite_ is rose-coloured, and contains manganese.
_Alshedite_ and _Eucolite-Titanite_ are rich in the trivalent metals;
_Grothite_ is a brown variety containing a considerable percentage of
ferric iron. _Yttrotitanite_, which contains a high proportion of rare
earths, is usually treated as a separate species (see above).
_Titanomorphite_ and _Leucoxene_ are white amorphous varieties chiefly
produced by alteration of rutile and ilmenite.

Titanite is a fairly widespread mineral; as an accessory rock
constituent it is common in the massive plutonic rocks in tiny crystals,
readily distinguished under the microscope by the high refraction and
birefringence, whilst in large embedded crystals it occurs in many
granular limestones, and in plutonic acid, as well as in some
metamorphic rocks. In good crystals it is found in many parts of
Switzerland and the Alps, in Dauphiné, the Tyrol, Piedmont, the Urals,
South Norway, and other European localities; it is also widely
distributed in the United States and Canada.

The mineral is important as a valuable source of titanium.

       *       *       *       *       *

The class of Titano-silicates is a very large one, and might be extended
almost at will by the inclusion of the numerous silicates which contain
titanium. Owing to the frequency with which small quantities of silica
are replaced by titanium dioxide, almost all the commoner silicate
minerals contain the latter oxide, so that titanium is one of the most
widely distributed of the elements. Relatively very few, however, of the
titanium-bearing minerals contain the element in considerable
quantities, and only two or three have any importance as commercial
sources of titanium compounds.

Only those additional titano-silicates which contain titanium as an
important constituent are mentioned below; short accounts will be found
in the alphabetical list.

_Johnstrupite_, _Mosandrite_, _Rinkite_, _Rosenbuschite_ and
_Tscheffkinite_ are complex titano-silicates containing yttrium or
cerium metals.

_Astrophyllite_, _Leucosphenite_, _Molengraafite_, _Neptunite_ and
_Rhönite_ are complex titano-silicates free from rare earth elements.

_Benitoite_ is a simple titano-silicate of barium; _Ænigmatite_ and
_Narsarsukite_ contain iron and sodium; _Lorenzenite_ has sodium and
zirconium. _Schorlomite_ is a titaniferous garnet. A variety of olivine
rich in titanium (_Titanium Olivine_) is also known.


(_b_) THE TITANATES


~Yttrocrasite.~[55]--This is a complex titanate of rare earths (chiefly
yttria earths) with lime, thoria, and oxides of lead, iron, uranium,
etc.; it has a considerable water content. An approximate formula is
R´´O,R^{iv}O₂,3R´´´₂O₃,16TiO₂,6H₂O, where R´´ = (Ca,Pb,Fe), R^{iv} =
(Th,U), and R´´´₂O₃ = rare earths. No constitutional formula can be
given; it will be noticed that the amount of titanium dioxide is
considerably more than is required to combine with the bases present
(cf. also Delorenzite below). It is radioactive.

  [55] Hidden and Warren, _Amer. J. Sci._ 1906, [iv.], ~22~, 515; also
  _Zeitsch. Kryst. Min._ 1907, ~43~, 18.

  Imperfect crystals only were found, apparently belonging to the
  orthorhombic system. No crystallographic data could be determined.

  The mineral is black, closely resembling polycrase and euxenite
  (_q.v._) in appearance. Hardness 5¹⁄₂-6; sp. gr. 4·80.

It is infusible, and not easily soluble in acids. Hydrofluoric acid
decomposes it, and the powdered mineral is also slowly attacked by
boiling concentrated sulphuric acid.

It was found in 1904 by Barringer, in Burnet Co., Texas.


~Delorenzite.~[56]--A compound similar to the above, but even richer in
titanium dioxide, which amounts to 66 per cent. Tin dioxide is also
present, with traces of columbic anhydride. The bases are the yttria
earths (almost free from ceria earths), uranium dioxide, and some
ferrous oxide, the formula being 2FeO,UO₂,2Y₂O₃,24TiO₂, with a little
SnO₂ replacing TiO₂. It is strongly radioactive. Its closest chemical
neighbour is yttrocrasite, but in appearance and angles it closely
resembles polycrase (_q.v._). Its discoverer, Zambonini, therefore
formulates it as a metatitanate with titanium acting also as a
base--polycrase is a mixed metatitanate and metacolumbate--thus, 2FeTiO₃
+ U(TiO₃)₂ + 2Y₂(TiO₃)₃ + 7(TiO)TiO₃.

  [56] Zambonini, _Zeitsch. Kryst. Min._ 1908, ~45~, 76.

  The crystals occur in aggregates of numerous individuals in
  sub-parallel growth. The system is orthorhombic; _a_ : _b_ : _c_ =
  0·3375 : 1 : 0·3412. Usual forms--the pinakoids _a_ {100} and _b_
  {010} with prism _m_ {110}, dome _d_ {201}, etc. Habit prismatic,
  elongated ∥ c axis. Hardness 5¹⁄₂-6; sp. gr. about 4·7.

It was found with struvite in a pegmatite at Craveggia, Piedmont, Italy.


~Ilmenite or Menaccanite~ (Specular Iron Ore, Titaniferous Ironstone,
etc.).--This is a titanate of iron, usually written FeTiO₃. Its
constitution has given rise to very considerable discussion[57]; not
only do the relative proportions of iron and titanium vary greatly, but
the iron is undoubtedly present in both the ferrous and the ferric
states, and in the former state is partly replaced in some specimens by
manganese and magnesium. In 1829 Mosander put forward the view that the
mineral consisted of FeTiO₃, ferrous titanate, with varying proportions
of ferric oxide, the forms and angles of ilmenite being very similar to
those of hæmatite, Fe₂O₃. This view was disputed by H. Rose, who
concluded that the mineral must have been originally an isomorphous
mixture of ferric oxide, Fe₂O₃, and titanic oxide, Ti₂O₃, which on
exposure to high temperature in the earth’s crust would change according
to the equation

  Fe₂O₃ + Ti₂O₃ = 2TiO₂ + 2FeO

so that the proportion of ferrous iron increases with the proportion of
titanium dioxide, as is actually found to be the case. This condition,
however, is also satisfied by Mosander’s view. The latter view was also
supported by Rammelsberg, who pointed out that the presence of magnesium
indicated the existence of ferrous iron as a primary constituent.
Additional support is lent to this view by the discovery of Pyrophanite,
MnTiO₃ (see list), which is found to be isomorphous with ilmenite, so
that there can be little doubt that MgTiO₃, which can be only a
titanate, would, if it existed in the crystalline form (see Geikielite
in list), also be isomorphous with ilmenite. Friedel and Guérin (1876)
prepared artificial titanium sesquioxide, Ti₂O₃, and found it to be
isomorphous with hæmatite, Fe₂O₃; they concluded that FeFeO₃, FeTiO₃ and
TiTiO₃ formed an isomorphous series, and that ilmenite was a mixture of
the second with the other two. In 1890 Hamberg pointed out that there
was no reason to suppose that hæmatite, Fe₂O₃, contains ferrous iron,
_i.e._ has the constitution Fe´´Fe^{iv}O₃, analogous to Fe´´Ti^{iv}O₃,
since in corundum, the analogous compound of aluminium, Al₂O₃, divalent
aluminium can hardly exist; nevertheless, strict analogy of constitution
is not necessary for isomorphism, as shown by the case of potassium
nitrate, KNO₃, and aragonite, CaCO₃, so that hæmatite, Fe₂O₃, and
ferrous titanate, FeTiO₃, might form solid solutions in varying
proportions without the strictly analogous formula FeFeO₃ being true for
the former. The balance of opinion inclines to the constitution
(_m_FeTiO₃ + _n_Fe₂O₃ in isomorphous mixture) originally proposed by
Mosander. The evidence in support of this view has been greatly
strengthened by the recent work of Manchot,[58] which has proved the
absence of titanium sesquioxide, Ti₂O₃; the mineral is therefore to be
regarded as a titanate.

  [57] For a full account of the earlier work on the constitution of
  ilmenite _vide_ Hintze, i. 1858 _et seq._

  [58] _Zeitsch. anorg. Chem._ 1912, ~74~, 79.

  Crystal system--rhombohedral; in forms and angles very close to
  hæmatite, but the two differ in symmetry (hæmatite has _t_, 3δ, _c_,
  3π; ilmenite has only _t_, _c_).

  _c_ = 1·38458; (111) ∧ (100) = 57° 58¹⁄₂´; habit, tabular, thick; or
  in thin laminæ. Usually in embedded grains or rolled crystals in sand.

  Hardness 5 to 6; sp. gr. 4·5 to 5·0, increasing with percentage of
  ferric oxide. Iron black, opaque; streak black to brownish-red. Lustre
  sub-metallic. Slightly magnetic.

The mineral is infusible; when powdered, it dissolves slowly in boiling
hydrochloric acid, the filtered yellow solution giving the
characteristic blue colouration of titanium salts on addition of
tinfoil. In fused potassium hydrogen sulphate it dissolves readily. The
variation in composition can be judged from the following limits:

  TiO₂    Fe₂O₃     FeO
   3·5     93·6     3·3 per cent.
  52·8      1·2    46·5     „

Ilmenite is a widely distributed mineral. In crystals it occurs chiefly
at Kragerö and Arendal in Norway, at Miask in the Ilmen mountains, in
Dauphiné, the St. Gothard, etc.; in the massive form at Bay St. Paul,
Quebec, and other localities in America; and in sands at Menaccan in
Cornwall, Iserwiese in Bohemia, Puy de Dôme, dép. Haute Loire, France,
and in Brazil, Australia, and New Zealand.

The mineral was discovered at Menaccan in Cornwall by McGregor, about
1790. He described it as containing iron and a new oxide; the unknown
oxide was obtained in 1795 from rutile by Klaproth, who gave the name
Titanium to the new metal it contained.

       *       *       *       *       *

Short descriptions of the following titanates are also given (see list):

_Davidite_ and _Knopite_; these are complex titanates containing
elements of the cerium and yttrium groups.

_Arizonite_ and _Pseudobrookite_--ferric titanates.

_Perovskite_, calcium titanate, and its variety _Hydrotitanite_.

_Pyrophanite_, a manganese titanate isomorphous with ilmenite, and
_Senaite_, a species intermediate in composition between these two.

_Geikielite_, the magnesium analogue of ilmenite, with the variety
_Picroilmenite_, which is rich in iron.

_Uhligite_, a titanate of zirconium, calcium and aluminium.

_Derbylite_, _Lewisite_ and _Mauzeliite_, an interesting series of
titano-antimonates.

_Warwickite_, a boro-titanate.


CHAPTER IV

THE TANTALO-COLUMBATES


(_a_) TANTALO-COLUMBATES CONTAINING NO TITANIUM DIOXIDE


~Samarskite~, Yttro-ilmenite or Eytlandite
(Urano-tantalite).--Samarskite is a tantalo-columbate[59] of the rare
earth metals, with iron, calcium, and uranium.

  [59] In this and all similar minerals, columbium (niobium) and
  tantalum are to be regarded as vicarious; they replace each other in
  all proportions. It seldom happens that a pure columbate is found free
  from tantalum, or _vice versa_; one or other may predominate, but the
  two are almost always found together.

Rammelsberg gives the formula R´´₃R´´´₂(Cb,Ta)₆O₂₁, where R´´ =
(Fe´´,Ca,UO₂), and R´´´ = rare earth metals. Groth regards it as
essentially a pyrocolumbate (tantalate) of rare earth metals
R₄[(Cb,Ta)₂O₇]₃ the iron, calcium and uranium being more or less
accessory constituents. Des Cloizeaux considers the formula indefinite.
The mineral has also been found to contain tin, thorium, germanium, and
helium. The yttria earths usually predominate (11·9 to 18·9 per cent.),
the percentage of ceria earths being low (2·4 to 5·2 per cent.). The
yttria earths contain the very rare oxide samaria.

The mineral is radio-active.

  Crystal system--orthorhombic; _a_ : _b_ : _c_ = 0·5456 : 1 : 0·5178.

  Forms--macro- and brachy-pinakoids _a_ {100} and _b_ {010}; prisms _m_
  {110} and _h_ {120}, the macrodome _e_ {101}, and pyramids _p_ {111}
  and _v_ {231}.

  Angles--(100) ∧ (110) = 28° 37´; (001) ∧ (101) = 43° 30´; (001) ∧
  (011) = 27° 22¹⁄₂´.

  Habit usually prismatic, with _e_ prominent; sometimes tabular
  parallel to _a_ or _b_. Cleavage ∥ _b_, imperfect. The faces are
  usually rough. The mineral commonly occurs massive, and in flattened
  grains embedded in granite. Conchoidal fracture. Brittle. Hardness 5
  to 6; sp. gr. 5·6 to 5·8.

  Colour velvet-black, streak reddish-brown. Opaque even in thin films.

Before the blowpipe it fuses at the edges; with borax it gives an iron
bead. It is decomposed by boiling concentrated sulphuric acid, better by
fusion with potassium hydrogen sulphate, and leaching the residue with
dilute hydrochloric acid--this leaves the insoluble oxides Cb₂O₅ and
Ta₂O₅. On heating it glows, with decrease in specific gravity (cf. p.
38).

Samarskite occurs with other columbo-tantalates in felspar, or in veins
in granite, near Miask in the Urals, near Quebec in Canada, and in
Mitchell County, North Carolina. From the last-named locality, masses up
to twenty pounds in weight have been obtained.

The mineral was first discovered in the Urals by Ewreinoff, captain of a
corps of Russian mountain engineers. He sent a specimen for
identification to the mineralogist Gustave Rose, who pronounced it to be
a tantalate of uranium containing manganese, and called it
Urano-tantalite.[60] In 1847 the chemist Heinrich Rose, brother of
Gustave, in the course of his researches on tantalic ‘acid’ (oxide),
analysed a specimen. He found the composition given above, and renamed
it Samarskite,[61] in honour of the Russian engineer who furnished him
with the specimen for analysis.

  [60] _Pogg. Ann._ 1839, ~48~, 555.

  [61] _Ibid._, 1847, ~71~, 157.

In 1907, Brögger[62] announced that _Annerödite_, of which he had
published an account as a new species in 1881, was a parallel growth of
the mineral columbite, (Fe,Mn)Cb₂O₆, on samarskite.

  [62] _Abstr. Chem. Soc._, 1907, ~92~, ii. 885.

Both minerals are orthorhombic, but they are not isomorphous. The
mistake was due to the fact that whilst the crystallographic data were
determined from the upper crystals of columbite, the crystals of
samarskite were used for analysis.


~Plumboniobite.~[63]--This is a recently discovered mineral closely
related to samarskite and yttrotantalite (_q.v._). It is essentially a
columbate[64] of yttrium metals, lead and uranium, with water, ferrous
oxide, titanium dioxide, stannic oxide, alumina, lime, and cuprous
oxide. The formula given is R´´₂Cb₂O₇,R´´´´₄(Cb₂O₇)₃, where R´´ =
(Fe,Pb,Ca,UO), and R´´´ = Al and yttria metals, with isomorphous (?)
metatitanate. The mineral is radio-active, and gives considerable
quantities of gas on being heated with sulphuric acid (carbon dioxide
0·19, helium and nitrogen 0·22 per cent.). The yttria earths are rich in
the oxides of gadolinium and samarium, and the mineral should prove a
valuable source of these elements. It is remarkable that the ceria
earths are almost entirely absent.

  [63] Hauser u. Finch, _Ber._ 1909, ~42~, 2270; Hauser, _ibid._, 1910,
  ~43~, 417.

  [64] It is to be understood that small quantities of columbium are
  replaced by tantalum.

  The mineral is massive, with some indication of crystalline structure.
  It is dark brown to black, transparent in flakes, and under the
  microscope is seen to be isotropic, with doubly-refracting inclusions,
  undoubtedly of a secondary nature. Hardness 5 to 5¹⁄₂; sp. gr. 4·80 to
  4·81. Unlike samarskite, it does not glow on ignition.

It occurs with mica and pitchblende in pegmatite veins in granite, at
Morogoro, in the Uluguru Mountains, German East Africa.


~Yttrotantalite.~--This is a tantalo-columbate similar in composition to
Samarskite, and isomorphous with it; though, as the name implies, the
acidic oxide is chiefly tantalum pentoxide, the percentage of columbic
anhydride being much lower than in the latter mineral. It is a pyro-salt
of the formula R´´R´´´₂(Cb,Ta)₄O₁₄ + 4H₂O,[65] where R´´ = (Fe,Ca) and
R´´´ = rare earth (chiefly yttrium) metals (Rammelsberg). Strutt found
thorium and radium in it. The manner in which the water is combined in
this, as in many other minerals, is at present undetermined.

  [65] Dana gives R´´R´´´₂(Cb,Ta)₄O₁₅ + 4H₂O; this appears to be an
  error.

  Crystal system--orthorhombic; _a_ : _b_ : _c_ = 0·5411 : 1 : 1·1330.
  Common forms--pinakoids _b_ {010} and _c_ {001}, prisms _m_ {110}, _o_
  {210}, _p_ {120}, domes _s_ {201} and β {011}. Habit, prismatic with
  _m_ and _b_ prominent, or tabular parallel to _b_. Colour yellow to
  black, white after strong ignition.

It is found at Ytterby in Sweden, and in South Norway.


~Fergusonite~, Tyrite, or Bragite.--A columbate and tantalate of the
rare earth metals, with uranium, iron, calcium, etc. The general formula
is that of an ortho-compound, R₂O₃,(Cb,Ta)₂O₅ or R(Cb,Ta)O₄, where R =
metals of the rare earths, chiefly of the yttrium group. Brögger
includes the other constituents in the more complex formula
(Th,U)(Si,Sn)O₄ + 12R(Cb,Ta)O₄; but the simpler formula agrees quite
well with specimens from the most widely separated localities, and is
usually adopted. The mineral is radio-active and contains helium.

  Tetragonal, polar (with tetrad axis of symmetry only) _c_ = 1·4643.
  (001) ∧ (101) = 55° 40´. Common forms--Basal pinakoid _c_ {001},
  tetragonal prism _g_ {320}, pyramids _s_ {111}, _z_ {321}. Brittle.
  Hardness 5 to 6; sp. gr. 5·84, decreasing on hydration. Lustre dull,
  brilliantly vitreous on broken surfaces. Colour brownish-black.
  Translucent to opaque.

Fergusonite was discovered by Hartwell. It occurs with samarskite, and
often with gadolinite and allanite, in Norway and Sweden, the Carolinas,
Texas, the Urals, W. Australia, etc.

On heating it glows suddenly between 500° and 600°C.,[66] losing all its
helium, and with decrease in density (5·619 to 5·375). At the same time
it gives out a considerable amount of heat--8·09 C.[67] for 1 gm. (see
p. 38).

  [66] Ramsay and Travers, _Zeitsch. physikal. Chem._ 1898, ~25~, 568.

  [67] The heat of combustion of a gram of hydrogen is 342 K.


~Sipylite.~--Essentially a columbate of rare earth metals, with oxides
of tantalum, tungsten, zirconium, uranium, iron and calcium, and some
water. Mallet, the discoverer, gives the formula as R₂O₃,Cb₂O₅, the
basic oxides including, besides the rare earths, Cb₂O₅ with Ta₂O₅ and
WO₃, and some water. An alternative formula, making it a complex
pyro-salt, is also given, but from its great similarity in form and
angles to fergusonite, the first formula is preferred. Strutt finds that
it contains not only uranium, radium and helium, but also thorium in
considerable quantity (ThO₂ = 4·9 per cent.), a fact which had been
overlooked by Mallet. The rare earths contain a high proportion of
erbia.

  It is tetragonal, _c_ = 1·4767, (001) ∧ (101) = 55° 54´. The crystals
  are octahedral, with the form _p_ {111}; _p_ ∧ _p_´ = 79° 15´, _p_ ∧
  _p_´´ = 128° 50´. Cleavage distinct ∥ _p_. It is usually granular and
  amorphous. Colour brownish-black to brownish-red, lustre resinous.
  Brittle. Hardness 6; sp. gr. 4·89. Translucent.

Its behaviour on heating has been already mentioned (see p. 39); it is
infusible. Boiling hydrochloric acid partially dissolves it; the
solution gives the turmeric test for zirconium, and on diluting and
adding metallic tin a sapphire-blue colour is developed, due to the
columbium present. Boiling concentrated sulphuric acid decomposes it
slowly.

It is found in Amhurst Co., Virginia, adherent to the allanite which
occurs there in large quantities. It was discovered there by Mallet in
1877, who named it, on account of the columbium (niobium) present, from
Sipylus, one of the sons of Niobe.[68]

  [68] See Mallet, _Amer. J. Sci._ 1877, [iii.], ~14~, 397.

       *       *       *       *       *

In this class, also, are to be included the following minerals (see
list):

_Nohlite_ and _Vietinghofite_, varieties of Samarskite.

_Hjelmite_ and _Kochelite_, minerals closely related to Yttrotantalite
and Fergusonite respectively.

_Koppite_, _Loranskite_, _Microlite_ and _Rogersite_, complex
tantalo-columbates containing elements of the cerium or yttrium groups.


(_b_) TANTALO-COLUMBATES CONTAINING TITANIUM DIOXIDE


~Æschynite.~--A columbate and titanate of the cerium metals, with
thorium, calcium, iron, etc. From the results of an analysis on a
specimen from Hitterö, Norway, Tschernik proposed the rather formidable
formula

  2(2Ce₂O₃,3TiO₂),4(ThO₂,TiO₂),Y₂(CbO₃)₆,3(CaO,TiO₂),3Fe(CbO₃)₂,
  Fe(TaO₃)₂,6TiO₂.

This can be simplified to Y(CbO₃)₃ + ThTiO₄ + ³⁄₂TiO₂, in which Y
represents rare earth metals partially replaced (2 atoms) by ferrous
iron (3 atoms), whilst thorium can be partially replaced by (2 atoms of)
ferrous iron or calcium. Strutt found it to contain the uranium-radium
combination and helium.

  The crystals are orthorhombic, holosymmetric; _a_ : _b_ : _c_ =
  0·4866 : 1 : 0·6737.

  Common forms--brachy- and basal pinakoids _b_ {010} and _c_ {001},
  prisms _m_ {110} and _r_ {120}, domes _d_ {101} and _v_ {021}, with
  pyramid _o_ {111}.

  (100) ∧ (110) = 25° 57´; (001) ∧ (101) = 54° 9´; (001) ∧ (011) = 33°
  58´.

  Habit prismatic, vertically striated, or tabular parallel to b with
  horizontal striations. Brittle. Hardness 5 to 6; sp. gr. 4·9 to 5·7.
  Colour nearly black. Nearly opaque.

It occurs at Miask, in the Urals, at Hitterö in Norway, and at
Fredriksvarn. The variety from the last locality is called Polymignite;
it was shown by Rose to be probably identical with Æschynite. Æschynite
was discovered by Berzelius at Miask and named by him from the Greek
αίσχύνη, shame, from the fact that its composition could not at that
time be determined.

If the ceria earths be largely replaced by yttria earths, a variety very
similar in appearance and angles, but approximating to polycrase
(_q.v._) in composition, is obtained. This mineral was found in 1879,
and referred to Æschynite; analysis subsequently showed its true
composition, and it was named Blomstrandine (_q.v._) by Brögger in
1907.


_The Isodimorphous Series Euxenite, Polycrase, Blomstrandine, and
Priorite._

Euxenite and Polycrase are members of an isomorphous series and vary
considerably in composition. The composition of the series is that of
mixed columbates and titanates of yttria earths (with, as usual, some
ceria earths), with uranium and zirconium, and water. Before the
isomorphous relation was recognised, Rammelsberg gave for Euxenite the
formula R´´´(CbO₃)₃,R´´´₂(TiO₃)₃,1¹⁄₂H₂O. The ratio of the acidic
oxides, Cb₂O₅ : TiO₂, is here 1 : 2. This is the greatest value of the
ratio, which varies for the series between 1 : 2 and 1 : 5.[69] The end
members, the pure metacolumbate and pure metatitanate respectively, are
unknown; all the members occurring in nature are to be regarded as
mixtures of these within the limits set by the ratios ¹⁄₂ and ¹⁄₅.
Brögger[70] suggests that the name Euxenite be retained for all members
for which the ratio is between ¹⁄₂ and ¹⁄₃, whilst for those minerals in
which it is less than ¹⁄₄ the name Polycrase be kept; these views have
been supported by Lange, who has analysed members of the series.

  [69] Lange (_Abstr. Chem. Soc. 1911_, ~100~, ii. 499) gives the limits
  ¹⁄₂ and ¹⁄₆.

  [70] _Abstr. Chem. Soc. 1907_, ~92~, ii. 885.

The members of this isomorphous series, however, are themselves
dimorphous, that is, can each crystallise in two different ways. The
second form corresponding to the Euxenites is known as Priorite, whilst
that corresponding to Polycrase is known as Blomstrandine; and these
second forms are themselves members of a parallel isomorphous series of
the same chemical composition, of course, as the first series. It is,
perhaps, undesirable to cite this as a typical example of an
isodimorphous series, since no end members of unmixed composition are
known. A perfect example of such a series is furnished by the oxides of
antimony and arsenic. Each of these compounds exists in two distinct
crystalline varieties, antimony trioxide, Sb₂O₃, as Valentinite
(orthorhombic) and Senarmontite (cubic), arsenic trioxide, As₂O₃, as
Claudetite (orthorhombic) and Arsenolite (cubic); and these two
modifications are isomorphous with one another, senarmontite with
arsenolite, and valentinite with claudetite.

In the case we are considering, the name Euxenite is applied to one
crystalline modification (A) of a number of isomorphous compounds within
certain limits of composition, the name Priorite to the second
crystalline modification (B) of the same compounds; the name Polycrase
is applied to compounds having the crystal form A, and a composition
varying within a second set of limits in the same chemical series,
whilst this second set of compounds in the crystalline form B is known
as Blomstrandine.

Stated as concisely as possible, the relationship is as follows: Each
member of this chemical series of continuously varying composition can
crystallise in two forms, which are the same for every member. The two
varieties at one end of the series are called euxenite and priorite, at
the other end polycrase and blomstrandine.

Thus, whilst euxenite and priorite, at the one end, and polycrase and
blomstrandine at the other, have the same compositions, euxenite and
polycrase have the same crystalline form, whilst priorite and
blomstrandine have the same second crystalline form.

All four minerals have the same bright black appearance, and bright
conchoidal fracture; they are all four isotropic, probably as a result
of hydration. All are orthorhombic, but the measurements for euxenite
and polycrase are different from those for blomstrandine and priorite.
The two latter are not so widely distributed as the two former.
Blomstrandine occurs at Hitterö, Arendal, and other localities in
Norway; priorite is found in Swaziland, South Africa.

The crystal system of the Polycrase-Euxenite series is orthorhombic, but
Dana gives slightly different axial ratios for the two minerals. This,
though Brögger gives the same values for both, is by no means
incompatible with isomorphism, as a glance at the axial ratios for the
minerals aragonite, strontianite, witherite, etc., of the series of the
orthorhombic carbonates, will show.

Brögger’s ratios for the two are _a_ : _b_ : _c_ = 0·3789 : 1 : 0·3527;
Dana gives for polycrase 0·3462 : 1 : 0·3124, for euxenite 0·364 : 1 :
0·303.


~Euxenite.~

  This species occurs usually in the massive form as a bright
  brownish-black mineral, of hardness 6¹⁄₂, and sp. gr. 4·6 to 5·0. The
  crystals are prismatic in habit; the common forms are the pinakoids
  _a_ {100} and _b_ {010}, the prism _m_ {110}, the unit pyramid _p_
  {111}, and the dome {201}. Ramsay, Collie and Travers found no helium
  in it; Boltwood found uranium, radium and helium, and Strutt found in
  addition to these thorium. As early as 1879, Blomstrand had observed
  zirconium in euxenite.

The mineral is infusible and with difficulty soluble in acids. It occurs
in many localities in Scandinavia (Hitterö, Arendal, Brevig, etc.), in
North Carolina, South Australia, etc. It was discovered by Scheerer at
Jölster, in Norway, in 1839.

The Euxenite-Polycrase series was studied by Hauser and Wirth in
1909,[71] in an endeavour to establish their theory that the proportions
in which the various earths and acids occur in this group of minerals is
subject to definite laws beyond the ordinary laws of combination. Thus
of the erbia earths they state that the proportion of holmia and
dysprosia increases relatively to erbia as titanium dioxide increases,
_i.e._ as we pass from the euxenites to the polycrases; at the same time
scandia and yttria increase relatively to the other yttria earths (the
terbia group), whilst in the ceria group samaria and praseodymia
decrease relatively to the others. Thus samaria is found in appreciable
quantities only when the titanium content is low. The original paper
must be consulted for full details.

  [71] _Ber._ 1909, ~42~, 4443.

It was stated above that zirconium was found in euxenite in 1879. In
1901 Hofmann and Prandtl[72] declared that zirconia was an unfailing
constituent of the mineral, and that it was always accompanied by a new
oxide, which they named Euxenia (‘Euxenerde’). This was characterised by
the solubility of its oxalate in acid solutions, the insolubility of
the precipitated hydroxide in excess of alkali, and the gradual
precipitation by hydrogen peroxide from a slightly acid solution of its
salts. In their paper quoted above, Hauser and Wirth state that zirconia
is never present in typical euxenites. In a second paper[73] they state
that after exhaustive treatment of every known zirconia mineral, they
can find no trace whatever of the ‘new earth,’ and conclude that Hofmann
and Prandtl must have made some experimental error. During this
examination, they observed radioactivity in some minerals which
contained no traces of uranium or thorium.

  [72] _Ibid._ 1901, ~34~, 1064.

  [73] _Ber._ 1910, ~43~, 1807.


~Risörite.~[74]--A columbate of yttria earths, with titanium; ferric
oxide, alumina, lime and lead monoxide are present in small quantities.
It resembles fergusonite in composition, but differs in the almost
complete absence of uranium, the high loss on ignition, and the amount
of titanium present, which is here considerable (TiO₂ = 6·5 per cent.).
Hauser regards it as an orthocolumbate, R´´´(Cb,Ta)O₄, with an
isomorphous admixture of metatitanate, R´´´₂(TiO₃)₃.

  [74] Hauser, _Ber._ 1907, ~40~, 3118; _Zeitsch. anorg. Chem._ 1908,
  ~60~, 230.

The rare earths are chiefly yttria, with some erbia earths and a little
terbia; ceria, lanthana and didymia are also present. The mineral
contains a considerable amount of helium, which is remarkable in view of
the very small content of uranium and thorium (cf. Thalenite). It is
radioactive, the active constituent being precipitated with the lead
(and to a very small extent with the rare earths).

It is infusible, but at a red heat it loses much water, and becomes very
brittle, with increase of specific gravity; no glowing is observed. It
is attacked by boiling concentrated sulphuric acid, and by fused
potassium bisulphate; also by hydrofluoric acid (40 per cent.), with
separation of the insoluble rare earth fluorides.

  No good crystals have been found, and no crystallographic data are
  known; examined by polarised light it appears isotropic, but this may
  be due to alteration. Colour, yellowish- to greenish-brown. Streak,
  yellowish-white. Hardness 5¹⁄₂; sp. gr. 4·179, increasing to 4·678
  after ignition (cf. p. 38).

The mineral was found in a granite-pegmatite at Risör, South Norway.


~Wiikite.~[75]--A mineral of very complex composition, for which no
definite formula can be assigned. Its chemical nature may be understood
from the following analytical data:

Columbic and tantalic anhydrides = 16·0; Dioxides of titanium and
zirconium = 23·4; Silica = 17·0; Ceria = 2·5; Yttria = 7·6; Scandia =
1·2; Thoria = 5·5; Ferrous oxide = 15·5; Uranic oxide = 3·6; water (and
gas) = 5·8 per cent.

  [75] Crookes, _Phil. Trans._ 1908, A, ~209~, 15.

Traces of lime, magnesia, stannic oxide and sulphur are also present.

The mineral is infusible; on heating, helium, sulphuretted hydrogen and
water vapour are given off, and a white sublimate is formed. The
evolution of gas is almost explosive, the mineral breaking with a
curious fracture.

  It is black and perfectly amorphous, showing no trace of crystalline
  structure or action on polarised light. Hardness, 6; sp. gr. 4·85.

Wiikite is partially attacked by acids, readily by fused potassium
bisulphate. It is radioactive.

The mineral was found with monazite in a felspar quarry at Impilaks,
Lake Ladoga, Finland. It is important as the source of scandium used by
Sir William Crookes in his investigations of that element; some
specimens of the mineral contain over 1 per cent. of the oxide (see p.
44).

       *       *       *       *       *

The following related minerals, of which descriptions are given in the
alphabetical list, are to be included here:

_Arrhenite_, _Chalcolamprite_, _Endeiolite_ and _Wöhlerite_, are complex
tantalo-columbates containing silica.

_Hainite_ contains both silicon and titanium.

_Dysanalyte_ is a titano-columbate believed by Hauser[76] to be merely
an impure form of perovskite (see p. 14).

  [76] Vide _Zeitsch. anorg. Chem._ 1908, ~60~, 237.

_Ilmenorutile_ and _Strüverite_ are closely allied minerals believed by
Prior[77] and Schaller[78] to be isomorphous mixtures of rutile with
Tapiolite or Mossite (ferrous tantalo-columbates).

  [77] _Min. Mag._ 1908, ~15~, 78.

  [78] _Abstr. Chem. Soc._ 1912, ~102~, ii. 773.

_Pyrochlore_ is a complex titano-columbate containing elements of the
cerium or yttrium groups.

_Blomstrandite_ is an hydrated titano-columbate of rare earth elements,
with calcium and uranium; it must not be confused with blomstrandine.


CHAPTER V

THE OXIDES AND CARBONATES


(_a_) THE OXIDES


~Uraninite~ or Pitchblende.--Uraninite consists essentially of oxides of
uranium (UO₂ + UO₃ = 75 to 85 per cent.), associated with thoria,
zirconia, rare earths, beryllia, and oxides of lead. Traces of lime,
iron oxides, silica, bismuth, and arsenic are also sometimes present,
with water in widely varying quantities. Nitrogen and helium are always
found in it, and, of course, radium. Groth regards pitchblende as
uranous uranate U^{iv}(U^{vi}O₄)₂, the uranium in the acidic radicle
being hexavalent and in the basic radicle tetravalent, and in the latter
condition partially replaced by lead, thorium, and rare earths.

Szilard[79] regards it rather as a loose compound or even a solid
solution of oxides of thorium and uranium,[80] with small quantities of
other oxides, he having obtained apparently homogeneous (though
non-crystalline) bodies by dissolving thorium hydroxide in solutions of
uranium salts and evaporating to dryness.

  [79] _Compt. rend._ 1907, ~145~, 463.

  [80] See under Thorianite, _infra_.

The cubic form of the crystalline varieties has been taken as indicating
that the mineral is really a spinel,[81] but it is difficult to see how
the general formula of that group can be considered comparable to the
uranyl uranate formula, UO₂,UO₃, for pitchblende.

  [81] The Spinels are an isomorphous family of cubic minerals of the
  general formula R´´O,R´´´₂O₃, where R´´ = Be, Fe, Mg, Ca, etc., and
  R´´´ = Fe, Al, Cr, etc.

  Crystals are rare, and belong to the cubic system, the common forms
  being the octahedron _o_ {111} and the dodecahedron _d_ {110}; the
  cube _a_ {100} is sometimes present. The mineral is massive, usually
  botryoidal. The crystalline or primary form is black, with hardness
  5¹⁄₂, sp. gr. 9·0 to 9·7; the altered varieties are grey to greenish-
  and brownish-black, sp. gr. 5·0 to 6·4.

It is infusible before the blowpipe, but readily soluble in nitric acid.

The mineral occurs both as a primary and secondary constituent of rocks;
as a primary mineral it is found in Norway, North Carolina, etc.; as a
secondary species it occurs in the massive and hydrated form, with ores
of lead, silver, tin, etc., in Saxony and Cornwall, and at the
celebrated mine of Joachimsthal, in Bohemia. The latter deposits,
consisting of the massive and altered varieties, for which the name
Pitchblende is generally reserved, have been much used as a source of
radium, especially those at Joachimsthal, and the Cornwall ore.

Several varieties of uraninite have been distinguished by special names.
Crystalline varieties from Anneröd and Arendal in Norway are known as
Bröggerite and Cleveite respectively; Nivenite is a third form. In these
varieties uranium oxides have been replaced to a considerable extent by
the rare earths and thoria. An amorphous variety of doubtful
composition, produced by alteration, is known as Gummite; Uranosphærite
is a similar altered form.


~Thorianite.~[82]--This interesting mineral consists chiefly of thoria,
ThO₂ (55-79 per cent.), with oxides of uranium (11-32 per cent.), and
ceria oxides (1-8 per cent.); oxides of lead and iron are also present
in small quantities, and zirconia with silica, probably due to
associated zircon.

  [82] Dunstan and Blake, _Proc. Roy. Soc._ 1905, A, ~76~, 253; Dunstan
  and Jones, _ibid._, 1906, A, ~77~, 546.

Helium is present, and the mineral is strongly radioactive. A careful
analysis by Hahn[83] shows traces of many metals; the same chemist has
also separated an extremely active component, 250,000 times as active
as thorium nitrate, which he calls Radiothorium.

  [83] Hahn, _ibid._, 1907, A, ~78~, 385.

The composition has been accounted for (Dunstan and Jones, _loc. cit._)
on the hypothesis that thoria (ThO₂) and uranous oxide (UO₂) are
isomorphous, the mineral being really a solid solution. Whilst, however,
the crystal system of the natural body is really rhombohedral (_vide
infra_) the two pure oxides appear to be cubic. Thus Troost and
Ouvrard[84] obtained artificial thoria in minute octahedra; and,
similarly, Hillebrand[85] obtained uranous oxide in octahedra by
reduction of uranyl chloride, UO₂Cl₂, though his work seems to be open
to objection. On the other hand, the same author[86] found that uranous
oxide and thoria, fused together in almost any proportions, gave a
homogeneous body crystallising in octahedra (cf. Szilard, _Compt. rend._
1907, ~145~, 463, quoted under Uraninite). The probability of the
isomorphism of the oxides is strengthened by the observation of
isomorphism in the sulphates. As early as 1886, Rammelsberg showed that
uranous sulphate, U(SO₄)₂, crystallises with nine molecules of water and
is isomorphous with the corresponding thorium sulphate, Th(SO₄)₂,9H₂O;
and six years later, Hillebrand and Melville[87] obtained mixed crystals
of the two sulphates which were exceedingly close in forms and angles to
those of pure uranous sulphate. It is then at least probable that the
two oxides are isomorphous, though the point cannot be regarded as
satisfactorily proved, by reason of the anomalous crystal forms of the
naturally occurring mixtures, thorianite and uraninite. The recent
results of Kobayashi[88] point to the conclusion that different
varieties of thorianite may exist, in each of which the oxides of
thorium and uranium bear definite simple ratios to one another.

  [84] _Compt. rend._ 1882, ~102~, 1422.

  [85] _Zeitsch. anorg. Chem._ 1893, ~3~, 243.

  [86] _Bull. U.S. Geol. Surv._ No. 113, 1893.

  [87] _Ibid._ No. 90, 1892, p. 30.

  [88] _Abstr. Chem. Soc._ 1912, ~102~, ii. 1181.

  Thorianite occurs in jet-black crystals with a bright resinous lustre.
  They are pseudocubic, and the twinning resembles that of the cubic
  mineral fluorspar--interpenetrant cubes, twin axis a cube diagonal.
  Close examination shows, however, that twinning can only take place
  about one of the four diagonals, and an optical examination makes it
  clear that the symmetry is really rhombohedral. The case is exactly
  analogous to that of the mineral chabazite, a zeolite which occurs in
  rhombohedra of which the angles differ but little from those of the
  cube, and which also forms the interpenetrant twins. In view of the
  fact that both uranous oxide and thoria have been obtained as
  octahedra, whilst a fused mixture of the two on cooling forms cubic
  crystals, it seems not unlikely that at high temperatures the
  pseudocubic thorianite would become truly cubic; but no experiments in
  this direction seem to have been tried.

  The crystals are brittle; hardness 7; sp. gr. 8·0-9·7.

Thorianite is infusible, incandescing before the blowpipe. When
powdered, it dissolves readily in nitric and sulphuric acids, with
evolution of helium. Gray[89] has shown that the helium content can be
reduced by 28 per cent. by fine grinding, thus showing that part at
least of the gas must be mechanically held.

  [89] _Proc. Roy. Soc._ 1908, A, ~82~, 306.

Thorianite was found in Ceylon, being originally mistaken for
pitchblende. A sample was supplied by the discoverer, Mr. Holland, to
the officers of the Mineral Survey, by whom it was sent to London for
examination. Its composition was determined by Dunstan, who named it. It
was found in the river gravels (gem-gravels), the matrix being a
pegmatite granite. It is a valuable source of thorium nitrate for
incandescent mantles, one ton of the mineral (with thoria content of 70
per cent.) having been sold for £1500; but the supply is small and
unreliable.


~Baddeleyite.~[90]-- Baddeleyite consists of almost pure zirconia (ZrO₂
= 96·5 per cent.) with small quantities of ferric oxide, alumina, lime,
magnesia, alkalies and silica. Thoria and rare earths are present in
traces, uranium is absent; the mineral is not radioactive, and contains
only traces of helium.

  [90] _Vide_ Fletcher, _Min. Mag._ 1893, 46, ~10~, 148; Hussak,
  _Zeitsch. Kryst. Min._ 1895, ~24~, 164, and ~25~, 298.

  Monoclinic--_a_ : _b_ : _c_ = 0·9871 : 1 : 0·5114. β = 98° 45¹⁄₂´.

  Common forms--all three pinakoids, _a_ {100}, _b_ {010}, and _c_
  {001}, with the hemi-prisms _m_ {110}, _k_ {120}, and _l_ {230}, and
  various pyramids and domes.

  Angles--(100) ∧ (110) = 44° 17¹⁄₂´; (100) ∧ (001) = 81° 14¹⁄₂´; (100)
  ∧ (101) = 55° 33¹⁄₂´.

  Cleavage ∥ _c_ and ∥ _b_, parting ∥ _m_ due to repeated twinning.
  Twinning is exceedingly common; of many hundred crystals examined by
  Hussak, only three were found untwinned. Twin planes _m_ (110), _a_
  (100), and _x_ (201).

  Colour brown, varying in zones by twinning, with distinct pleochroism.
  Hardness; sp. gr. varies from 4·4 to 6·0, being about 5·5 to 5·6 for
  fairly pure material. Double refraction negative, 2 E = 70-75°. Acute
  bisectrix nearly coincident with _c_ axis, plane of the optic axes
  _b_, (010).

The mineral is insoluble in acids, readily soluble in fused potassium
hydrogen sulphate. Before the blowpipe it is almost infusible; it
dissolves in the fused borax bead, rapid cooling causing separation of
crystals. If a bead containing zirconia be heated until the borax is
partially volatilised, zirconia crystallises on cooling in tetragonal
crystals, isomorphous with those of rutile.[91]

  [91] Nordenskiöld, _Pogg. Ann._ 1861, ~114~, 625; for tetragonal
  zirconia see also Troost and Ouvrard, _Compt. rend._ 1888, ~102~,
  1422.

The mineral was discovered in 1892 by Hussak and L. Fletcher
independently. The former, who obtained it from the pyroxenite sand of
São Paulo, South Brazil, believed it to be a tantalo-columbate, and
called it Brasilite. Fletcher found it in a gem-gravel from Rakwana,
Ceylon, and named it Baddeleyite. An analysis by Blomstrand of Hussak’s
mineral showed it to be identical with the Ceylon mineral, and Hussak
withdrew his name and accepted Fletcher’s. It has recently been
found[92] in a corundum-syenite, near Bozeman, Montana, U.S.A.

  [92] Rogers, _Amer. J. Sci._ 1912, [iv.], ~33~, 54.

The mineral now comes on the market in commercial quantities; pure
zirconia almost entirely free from iron can be obtained by leaching with
acids. The pure oxide is extraordinarily refractory, and promises to be
of great use for crucibles, furnace linings, etc. (_vide_ p. 324).


~Rutile.~--Titanium dioxide, TiO₂, occurs crystallised in nature in the
three minerals Rutile, Brookite, and Anatase (Octahedrite), which
therefore form a trimorphous series. They are all stable minerals,
though rutile appears the most stable, being occasionally found in
pseudomorphs after the other two. The family is remarkable in that it is
not unusual to find two of them occurring together--an uncommon
phenomenon with polymorphous minerals.

Rutile often contains small quantities of iron and chromium. The
ferriferous varieties are distinguished as Nigrine, which is black, with
2-3 per cent. ferric oxide, and Ilmenorutile, with up to 10 per cent. of
ferric oxide, and specific gravity up to 5·13.

  Crystal system--tetragonal, holosymmetric; _c_ = 0·6442; (001) ∧ (101)
  = 32° 47´.

  Common forms--prisms _a_ {100}, _m_ {110}, and _l_ {310}; pyramids _e_
  {101}, _s_ {111}, and many others. The basal pinakoid _c_ {001} is
  very rare. Habit, prismatic, with vertical striations; or in slender
  needles. Twinning very common and varied; usually on the cassiterite
  law--twin plane _e_ (101)--forming the knee-shaped twins, and
  irregular rosettes by repetition, and many contact twins. Contact
  twins on the law--twin plane _v_ (301) are less common.

  Cleavage ∥ _a_ (100) and _m_ (110), distinct. Hardness 6-6¹⁄₂; sp. gr.
  4·18-4·25, and up to 5·2 if much iron is present. Colour reddish-brown
  to black, with good metallic lustre; transparent to opaque. The
  refraction and double refraction are very high--ω = 2·6158, ε = 2·9029
  for sodium light--and allow the crystals to be readily distinguished
  in rock-sections.

The mineral is insoluble in acids, but can be dissolved after fusion
with alkalies or alkali carbonates.

Rutile is a member of the isomorphous series, cassiterite, zircon, etc.
(see under Thorite), and in particular it has the colour, appearance,
and twinning of cassiterite, from which, however, it is readily
distinguished by its lower specific gravity. In this connection it is
interesting to note that an apparently pure specimen, quite free from
inclusions, was found (1904) to contain 1·7 per cent. of tin
dioxide.[93]

  [93] Friedel et Grandjean, _Bull. Soc. franc. Min._ 1909, ~32~, 52.

As an accessory rock mineral, and also as an important constituent of
many sands, rutile is of very wide distribution. It occurs, usually
imbedded in quartz or felspar, in many granites, syenites, gneisses,
slates, and allied rocks; in acicular crystals penetrating quartz it
forms the ‘Veneris Crinis’ of Pliny. At Risör and other localities in
Norway, it is found in the massive form, and it is largely worked at
Risör as a source of titanium. It occurs in all the countries of Europe,
and largely in America. Arendal, Kragerö, and Risör, in Norway, the
Binnenthal, the Urals, the St. Gothard, Castile, Magnet Cove in
Arkansas, Alexander Co. in N. Carolina, Barre and Shelburne in
Massachusetts, and Chester Co. in Pennsylvania are the chief localities.

It was in this mineral that the element titanium was first recognised by
Klaproth (1795).


~Anatase~ (Octahedrite) is the second crystalline modification of
titanium dioxide.

  Tetragonal _c_ = 1·7771. (001) ∧ (101) = 60° 38´, (111) ∧ (11̅1) = 82°
  9´.

  Common forms--Prisms _a_ {100} and _m_ {110}, pyramids _p_ {111}, _e_
  {101}, and many other complex forms; the basal plane _c_ {001} is
  occasionally found. Habit usually octahedral, with _p_ or _v_
  prominent; sometimes tabular with _c_, more rarely prismatic with _a_
  well developed. Cleavage ∥ _c_ and _p_ perfect. Hardness 5¹⁄₂-6; sp.
  gr. 3·82-3·95, usually increasing after heating. Lustre adamantine, so
  splendent that in Brazil detached crystals have been mistaken for
  diamonds. Colour, some shade of bluish-black to brown; by transmitted
  light, greenish-yellow. Transparent to opaque. Double refraction
  negative, strong; for sodium light ω = 2·554, ε = 2·493.

It is found at Bourg d’Oisans in Dauphiné, and in Norway, the Urals,
Brazil, etc. In Switzerland it occurs as the variety Wiserine, which was
at one time believed to be xenotime. It was named Octahedrite by de
Saussure, in 1796, from the prevailing habit, and Oisanite, from its
occurrence in Dauphiné, by Delamètherie, in 1797. The name anatase
(ανατασις = erection) was proposed by Haüy, being intended to denote
that the vertical axis (_c_ : _a_) is greater than that of rutile, the
other tetragonal modification of the dioxide.


~Brookite~, the third form of this compound, is orthorhombic.

  _a_ : _b_ : _c_ = 0·8416 : 1 : 0·9444.

  Common forms--the three pinakoids _a_ {100}, _b_ {010}, and _c_ {001},
  prisms _m_ {110}, _l_ {210}, pyramids _e_ {122}, _z_ {122}, and
  numerous others.

  Angles--(100) ∧ (110) = 40° 5´; (001) ∧ (100) = 48° 18´; (001) ∧ (011)
  = 43° 22´.

  The habit is varied; it occurs usually in bipyramids with _e_ and _m_
  or prismatic with _m_, _a_, and terminating pyramids. Cleavage ∥ _m_
  indistinct, ∥ _c_ very poor.

  Hardness 5¹⁄₂-6; sp. gr. 3·87-4·01. Lustre metallic. Colour brown to
  reddish- and yellowish-brown and black. The optical behaviour is
  interesting. The acute bisectrix is perpendicular to _a_ (100), but
  while for red light the plane of the optic axes is (001), for blue it
  is (010); for an intermediate light, therefore (λ = 5550 µµ), the
  mineral appears uniaxial.

The chief localities are Bourg d’Oisans, Miask, the St. Gothard, the
Tyrol, Magnet Cove in Arkansas, and Tremadoc in Wales.

Titanium dioxide can be obtained crystalline by the action of steam on
titanium tetrafluoride, TiF₄, at high temperatures; it is stated that by
varying the temperature of the reaction, any one of the three
crystalline modifications can be obtained.

       *       *       *       *       *

The only other minerals which need be mentioned in this class (see list)
are:

_Zirkelite_, a complicated mixture of oxides, in which thoria, zirconia,
and titanium dioxide act as acidic oxides, and

_Mackintoshite_, a mixture of several oxides, of which those of thorium
and uranium are the most important.


(_b_) THE CARBONATES


~Lanthanite~, Hydrocerite.--This mineral is a carbonate of ceria earths,
chiefly lanthana, of the formula La₂(CO₃)₃,9H₂O.

  Orthorhombic; _a_ : _b_ : _c_ = 0·9528 : 1 : 0·9023. Common forms--the
  pinakoids _a_ {100} and _c_ {001}, with _m_ {110} and _o_ {111}.

  Angles--(100) ∧ (110) = 43° 37´; (001) ∧ (101) = 43° 26¹⁄₂´; (001) ∧
  (011) = 42° 3¹⁄₂´.

  Habit tabular, parallel to _c_; cleavage perfect, ∥ _c_.

  Double refraction negative; optic axis plane _a_ (100).

  Usually amorphous, being probably an alteration product of a mineral
  rich in lanthanum. Hardness 2; sp. gr. 2·6-2·7.

  Colour white to yellowish-white, usually opaque; infusible before the
  blowpipe (being converted to the oxide), readily soluble in acids.

Lanthanite occurs with cerite at Bastnäs, and at Bethlehem,
Pennsylvania.

Morton[94] states that he prepared a crystalline didymium carbonate in
the laboratory, of the formula Di₂(CO₃)₃,8H₂O, which was isomorphous
with lanthanite; he concluded that the latter had only eight instead of
nine molecules of water.

  [94] See abstract in _Zeitsch. Kryst. Min._ 1886-87, ~12~, 518.


~Parisite~ (~Synchisite~), and ~Cordylite~.--_Parisite_ is a
fluocarbonate of calcium and cerium metals; _Cordylite_ is an analogous
compound in which barium replaces calcium, and is isomorphous with
Parisite. The formula of Parisite is CaR₂F₂(CO₃)₃, where R = cerium
metals. Groth formulates this as (CaF)(RF)R(CO₃)₃, Penfield and Warren
as (RF)₂Ca(CO₃)₃, whilst Schilling gives Ce₂(CO₃)₃,CaF₂. Analogous
formulæ may be proposed for Cordylite, BaR₂F₂(CO₃)₃. Since the two
minerals are very similar in crystallographic properties, one
description will be sufficient for both. The following are Dana’s data
for Parisite:

  Hexagonal, _c_ = 3·2891. (0001) ∧ (101̅1) = 75° 15´.

  Forms are extremely numerous, and have remarkably high indices. Among
  the simplest are the base _c_ {0001}, the prism _m_ {101̅0}, pyramids
  _q_ {101̅2}, and _h_ {112̅2}; the other forms are chiefly rhombohedra
  and pyramids. The usual habit is that of an acute double hexagonal
  pyramid, with form _o_ {202̅1}, terminated by _c_. Cleavage ∥ _c_,
  perfect.

  It is brownish-yellow to red. Hardness 4¹⁄₂; sp. gr. 4·36.

  The double refraction is strong, positive. Soluble in hydrochloric
  acid with effervescence.

Both minerals are characteristic pneumatolytic species of the
riebeckite-ægirine rocks. Parisite was discovered by Paris in the
emerald mines of the Muso valley, Colombia, in 1835, and first correctly
analysed by Bunsen in 1845. Before the blowpipe it glows, remaining
infusible (the glow does not appear to have been investigated in this
case).

_Cordylite_ was discovered by Flink in 1900, in Greenland.

  It is yellow to brownish-yellow and colourless. Hardness 4¹⁄₂; sp. gr.
  4·31. Before the blowpipe it decrepitates, and is infusible; moistened
  with hydrochloric acid, it gives the characteristic barium flame.

The so-called Synchisite was discovered by Nordenskiöld who correctly
described it as Parisite. Flink found it in Greenland, and announced it
as a new species, with the formula R₂F₂Ca₂(CO₃)₄, _i.e._ the formula for
parisite plus one molecule of calcium carbonate, CaCO₃. From its
extraordinary resemblance to parisite in physical and crystallographic
properties, Palache and Warren[95] believe that the specimens selected
by Flink for analysis must have consisted, in reality, of parisite with
admixed calcium carbonate. This conclusion has now been confirmed by
Quercigh, by a careful comparison of the optical properties.[96] The
minerals are usually found together, the chief localities being S.
Norway, the gold districts of the Urals, Narsarsuk in S. Greenland, and
Montana, U.S.A.

  [95] _Amer. J. Sci._ 1911, [iv.], ~31~, 533.

  [96] _Abstr. Chem. Soc._ 1912, ~102~, ii. 773.

       *       *       *       *       *

The following rare earth carbonates are described in the alphabetical
list:

_Ancylite_, a basic hydrated carbonate.

_Tengerite_, a hydrated carbonate formed by the weathering of
gadolinite.

_Kischtimite_, a fluo-carbonate related to parisite.

_Bastnäsite_ (Harmatite) and _Weibyite_, hydrated fluocarbonates of the
cerium elements.


CHAPTER VI

THE PHOSPHATES AND HALIDES


(_a_) THE PHOSPHATES


~Monazite~, Phosphocerite.--Monazite, by far the most important,
commercially, of all the rare earth minerals, is essentially an
orthophosphate of the ceria earths, of the formula R´´´PO₄.[97] The
yttria earths are usually present in small quantities. Silica and
thoria, in quantities varying from traces up to 6 per cent. of the
former and from 1 to 20 per cent. of the latter, are invariable
constituents; it is almost entirely to the percentage of thoria that the
mineral owes its commercial value. The following also are common
constituents, though usually in very small quantities only--stannic,
ferric and manganous oxides, alumina, lime, magnesia, zirconia and
water. Helium was observed in it by Tilden, and by Ramsay, Collie and
Travers.[98] Boltwood[99] and Zerban[100] found uranium in it; the
latter attributed this to impurities, the former regarded it as an
essential constituent. Strutt[101] found uranium in a pure monazite.
Haitinger and Peters[102] detected radium, their result being confirmed
by Boltwood and Strutt.

  [97] For the composition of the earths in monazite, see James, _J.
  Amer. Chem. Soc._ 1913, ~35~, 235.

  [98] _Trans. Chem. Soc._ 1895, ~67~, 684.

  [99] _Phil. Mag._ 1905, [vi.], ~9~, 599.

  [100] _Ber._ 1905, ~38~, 557.

  [101] _Proc. Roy. Soc._ 1905, A, ~76~, 88 and 312.

  [102] _Sitzungsb. kaiserl. Akad. Wiss. Wien_, May, 1904.

  Monazite occurs in small crystals belonging to the monoclinic system.
  _a_ : _b_ : _c_ = 0·9693 : 1 : 0·9256, β = 76° 20´. These values vary
  slightly with different specimens. Common forms--Ortho- and
  clino-pinakoids _a_ {100}, _b_ {010}, hemi-prisms _m_ {110}, and _n_
  {120}, hemi-ortho-prisms _w_ {101} and _x_ {1̅01}, hemi-clino-prism
  _e_ {011}, hemi-pyramid _v_ {1̅11}, etc.; the basal pinakoid _c_ {001}
  is rare.

  Angles--a ∧ _m_ = 43° 17´, _c_ ∧ _w_ = 37° 8´, _c_ ∧ _e_ = 41° 58´.

  Habit tabular, parallel to _a_, needle-shaped by elongation parallel
  to _b_ axis, or prismatic by good development of _v_.

  Cleavage ∥ _c_, perfect, ∥ _a_, distinct, ∥ _b_, difficult.

  Twin plane _a_ (100). Birefringence moderate, positive; plane of optic
  axes perpendicular to _b_, nearly parallel to _a_. Acute bisectrix
  inclined to _c_ at angle of 1°-4°. Dispersion feeble, ρ < υ. Brittle.
  Hardness 5-5¹⁄₂; sp. gr. 4·9-5·3; conchoidal fracture. Lustre
  resinous. Colour, red to brown, yellow, yellowish- and greenish-brown.
  Transparent when pure; more often translucent to opaque.

Monazite is with difficulty soluble in acids; before the blowpipe it is
infusible; when moistened with sulphuric acid it colours the flame
greenish-blue.

The mineral often occurs massive, yielding angular fragments, but is
most common in rolled grains. It occurs in the gneiss of the Carolinas
and Georgia, and in sands derived from the gneiss, in Idaho and many of
the Pacific States; in Brazil, at various localities in the provinces of
Minas Geraes, Bahia, Espirito Santo; in Queensland, Australia; in
Madagascar; in Ceylon; near Travancore in India; in the Urals; in
Scandinavia, etc. The deposits of commercial value will be treated more
fully in the next chapter. It is of wide distribution as an accessory
constituent of granites, diorites, and gneisses.

Monazite was first described, under the name Turnerite, by Lévy,[103] in
1823; the specimen was from the collection of the English chemist
Turner, who thought it a variety of sphene (titanite), and was named
after him at the suggestion of the mineralogist Heuland. The specimen
was stated to have been found in Dauphiné, but in spite of considerable
examination of the question, the precise locality is still unknown. The
resemblance between Turnerite and the mineral later described as
monazite (μοναζειν = to be solitary) was pointed out by Dana in 1866,
and confirmed by Pisani, 1877. The name Monazite was first used by
Breithaupt[104] in describing a mineral found by Menge (1826)
accompanying zircon in a granite from Miask in the Urals. Breithaupt
concluded, from the high specific gravity, that the mineral contained a
heavy metallic oxide. It was again described as Mengite by Brooke[105]
in 1831. It was re-discovered by Shephard[106] in South Carolina in
1837, and described by him under the name Edwardsite, a variety from
Connecticut being called Eremite. To Shephard belongs the honour of
having discovered its true nature; after analysis he described it as a
‘Basic Sesquiphosphate of the Protoxide of Cerium,’ giving the formula
(modern notation) 3CeO,2P₂O₅, and finding also zirconia, alumina, and
silica in it (his specimen was probably very impure). Gustav Rose[107]
showed this to be identical with monazite in 1840. In 1846 Wöhler
described, under the name Cryptolite, a variety of tetragonal habit
closely resembling zircon. This occurs at Arendal in Norway, enclosed by
apatite, in the granite; it may be obtained by treatment with dilute
nitric acid, which dissolves the apatite.

  [103] _Annals of Philosophy_, 1823, ~21~, 241.

  [104] _Schweigg. J._ 1829, ~55~, 30.

  [105] _Phil Mag._ 1831, [ii.], ~10~, 139.

  [106] _Amer. J. Sci._ 1837, ~32~, 162.

  [107] _Pogg. Ann._ 1840, ~49~, 223.

The question of the manner in which the thorium is combined in monazite
is of considerable importance, in view of the fact that it is to this
element that the mineral owes its commercial value. The amount present
varies from traces up to over 20 per cent., but the usual value is
between 5 and 7 per cent. The first explanation of its presence was
advanced by Dunnington[108] who suggested, on the result of only one
analysis, that orangite (ThSiO₄) was present mechanically mixed with the
monazite. Penfield[109] supported this suggestion, and stated that in
three analyses of pure material he found the ratio of rare earths to
phosphorus pentoxide and that of thoria to silica exactly equal to
unity, though the actual amounts of thoria varied considerably. He also
quotes an analysis made by Rammelsberg in 1877, in which no thoria was
found, to show that it is not an essential constituent. In a microscopic
examination he found dark resinous particles scattered throughout the
section; after moistening with hydrochloric acid, warming, and washing,
these dark spots became white, and could be stained with fuchsine, the
monazite remaining unaffected throughout. He concluded that these
particles were thorite or orangite.

  [108] _Amer. Chem. J._ 1882, ~4~, 138.

  [109] _Amer. J. Sci._ 1882, [iii.], ~24~, 250; 1888, ~36~, 322.

Blomstrand[110] disputed Penfield’s conclusions. In twelve analyses of
monazite from various parts of Scandinavia he never once found either
thoria or silica absent. Of these twelve analyses, two give the ratio of
thoria to silica, ThO₂ : SiO₂, exactly unity, in seven cases the ratio
is not greater than 1·25, in five cases it varies considerably. He
summed up his results in three statements:

  (_a_) Silica is never absent; its amount depends not on the amount of
  thoria, but on the amount of phosphorus pentoxide present.

  (_b_) The thoria which is always present is combined partly with
  silica, partly with phosphorus pentoxide.

  (_c_) In most cases, the rare earths alone are insufficient to satisfy
  the ratio R₂O₃ : P₂O₅ = 1.

  [110] _J. pr. Chem._ 1890, ~41~, 266.

An exhaustive examination of the question has been made more recently by
Kress and Metzger.[111] They made in all over fifty analyses, using
thirty different specimens of monazite; they estimated silica both as
quartz and as silicate silica, and determined thorium by the fumarate
method--the other investigators had used the thiosulphate method of
Hermann (_vide_ p. 286). Their results may be summarised as follows:

  (i.) Silica is always present.

  (ii.) The amount of silica usually increases with the thoria, but not
  regularly.

  (iii.) By far the majority of cases showed insufficient total silica
  to combine with the thoria present.

  (iv.) In about 9 per cent. of the cases, the thoria present was
  insufficient to combine with the silicate silica, from which it
  follows that some foreign silicate must be at least occasionally
  present.

  (v.) A careful microscopic examination showed conclusively that no
  thorite (ThSiO₄) was present, the silicate being biaxial; quartz is
  present as such.

  [111] _J. Amer. Chem. Soc._ 1909, ~31~, 640.

They conclude that thorium is present as phosphate, and is an essential
constituent, but that there is always some admixed silicate, most
probably a felspar.


~Xenotime.~--Chemically this mineral is closely allied to monazite,
being an orthophosphate of rare earths, containing silica and thoria;
whereas, however, in monazite the content of yttria earths does not rise
above 4 per cent., in xenotime these constitute by far the greater part
of the bases, the content of ceria earths ranging from 8·2 to 11 per
cent. The yttria earths, chiefly oxides of yttrium and the erbium group,
vary from 54·1 to 64·7 per cent. There are traces of zirconia; Ramsay,
Collie and Travers detected helium, whilst Boltwood, and also Strutt,
found uranium and radium. It also appears to contain traces of sulphuric
anhydride.

  The crystals are tetragonal, holosymmetric. _c_ = 0·6187; (001) ∧
  (101) = 31° 45´.

  Common forms are the prisms _a_ {100} and _m_ {110}, the basal
  pinakoid _c_ {001}, the pyramids _e_ {101}, _f_ {201}, _z_ {111}, etc.

  Cleavage ∥ _m_, perfect. Uniaxial, double refraction strong, positive.
  Transparent to opaque. Colour, brown to reddish-brown and yellow.
  Hardness 4-5; sp. gr. 4·45-4·56.

It is insoluble in acids, and infusible before the blowpipe; when
moistened with sulphuric acid, however, it turns the flame bluish-green,
like most mineral phosphates (_vide_ monazite).

It is not so widely distributed as monazite, but is not uncommon. It
often occurs with zircon--to which it is very closely allied in crystal
form, if the two are not actually isomorphous--in parallel growth, in
granitic rocks. The diamond sands of Diamantina, Brazil, form the
richest source of the mineral, but it is also found in Scandinavia, at
Hitterö, Åro, etc.

The mineral is of considerable importance, chemically, on account of the
high percentage of erbia earths.

In the works of Bauer, Rosenbusch, Weinschenk, Schilling and Iddings
will be found accounts of a mineral named ‘Hussakite.’ These accounts
rested on the work of Kraus and Reitinger,[112] who in 1901 announced
the discovery of a new species. The crystals were obtained as a specimen
of xenotime by Prof. Muthmann from Dr. E. Hussak, in São Paulo, and had
the crystallographic properties of that mineral. On analysis, the amount
of sulphur trioxide present was found to be remarkably high (6·3 per
cent.), and Kraus and Reitinger concluded that the substance was
distinct from xenotime. They announced it as a new mineral, with the
name Hussakite, and the formula 3R₂O₃,3P₂O₅,SO₃ or 6RPO₄,SO₃, and stated
that by the action of dilute alkalies the sulphur trioxide could be
easily and completely removed. They therefore regarded xenotime as a
pseudomorph[113] after hussakite, the sulphur trioxide having been
removed from the latter by the action of the alkaline waters of the
earth’s crust. In support of this view, they gave analyses of opaque
crystals from a Bahia sand represented as containing 2·6 to 2·7 per
cent. of sulphur trioxide, and so as being intermediate forms produced
during the change.

  [112] _Zeitsch. Kryst. Min._ 1901, ~34~, 268.

  [113] One mineral is said to be pseudomorphous after another when the
  first is produced from the second by a chemical change which proceeds
  so slowly that the original structure and crystalline form are
  unaltered (_i.e._ a change proceeding molecule by molecule). The
  pseudomorph is usually opaque and shows clear signs of the alteration.

The latter conclusion was quickly challenged by Brögger, who found no
sulphur trioxide in a perfectly fresh and transparent xenotime from Åro
in Scandinavia. Brögger concluded that the Hussakite of Kraus and
Reitinger was an independent species of the formula 5YPO₄,(YSO₄)PO₃, and
that xenotime was not derived from it.

Basing his work on the barium chloride test given by Kraus and Reitinger
(see below) Rösler[114] declared that ‘Hussakite’ was a common accessory
constituent of igneous rocks, having been previously mistaken for
zircon, which it resembles in appearance and optical properties.

  [114] _Zeitsch. Kryst. Min._ 1902, ~36~, 258.

In 1907 Hussak[115] published a paper in which he showed that the
mineral named after him was not a new species at all, but a xenotime of
prismatic habit. Analyses made at his request by Florence in Brazil, G.
T. Prior in London, and Tschernik in St. Petersburg, confirmed the
original values given by Gorceix (sulphur trioxide up to 0·25 per
cent.). He mentions Brögger’s analysis of the Norwegian specimen in
which Kraus and Reitinger had found 2-3 per cent. of sulphur trioxide,
but in which Brögger found none. He explains the results of Kraus and
Reitinger as due to the addition of barium chloride to the acidified
solution of the carbonate fusion of the mineral, by which barium
phosphate was precipitated; this was dried and weighed as barium
sulphate. Rösler’s tests are declared doubtful; xenotime is not a widely
spread rock constituent, the mineral in question being really zircon.

  [115] _Centr. Min._ 1907, 533.

In face of these results, there can be little doubt that the name
‘hussakite’ is unnecessary and undesirable, since the mineral to which
it was applied is proved to be xenotime.

       *       *       *       *       *

In the alphabetical list, particulars of the following rare earth
phosphates will be found:

_Castelnaudite_, a variety of xenotime containing zirconia.

_Churchite_ and _Rhabdophane_ (Scovillite), hydrated phosphates.

_Gorceixite_, an alumino-phosphate of alkaline and ceria earths.

_Retzian_, an hydrated arsenate of manganese, calcium and rare earth
metals.


(_b_) THE HALIDES


~Yttrocerite.~--This mineral is a fluoride of calcium and rare earth
metals, with water. A recent analysis by Tschernik[116] gives the
formula Ce₂F₆,2Y₂F₆,9CaF₂,2H₂O. Putting the rare earth metals together,
this gives 6RF₃,9CaF₂,2H₂O, or R₂Ca₃F₁₂,²⁄₃H₂O. Yttrocerite is of
interest since it was probably in the analysis of this mineral by the
discoverers, Berzelius and Gahn, that the double sulphate method of
separating the yttria from the ceria earths was first employed[117]
(_vide_ p. 156).

  [116] _Abstr. Chem. Soc._ 1907, ~92~, ii. 362.

  [117] _Schweigg. J._ 1816, ~16~, 244.

  It is found only massive or granular. Colour usually white to
  violet-blue, sometimes reddish-brown. Hardness 4¹⁄₂; sp. gr. 3·45.
  Infusible, but loses colour before the blowpipe. When powdered, it
  dissolves completely in boiling hydrochloric acid, and readily in
  sulphuric acid with evolution of heat. It has been found at various
  localities in Scandinavia.


~Yttrofluorite.~[118]--This is a fluoride of varying composition, very
similar to yttrocerite, but characterised by the absence of water, and
the very small ceria content (1·7 per cent.). It is thus a fluoride of
calcium and the yttrium metals.

  [118] T. Vogt, _Centr. Min._ 1911, 373.

  Cubic, with poor octahedral cleavage. Colour, yellow to brown and
  yellowish-green; transparent to translucent, bleached by weathering.
  Very brittle. Hardness 4¹⁄₂; sp. gr. 3·54-3·56.

It is very similar to fluorspar (except that the octahedral cleavage of
the latter is very good), and is regarded by Vogt as an isomorphous
mixture of the latter with yttrium fluoride (or with a double yttrium
calcium fluoride, which is less probable). This view would account for
the variations in composition, and also for the remarkable frequency
with which traces of rare earths are found in fluorspar (_vide_ p. 2).
Yttrocerite is regarded as a similar isomorphous mixture, but containing
cerium metals in addition to the yttrium group.

Yttrofluorite occurs in pegmatite veins in granite in Northern Norway,
with gadolinite, fergusonite, allanite, fluorspar, and the usual vein
minerals.

       *       *       *       *       *

The other members of this family (see list) are:

_Fluocerite_, a basic fluoride of yttrium and cerium metals.

_Tysonite_, a hydrated fluoride containing carbonates.

It is to be noticed that fluorine is the only member of the halogen
family which occurs in nature in combination with rare earth elements.
This fact is possibly connected with the great age of the rare earth
minerals, and their formation during pneumatolytic metamorphism of
plutonic rocks (_vide_ Chapter I).


CHAPTER VII

THE MONAZITE SANDS

It has been stated that monazite is a not uncommon accessory constituent
of many rocks, particularly of granites, gneisses, diorites, etc. The
crystalline material, of which an account has been given, is found
sometimes in veins in these rocks, more often in tiny crystals
disseminated throughout the mass. Most of these monazite-bearing rocks
are extremely old, belonging to the Archæan or pre-Cambrian age, and
probably none are of secondary (Mesozoic) or later age. It follows,
then, that they have been subjected to erosion during practically the
whole immense period of which geology can give us any detailed
knowledge. Heat, frost, wind, the action of vegetation and of
percolating water, the innumerable weathering agents known to the
geologist, have been at work on them during countless ages, breaking,
crushing, dissolving; rains, brooks, rivers, even ocean-waves have
dissolved or washed away the fragments, sorted them out unerringly
according to density, and re-deposited them, now in a river-bed, now at
the base of some sea cliff, now in a wide alluvial plain from which the
water has long since retired. It is in deposits of this nature that the
monazite has been concentrated. Its relatively high specific gravity
(about 5·0) has secured its separation from the lighter mica, quartz,
and felspar of the parent-rock; but the heavier vein or accessory
minerals have, of course, been concentrated with it. Zircon is an
invariable constituent of these ‘monazite sands,’ as such deposits are
called; and others almost as frequently found are rutile, ilmenite,
sphene (titanite), and apatite. Common, too, are the characteristic
minerals of the metamorphic rocks, garnet, epidote, sillimanite,
tourmaline, etc. Rare earth minerals found in the monazite sands include
xenotime, fergusonite, samarskite, gadolinite, and allanite. The
remaining minerals are oxides of iron and tin, with, of course, a
considerable amount of quartz.

It is apparent, from what has been said above, that monazite will be
concentrated with the heaviest constituents of the rocks from which it
is derived. Very often, indeed usually, these rocks are precisely those
in which gold occurs, disseminated sometimes in tiny particles,
sometimes collected into nuggets in veins of quartz and pegmatitic
minerals. The erosion of these rocks concentrates the gold with the
heaviest minerals; and hence it happens that monazite is an almost
universal constituent of the gold- and gem-bearing sands and gravels. In
the Carolinas and in Brazil, monazite is found in the gold washings; and
though in the past the two have always been extracted separately, the
gold first and the monazite from the washings or tailings, there appears
to be no reason why a system calculated to extract both--where, of
course, the content is high enough--should not be put into operation in
the future.

A chemical test affords the only reliable method of detecting monazite
in a sand. A little of the sand is washed with water to remove the
lighter minerals and warmed with concentrated sulphuric acid. A few
drops of the liquid are poured off, evaporated to small bulk, and one
drop placed on a glass plate. This is placed under a microscope and one
drop of a concentrated solution of sodium acetate is added. If monazite
is present in the sand, tiny pointed oval crystals of sodium cerium
sulphate will separate.

On the commercial scale, monazite is extracted from the sands only, in
the manner described below. An effort was made in North Carolina in 1906
by the British Monazite Company, representing the South Metropolitan Gas
Light Company of London, to extract monazite from the rock in which it
occurs disseminated. The rock was crushed and powdered, and the monazite
separated by washing off the lighter particles on concentration tables
(see below). In the same year, however, the price of thorium nitrate was
suddenly lowered 50 per cent. by the German Thorium Syndicate, which
largely controls the Brazilian output of monazite, and the British
company stopped operations in 1907. At present it may be said that only
the sands are available for profitable extraction.

Up to 1895, the Carolina deposits, which were worked chiefly by the
Welsbach Light Company of New York, either directly or indirectly,
supplied all the demand, but in that year the Brazilian sands were first
worked, and a keen struggle commenced for the market. The American
companies, after keeping up a considerable output for some years, were
forced to suspend operations in May 1910. The Brazil deposits, worked by
the German Thorium Syndicate and the Austrian Welsbach Company, which
have an agreement, now meet practically the whole demand. The Brazilian
sand occurs chiefly along the shores of the southern provinces, having
been concentrated by the action of the tides from the products of
erosion of the cliffs; it is very uniform and considerably richer than
the Carolina sand, and owing to its occurrence on the sea-shore, the
cost of transporting it is very low. It is exported chiefly to Germany,
recently also to the United States, and to a small extent lately to
England. The method of working it is similar to that employed in
Carolina--namely, concentration by washing and magnetic separation.


THE NORTH AMERICAN DEPOSITS[119]

  [119] See Nitze, _Bull._ No. 9, _N. Carolina Geol. Survey_, 1895; also
  Test, _Colorado School of Mines Bull._ Vol. IV. No. 2, p. 125, Jan.
  1908.

There are two important regions in North America within which monazite
sands occur; one extends over the Carolinas, and the north-western part
of Georgia, the other over the Idaho basin and neighbouring counties of
the Pacific Slope. It will be best to treat these separately, as the
deposits are somewhat different in character.


(_a_) ~The Carolina Deposits~, including the unimportant Georgia
deposits, which belong to the same field, occur over an area approaching
4000 square miles. The area is occupied chiefly by the Piedmont plateau,
which is drained by a number of streams rising in the South Mountains,
an eastern outlier of the Blue Ridge; it is in the basins and valleys of
these streams, particularly at the head-waters, that the monazite is
chiefly found. The geology of the district is very complicated,[120] the
rocks being very highly altered granites. The chief bed is known as the
Carolina gneiss, and includes several types of gneiss, usually very much
weathered. The sands, which average about 1 per cent. of monazite, are
worked in and near the stream beds; they occur in the beds, and in
layers 1 to 2 feet in thickness a few feet below the surface of the
surrounding soil.

  [120] See Sterret, _U.S. Geol. Survey_ (_Minerals_), 1906, p. 1195.

Concentration was formerly effected chiefly by a crude process of
washing. In this process the sand is thrown on to a sort of sieve, fixed
over the upper end of a long wooden trough, by one workman; a jet of
water is directed on to the sieve, washing the sand through it. The
heavier particles fall to the bottom of the trough, whilst the lighter
are washed right through. A second workman continually turns over the
sand left in the box and on the sieve; at the end of a day’s work the
‘concentrate’ is collected. This averages from 15 to 70 per cent. of
monazite, according to the nature and amount of the heavy minerals
accompanying it in the sand. The concentrate is dried either on rubber
or oiled cloths in the sun, or on an iron plate covering a trough in
which a fire is lighted. The iron minerals are then picked out by means
of a magnet, and the sand filled into sacks for transport.

Before treatment for thorium nitrate, the sand is at the present day
further concentrated by powerful magnetic separators. In a few cases the
older method of concentration by hand-washing has been abandoned for
machine concentration, the Wilfley table being sometimes employed. The
principle here is exactly the same, the sand being fed into a hopper by
means of a moving belt and thence on to a machine-shaken table from
which running water constantly removes the particles, sorting them
according to their specific gravity.

Further separation of the dried concentrate has been effected by three
kinds of separators.[121] The first was of the Edison, or
fall-and-deflection type; in this the sand is allowed to flow in a thin
vertical stream past a horizontal magnet, which deflects the minerals
containing iron; these fall on one side of a partition, the part richer
in monazite on the other. The second was an electrostatic machine; the
heated sand is borne on a moving belt underneath a rotating vulcanite
cylinder, excited by felt-covered rubbers; the lighter particles are
attracted to the cylinder, and dropped on one side, the heavier passing
on. Neither of these machines is of much value in effecting
concentration, and neither is in general use.

  [121] See Pratt and Sterrett, _Trans. Amer. Inst. Min. Eng._ 1909,
  ~40~, 313.

The third, and by far the most efficient and most widely-used machine,
is known as the Wetherill electro-magnetic separator. It depends on the
principle, first applied by the American engineer Wetherill, that not
only the iron minerals, but a large number of other minerals may be
attracted if the magnetic field be sufficiently strong. In all types of
this machine used in cleaning monazite concentrates, four magnetic
fields of increasing intensity are traversed by the sand; the first
removes magnetite, ilmenite, and the larger fragments of garnet; the
second removes all the remaining garnet and ilmenite; the third removes
the coarser, and the fourth the finer monazite, tailings of zircon,
rutile, and silica passing on. Careful adjustment of the magnetic fields
will readily give a 97-99 per cent. monazite.

Two types of this machine are in common use. In the first the magnetic
fields are obtained by four successive electro-magnets, arranged so that
a broad horizontally-moving belt passes between the poles of each in
succession. The upper poles are ground down to a fine edge perpendicular
to the direction of the belt, to secure a more powerful field. Just
beneath these edges, and just above the broad belt are four rapidly
driven horizontal belts moving at right angles to the first or main
belt; these carry off and deposit in separate bins the minerals
attracted by their respective magnets. This type is known as the Rowand
separator.

In the second type four horizontal belts are arranged in the form of
descending steps, as shown in the diagram (Fig. 2). The magnets are
placed at the end of each belt, and within it. The attracted mineral is
held to its own belt, whilst the remainder drops on to the next; the
attracted mineral falls into a bin as soon as its belt carries it out of
the magnetic field. The sand to be cleaned is fed on to the first belt
by means of a hopper.

The almost pure monazite so obtained is now treated chemically for its
thorium. The processes proposed and in use are described in Chapter
XVIII.

[Illustration: FIG. 2.]

As already stated, the extraction of monazite in the United States has
practically ceased; but the processes outlined above, which were first
brought into use in the Carolinas, have been adopted for the treatment
of the Brazilian sands.


(_b_) ~The Idaho Deposits.~--Monazite was first observed in placer-gold
deposits in the vicinity of Boise city near the Snake river. This
deposit was a gold-bearing sand derived from granite. Later the
gold-bearing sands of Oregon were also found to contain monazite; these
sands are rich in zircon, and contain platinum and allied metals as well
as gold. The sands of the Pacific slope are the so-called black sands,
derived from hornblende, and augite-granites, usually porphyritic, which
are much weathered at the surface. The soil is loose and is largely
composed of granite fragments; the rain and streams constantly bring it
down to the valleys, and continually renew the deposits. The
concentrates obtained by washing are rich in well-crystallised zircon,
with titanite and garnet.

In 1906 a company was formed to extract monazite from the black sand
residues left after the extraction of gold. By 1909 they had erected
plant and commenced operations at Centerville, and proposed to work the
poorer auriferous sands for gold during the monazite washing. This, it
was expected, could be done by washing the sands in boxes lined with
amalgamated copper plates, which would retain the gold. Considerable
amounts of monazite had already been extracted from the tailings when a
disastrous fire put a stop to the operations in 1910.

Since then the production of monazite in the United States has
practically ceased.


THE BRAZILIAN DEPOSITS

Brazil first became a serious competitor in the world’s market with the
United States, for the supply of monazite, in 1895. The greater
percentage of thorium, the more even quality of the sands, and above all
the occurrence on the sea-coast, rendered the Brazilian monazite cheaper
from the beginning, so that it soon ousted the Carolina sand, and since
1910 has supplied the whole demand. The deposits at present worked lie
along the coasts of Bahia, Minas Geraes and Espirito Santo, and whilst
they are very rich in monazite, there is the disadvantage that their
position and extent, and so also the possibility of working them, depend
very largely on the variations in the tides, etc. The largest of these
deposits is on the shores of a bay near the island of Alcobaca, on the
southern coast of Bahia.

Monazite also occurs to a considerable extent in the diamond sands and
gold-bearing sands of many of the interior provinces. In Minas Geraes it
has long been known to occur at the celebrated mining centres of
Diamantina and Ouro Preto, where xenotime and other rare earth minerals
are also found; it is also known at various localities in the
surrounding mining provinces of São Paulo, Goyaz, and Matto Grosso. More
recently, extensive inland deposits have been found by Freise, in the
province of Espirito Santo.[122] In the plateau-basin of the Muriahé and
Pomba rivers he found a sand known locally as ‘catalco’ which carries an
average of 2·1 per cent. of monazite and a gold-content of 1·75 grams
per ton. In the Aymoré’s mountains he found monazite, both massive and
granular, in pegmatite veins in granite; analysis showed a thoria
content of 9·23 per cent., which is very high. These deposits would form
a very valuable and extensive source of thoria, if the difficulties of
transport could be overcome.

  [122] _Zeitsch. pr. Geol._ 1909, ~17~, 514; _ibid._, 1910, ~18~, 143.

At present, as stated above, only the beach deposits are worked. The
Brazilian Government has laid a very heavy tax on all monazite exported;
it is stated[123] that the German Thorium Syndicate pays 50 per cent. of
its profits in royalties to the Government. In spite of this, the high
quality of the sand and the low cost of transport have enabled this
combine to lower the price of thorium nitrate to a point at which the
Carolina sands cannot be worked, and it appears probable that the
world’s markets will be supplied for some time, at least, entirely from
Brazil. The methods employed in working the sand are similar to those
already described.

  [123] _U.S. Geol. Survey_ (_Minerals_), 1906, p. 1195.

In the last few years monazite deposits have been found in various
places, notably in Australia, India, and Ceylon. In the latter locality
it occurs sparingly in the gem-gravels, in association with the much
more valuable thorianite and thorite, but the supply is uncertain, and
the minerals cannot be worked regularly. In Australia it occurs in
Victoria and in Queensland. In Victoria the deposits are poor in
monazite--about 0·025 per cent.--so that working is not profitable. In
Queensland it occurs in beach sands on the southern coast, with gold,
platinum, and cassiterite; there seems to be no reason why these
deposits should not be profitably worked when sufficient labour is
forthcoming. It also occurs in North Queensland, on the Walsh and
Tinaroo mineral fields; here it is found massive and granular in veins
in granite, associated with wolframite, molybdenite, and cassiterite.

Quite recently, deposits of considerable extent have been found near
Travancore, India.[124] These sands contain about 46 per cent. of the
mineral, which is itself very rich in thoria, containing about 10 per
cent. of the oxide; the unconcentrated sand is therefore as valuable as
a source of thorium nitrate as the ordinary Brazilian concentrates,
which average 4 per cent. or less of the oxide.

  [124] _Bull. Imp. Inst._ 1911, vol. ix., No. 2, p. 103.

Monazite has also been observed in the tin-bearing sands of Embabaan,
Swaziland, South Africa, and in the province of Ottawa, Canada.


CHAPTER VIII

RADIOACTIVITY OF THE MINERALS

In the present chapter no attempt will be made to give a complete
account of all the phenomena of radioactivity which have been observed
in the mineral world. There are, however, a few problems of the highest
scientific interest which centre about the rare earth minerals, and
mention of these can hardly be avoided in a work which professes to give
a general account of the rare earth group. It is obvious that a detailed
treatment cannot be given without entering into phenomena which would be
quite beyond the range of the present work, and an excuse is hardly
needed, therefore, for the fragmentary and abbreviated account which
follows. The reader’s acquaintance with the general phenomena of
radioactivity is of necessity assumed.

Radioactivity (the spontaneous emission of special radiations) was first
observed by Becquerel, in 1896, in the case of potassium uranyl
sulphate, and was soon found to be common to all uranium compounds, and
to the metal itself. Mme. Curie showed that whilst in uranium salts the
degree of activity varies directly with the percentage of uranium, in
minerals containing the element the same rule does not hold. The
observation that pitchblende is considerably more active than the
uranium it contains led to the discovery of polonium[125] and radium in
1898. Exactly analogous phenomena were shown to hold for thorium salts
and thorium-containing minerals by Mme. Curie and Schmidt in 1898, and
in 1905 Hahn separated Radio-thorium from thorianite. In 1899 Debierne
discovered that the rare earths precipitated from the solution obtained
on treatment of pitchblende are associated with another extremely active
body, which he named Actinium; Giesel found that in the separation of
the rare earths this remains with lanthanum. In 1903 Ramsay and Soddy
experimentally confirmed the prediction of Rutherford and Soddy, that
radium would be found to produce helium continuously. The discovery of
these remarkable phenomena has modified many fundamental physical
conceptions, and has opened up a new field of scientific enquiry, which
is being developed with unexampled rapidity.

  [125] Polonium, which was named by Mme. Curie in honour of her native
  country, has been shown to be identical with Marckwald’s
  ‘Radio-tellurium,’ which was named by Rutherford ‘Radium F’; it is one
  of the degradation products of radium.

It has been mentioned, in the accounts of the rare earth minerals given
above, that almost all these minerals are radioactive, _i.e._ have the
property of emitting specific radiations. Moreover, radioactivity, to
any considerable extent at least, is, with a few important exceptions,
confined to the minerals which have been already described. It has been
shown by many investigators, chief amongst whom are Strutt and Boltwood,
that the activity is usually due to the presence of uranium or thorium,
or both.[126]

  [126] Hauser and Wirth (_Ber._ 1910, ~43~, 1807) observed activity in
  some zirconium minerals containing neither thorium nor uranium.

After the discovery of helium in Cleveite (a variety of pitchblende,
_vide_ p. 13) in 1895 by Ramsay, a large number of minerals were
examined for this gas, and it was found that almost all the rare earth
minerals contain helium. The fact that these minerals are also for the
most part radioactive, naturally suggested some relation between the
activity and the presence of helium, and led directly to the discovery
that radium is continuously producing helium; and it became apparent
that helium has been accumulating in these minerals since their
formation, by the decay of radioactive elements. The question of the
origin of helium in minerals will be touched on again.

In 1904 Boltwood advanced the theory that radium is produced by the
degradation of uranium, the parent-element having, however, a much
greater half-life period. If uranium continuously produces radium,
whilst the latter decays much more rapidly than the former, it must
follow that in minerals containing uranium a state of equilibrium is
reached between uranium and radium, and the ratio of these two in all
minerals should therefore be constant, and independent of the geological
age. Boltwood examined a number of the minerals of which descriptions
have been given in the preceding chapters, and found the ratio to be
surprisingly constant.[127] Strutt also examined a large number of
minerals,[128] and whilst on the whole his results seemed to support the
theory, his values for the ratio were by no means so constant as those
of Boltwood. Strutt included in his examination the interesting
radium-containing mineral observed by Danne at Issy l’Evêque.[129] This
was a pyromorphite (lead chlorophosphate) containing neither uranium nor
thorium. Danne suggested that the radium was not an original
constituent, but had been introduced by the action of percolating
waters. This view was confirmed by McCoy and Ross,[130] who found that
the activity was entirely confined to the surface layer.

  [127] _Amer. J. Sci._ 1904, [iv.], ~18~, 97; _Phil. Mag._ 1905, [iv.],
  ~9~, 599.

  [128] _Proc. Roy. Soc._ 1905, A, ~76~, 88 and 312. _Ibid._ 1907, A,
  ~80~, 56.

  [129] _Compt. rend._ 1905, ~140~, 241.

  [130] _J. Amer. Chem. Soc._ 1907, ~29~, 1698.

Mlle. Gleditsch has also examined the question of the uranium-radium
ratio in minerals. Her earlier work[131] gave ratios which, whilst
constant for each mineral species, varied in much the same manner as
Strutt’s for different species, and afforded very little support to
Boltwood’s theory. Her more recent results,[132] however, are much more
closely in accord with the theory, which has been still further
strengthened by the work of Pirret and Soddy[133] and of Marckwald and
Russell.[134] It may now be regarded as firmly established that radium
is in the line of direct descent from uranium.

  [131] _Compt. rend._ 1909, ~149~, 267; _Le Radium_, 1909, ~6~, 165.

  [132] _Le Radium_, 1911, ~8~, 256.

  [133] _Phil. Mag._ 1911, [vi.], ~21~, 652.

  [134] _Ber._ 1911, ~44~, 777.

Boltwood had assumed that the helium in radioactive minerals is produced
from the uranium, during its disintegration. Strutt, however, disputed
this; his experiments showed that very little helium is found even in
the richest radium-uranium minerals unless thorium is also present.
Thus pitchblende contains a very high percentage of uranium, but
relatively little helium (there is usually a considerable thorium
percentage here too, so that nothing conclusive can be deduced from
this). Adams[135] found that carnotite, a mineral very rich in uranium,
but containing no thorium, contains no helium at all; he explained its
absence by the very loose texture and permeability of the mineral, which
would allow the gas to escape. Strutt concluded that whilst helium is
undoubtedly produced by disintegration in the uranium series, in
minerals it is produced more by thorium or, as more recent work
indicates, by radio-thorium, than by uranium.

  [135] _Amer. J. Sci._ 1905, [iv.], ~20~, 256.

The question of the origin of helium in minerals is, however, not
definitely settled, for several anomalous cases are known. Thus the
yttria silicate, Thalénite (_q.v._), contains quantities of helium, but
no uranium or thorium is given in the analyses. Similarly, Risörite
contains a relatively large quantity of helium, but only traces of
uranium and thorium. In the last mineral, the active constituent is
precipitated with the lead, so that no radio-thorium appears to be
present. Further, Thomsen analysed a fluorspar from Ivitgut in Greenland
which he found to contain 27 c.c. of helium per kilogram. This specimen
contains no uranium, but gives off the thorium emanation in quantities
which suggest the presence of radio-thorium; moderate quantities of
thorium are also present. Since the α particle has been definitely
identified as a positively charged helium atom, it appears certain that
disintegration in all three series (uranium, actinium, and thorium
series) produces helium, and a mineral containing a member of any of
these series (which gives α rays or α ray-giving products) would also
contain helium.

Even so, there is a case in which the helium content is anomalous, if
not altogether beyond explanation at the present stage. In examining a
large number of minerals for helium, Strutt[136] found that some samples
of beryl, a beryllium aluminium silicate, contain a relatively very
large amount of helium, but only traces of thorium, and was altogether
inactive. The absence of any active constituent renders untenable the
ordinary explanations of the presence of such a surprising quantity of
helium. Boltwood has put forward a suggestion which in the present state
of our knowledge must be regarded as a provisional explanation. He
conceives that in the concentration of beryllium from the parent magma,
it may have become associated with some short-lived intermediate
radioactive element, which had been altogether separated from its
long-lived parent element in the process of concentration; this
intermediate element, having collected in the crystallised beryl,
decayed completely in the course of the great period which must have
elapsed, leaving the helium to which it had given rise during its
disintegration enclosed in the mineral. It is difficult to see how two
substances which must be so intimately connected as a parent-element and
its product could be completely separated in the process of cooling of a
magma; but since so little is known of the process of crystallisation of
minerals, the suggestion can hardly be rejected on geological grounds.
In any case, we have here only one strongly marked exception to the very
definite rule that in all cases in which helium occurs in minerals, it
is accompanied by and undoubtedly produced from, a radioactive element
or elements; and in the majority of cases, the helium in minerals is
produced by disintegration of uranium or thorium and their products.

  [136] _Proc. Roy. Soc._ 1908, A, ~80~, 572.

Strutt found that traces of helium are universal in the mineral world.
His method of determining helium was approximate only. He obtained the
gas content by heating the powdered mineral--a method which, as Wood has
shown,[137] will only give all the gas when very high temperatures (up
to 1000°C.) are employed. The gases were freed from oxygen and hydrogen
by passing over a heated, partially oxidised, copper spiral, and from
carbon dioxide by means of potash. Nitrogen was removed by sparking with
excess of oxygen and shaking over potash; the excess of oxygen was
removed by melted phosphorus. The inert gases so obtained were freed
from all impurities by the use of the liquid alloy of sodium and
potassium for the electrodes of the spectrum tube in which the gases
were examined spectroscopically.[138] Argon, if present--it seems to be
a universal constituent of igneous rocks, into which it may have been
absorbed from the air--was removed by charcoal at a temperature of
-80°C. The helium so left was examined spectroscopically, and measured
in a MacLeod gauge.

  [137] _Proc. Roy. Soc._ 1910, A, ~84~, 70.

  [138] As soon as the discharge is started in such a tube, all the
  gases present other than those of the helium family are absorbed by
  these electrodes.

As stated, helium was found in traces in nearly all minerals, and its
presence is to be attributed to traces of radium, which also appears
universal. In minerals containing uranium or thorium, or rare earths
(the latter are almost always accompanied by uranium and thorium),
helium is found to a much greater extent, and Ramsay considers it
possible that some fraction of the helium content may arise from the
rare earth metals. There is, however, no positive evidence to support
the conjecture. He found that the helium ratio, _i.e._ the volume of
helium per gram of uranous oxide, UO₂, varies with the amount of thoria
present; but where the latter is absent the variations are much less
marked. If helium were produced in a mineral from uranium alone, and
none escaped, it is obvious that the helium ratio would depend only on
the age of the mineral. For minerals of about the same age, and
containing no thorium, the helium ratio would be roughly constant, if no
disturbing factor required consideration.

In 1905 Strutt pointed out that in all the minerals he had examined,
thorium was never present unless accompanied by uranium and radium,
whilst uranium and radium often occurred without thorium. He suggested
that the present atomic weight of thorium, 232·5, was too low, and that
it was really the parent of uranium (at. weight 238·5); he further
supposed that the next permanent member in the line of descent was one
of the cerium metals. These suggestions have been negatived by later
work of Boltwood and Holmes. The former pointed out[139] that it was far
more likely that thorium is a disintegration product of uranium of
considerably longer life. On the whole, however, there is very little
positive evidence to connect thorium with uranium.

  [139] Boltwood, _Amer. J. Sci._ 1905, [iv.], ~20~, 256.

In the same year Boltwood (_loc. cit._) drew attention to the persistent
appearance of traces of lead, bismuth, barium, etc., in the radioactive
minerals, and also pointed out that the variations of the ratio of
helium to uranium in pitchblende might be used to determine the age of
the mineral. In 1907 he suggested[140] that lead was the final product
of the degradation of uranium, from which it follows that the ratio of
uranium to lead should be constant for minerals of the same age (since,
lead decays, if at all, at an infinitely slower rate than uranium). He
collected all the available analyses, and classified the minerals dealt
with into six groups according to the value of the ratio. The order
given by the ratio was declared to be in accordance with the order of
age as given by geological evidence.

  [140] _Amer. J. Sci._ 1907, [iv.], ~23~, 77.

Holmes[141] has further extended this work. He examined a number of rare
earth and allied minerals from the Christiania district, which Brögger
considers to be of approximately Lower Devonian age, and found the ratio
of lead to uranium to approximate quite closely, for almost all the
minerals examined, to 0·045. Representing the change in the usual way as

    U   → 8He + Pb

  238·5 → 31·92 + 207·1

and using the data calculated by Rutherford and others for the rates of
decay, he gives the age of Lower Devonian strata as about 370 million
years. This figure is about twice as great as that deduced by
palæontologists from the flora and fauna, and greater still than the
times based on physical data, _e.g._ rates of cooling, precession and
nutation, etc. His figures for pre-Cambrian rocks, based on the same
ratio, range between 1000 and 1640 million years, the later being
deduced from a thorianite from the Archæan rocks of Ceylon. Strutt’s
figure for Archæan rocks is about 700 million years; this was derived
from work on the helium ratio, which must now be considered.[142]

  [141] _Proc. Roy. Soc._ 1911, A, ~85~, 248.

  [142] See Strutt, _Proc. Roy. Soc._ 1908, A, ~82~, 166; 1909, ~83~,
  96; 1909, ~83~, 298; 1910, ~84~, 194.

In 1898 Travers[143] had examined the effect of heat on cleveite and
fergusonite, and found that about half the total helium, together with
hydrogen, is given off at a bright red heat. He considered it likely
that the helium was combined with a metal (though he recognised no
distinction between occlusion and combination) and remarked: ‘The
results of such experiments cannot therefore serve as a basis for
speculation as to the origin or history of the substances in question.’
The chemical inactivity of helium, however, as well as the experiments
of Moss and Gray, who showed that helium was evolved on grinding the
materials,[144] indicate that the gas is mechanically bound only. This,
however, introduces the difficulty, if an attempt be made to use the
helium-uranium ratio to calculate the age of minerals, that the gas
would be expected to escape from a porous material, so that its amount
is never so great as it should be. Strutt himself found that helium
escapes rapidly from powdered monazite, whilst even the solid mineral
was found to evolve helium at a rate much in excess of the probable rate
of production by radioactive changes. Similar results were found with
thorianite, and the only conclusion, since helium is found in the
minerals, is that under the conditions under which these minerals exist
in the earth’s crust, this escape is checked or altogether prevented. It
follows, however, that any age determined from the helium ratio must be
a minimum age, since there is always the chance of loss; this of course
is not the case--except where the minerals have suffered chemical
changes--with the lead ratio, and may account for the discrepancies
observed.

  [143] _Proc. Roy. Soc._ 1898-99, ~64~, 140.

  [144] _Vide_ Gray, _Proc. Roy. Soc._ 1908, A, ~82~, 306.

Strutt’s earlier work on the helium ratio was made with phosphate
minerals (coprolites and fossil bones) of known ages. The ratios found
were not in order of age, the minerals being very permeable, so that
helium had probably been lost. He next turned his attention to igneous
rocks, and selected zircon for the work. Here he obtained some sort of
regularity in the order of age and the order given by the ratio, and
assumed that if helium were lost at all, it must be lost in roughly
proportional amounts by reason of the similarity in conditions.
Geological criticism tends to lessen the trustworthiness of the
conclusions; it is pointed out that the age of a specimen of zircon is
not necessarily that of the rock in which it occurs, for zircon is an
extremely stable mineral, and might survive unchanged several fusions
and re-crystallisations of the magma. Strutt replies to this that at the
temperature of fusion of a rock, zircon would certainly give up its
accumulated helium, so that the age determined from the helium content
would be that of the last fusion, _i.e._ the age as given by geological
data. On the other hand, our ignorance of the real mechanism of the
crystallisation of a magma, and especially of the amount and effect of
the pressures obtaining, robs this reply of its force, and the objection
must be counted valid.

In still later work Strutt used sphene and thorianite, and his results
agree as well as can be expected. The sphenes used were all from Archæan
rocks, except one, which was from a Tertiary volcanic deposit of the
Laacher See, near Coblenz (the lake is in the crater of an extinct
volcano). In this case the helium ratio was very much smaller (about
¹⁄₄₀₀₀ of the values for Archæan rocks) indicating the (comparatively)
extremely recent formation of the deposit.

The most recent results in the study of radioactivity point to the
conclusion that elements which differ in atomic weight and radioactive
properties may be chemically identical, or at least chemically
inseparable; such elements have been termed isotopes. The end product of
the thorium series of radio-elements should have an atomic weight of
about 208·4, and it has been suggested that the element actually
produced in this series of changes may be bismuth. The latest results,
however, rather point to the conclusion that disintegration in the
thorium series gives rise to an isotope of lead. If this hypothesis be
true, the lead derived from a mineral rich in thorium and poor in
uranium should have an atomic weight appreciably higher than that of
ordinary lead. Experiments to test this conclusion have recently been
carried out by Soddy and Hyman.[145]

  [145] _Proc. Chem. Soc._ 1914, ~30~, 134.

These authors have made analyses of Ceylon thorite, which they find to
contain 0·35 per cent. of lead; from the ratio of thorium to uranium in
the mineral, they calculate that the lead should have an atomic weight
of 208·2, that of ordinary lead being 207·1. Preliminary comparative
experiments on 1 gram of pure lead chloride extracted from the mineral
point to an atomic weight for the thorite lead of 208·4, a result
surprisingly in accord with theory. More extended experiments on this
most interesting question are in progress.

The present chapter would be incomplete without a reference to the
interesting work of Goldsmidt on radioactivity as an aid in identifying
mineral species.[146] He describes a simple method by which the activity
of a mineral may be rapidly and easily measured to a sufficient degree
of approximation, and shows how the determination enables a line to be
drawn on a diagram already mapped out; this line will intersect an area
on the diagram which corresponds to the particular mineral. Owing to
lack of analytical data, and to the great difficulty of determining with
accuracy small quantities of uranium and thorium, the method is at
present of scientific interest only; but it is capable of development,
and its development would be of undoubted value in the further study of
this branch of radioactivity.

  [146] _Zeitsch. Kryst. Min._ 1907-8, ~44~, 545; _ibid._ 1908, ~45~,
  490.

In order to make this part of the subject as clear as possible, the
chief points in this chapter are summarised as follows:

1. Radioactivity is only observed to an appreciable extent in some
rather rare minerals. These minerals as a rule contain radium, uranium,
thorium, rare earths, and helium.

2. The helium has been produced during geological time by the
degradation of one or more members of the three series of active
elements (the Uranium, Actinium, and Thorium series).

3. Radium is a degradation product of uranium, and itself is degraded
continuously; the final product of degradation is probably lead.

4. The age of minerals has been calculated from the ratio of lead to
uranium; the figures obtained are much greater than those put forward by
geologists and physicists.

5. The helium ratio has also been used, but appears less trustworthy,
owing to escape of helium, and uncertainty as to geological age of the
minerals employed.

6. Some connection between radioactivity and the presence of the yttrium
or cerium metals appears highly probable, but no satisfactory theories
have been advanced on this point; it has been shown that actinium is
very closely allied to lanthanum.




PART II

THE CHEMISTRY OF THE ELEMENTS


CHAPTER IX

GENERAL PROPERTIES OF THE CERIUM AND YTTRIUM GROUPS

The chemistry of the rare earth elements begins in the year 1794, with
Gadolin’s discovery of the new oxide ‘Ytterbia,’ for which the name
Yttria was subsequently proposed by Ekeberg, and generally adopted (see
Chapter I, and under Gadolinite, p. 35). The discovery of Ceria followed
in 1804 (see under Cerite, p. 32). The classical work of Mosander,
carried out between 1838 and 1842, showed the complex nature of the new
oxides. From ceria he separated three new earths, Ceria proper,
Lanthana, and Didymia. Yttria was shown to be a mixture of at least
three oxides, for which the names Yttria, Erbia, and Terbia were
proposed. These oxides were believed to have the general formula RO, by
analogy with the alkaline earths, which they were found to resemble in
many respects, notably in their strongly basic character.

The properties of the new oxides were examined during the next twenty
years by many chemists, the chief workers being Marignac, Rammelsberg,
and Hermann, but the next important advance was the investigation of the
absorption spectra of solutions of the rare earth salts, first suggested
by Gladstone in 1856, and developed more fully by Bunsen and Kirchhoff
in 1860 and the following few years. The introduction of the methods of
spectrum analysis furnished a very delicate and valuable method of
examining and identifying the various oxides, and so greatly assisted
the laborious processes of separation.

Sixteen elements (excluding thorium and zirconium) are at the present
time recognised as belonging to the rare earth group. With one or two
exceptions, these show the closest resemblance to one another, both in
chemical behaviour and in the properties of their compounds, so that the
difficulties of separating and purifying them are very great. They may
be said to form a series, in which the properties vary continuously but
gradually from member to member, so that no sharp differences are
anywhere perceptible. The method of division into groups is, therefore,
almost entirely one of convenience, and has arisen from the course which
the separations have followed.

The elements are divided into two chief families or groups, that of the
cerium metals and that of the yttrium metals respectively. The cerium
elements are separated by a process depending on the relative
insolubility of their alkali double sulphates; in this group are
included cerium, lanthanum, praseodymium, neodymium, and samarium. The
yttrium family is further divided into four sub-groups: the first
consists of scandium and yttrium; the second or terbium group of
europium, gadolinium, and terbium; the third or erbium group of
dysprosium, holmium, erbium, and thulium; and the fourth or ytterbium
group of ytterbium and lutecium--the element celtium, recently
discovered by Urbain, will also fall into this sub-group, but the
discovery awaits confirmation. Whilst scandium and yttrium fall into
somewhat abnormal positions, corresponding to their low atomic weights,
the terbium elements occupy an intermediate position between the cerium
elements and the remaining yttrium elements, or yttrium group proper,
and so are frequently classified as a third or intermediate group.

This list does not include all the names which have been put forward to
designate what have been claimed from time to time as new elements;
whilst the individuality of some of those included is not yet fully
established, and the homogeneity of others has been called in question.
The uncertainty is more pronounced among the yttrium elements than among
the cerium elements; owing to the opportunities for investigation
furnished by the commercial treatment of monazite, the chemistry of the
cerium group may be regarded as complete.

In the following table the elements are arranged in order of increasing
atomic weight, and it can be seen at once how closely the division into
groups follows this order:

              ELEMENT         ATOMIC WT.  COLOUR OF SALTS
            {Scandium, Sc        44·1     Colourless
            {Yttrium, Yt         89·0     Colourless

            {Lanthanum, La      139·0     Colourless
            {Cerium, Ce         140·25    Cerous, colourless; ceric,
  Cerium    {                             orange to red
  Group.    {Praseodymium, Pr   140·6     Green
            {Neodymium, Nd      144·3     Red to reddish-violet
            {Samarium, Sa       150·4     Topaz yellow

  Terbium   {Europium, Eu       152·0     Faint rose
  Group.    {Gadolinium, Gd     157·3     Colourless
            {Terbium, Tb        159·2     Colourless

            {Dysprosium, Dy     162·5     Bright green
  Erbium    {Holmium, Ho        163·5     Yellow to orange
  Group.    {Erbium, Er         167·7     Deep rose
            {Thulium, Tm        168·5     Bluish-green

  Ytterbium {Ytterbium, Yb      172·0     Colourless
  Group.    {Lutecium, Lu       174·0     Colourless

In their chemical relations, the rare earth elements may be placed
between the metals of the alkaline earths, and the trivalent metals
iron, aluminium, and chromium. With the exceptions of cerium in the
ceric salts, and of samarium and europium in the recently discovered
dichlorides, they are uniformly trivalent, but the oxides are very
strong bases, and the salts very slightly hydrolysed in dilute
solutions; generally, therefore, they resemble the calcium family rather
than the aluminium group. Among the common salts, the oxalates,
phosphates, chromates, iodates, fluorides, carbonates, tartrates, and
borates are almost insoluble; the sulphates are only sparingly soluble
at ordinary temperatures. Among the double salts, the alkali double
sulphates are of great importance from their employment for separations;
the tendency to the formation of complex salts is greater among the
yttrium than among the cerium elements, increasing with the atomic
weight, and with the decrease in basic strength of the oxides.

The great similarity in chemical behaviour of the rare earth elements is
apparent not only in the similarity in composition, solubility and
chemical properties of the salts--which is so great that the general
account of the compounds which follows applies almost in its entirety to
each member of the group--but also in the crystallographic relations
between corresponding compounds. Many of the salt hydrates form
isomorphous series; the sulphate octohydrates, for example, appear to be
isomorphous throughout the whole group, and probably the relation would
be found to apply even more completely than is generally accepted, if
the necessary data were forthcoming. Of great interest and practical
importance is the isomorphism between the nitrates and double nitrates
of the cerium elements and bismuth, which has been utilised with such
valuable results in the processes of fractional crystallisation.


~The Metals.~--The earlier attempts to reduce compounds of the rare
earth elements to the metallic condition, by means of metallic sodium or
potassium, did not yield pure products; nor did the use of aluminium or
magnesium lead to results of practical importance. The metals were first
obtained in a coherent physical condition by Hillebrand and Norton,[147]
by electrolysis of the fused chlorides. These investigators obtained
cerium, lanthanum, and the so-called didymium, and measured their
specific heats; their results confirmed the atomic weights assigned to
the elements by Mendelejeff, except in the case of lanthanum. Their
method has since been elaborated by Muthmann, Hofer and Weiss,[148] who
have prepared large quantities of the cerium elements in the pure
state. More recently, Hirsch has prepared metallic cerium in large
quantities,[149] and has studied its properties.

  [147] _Pogg. Ann._ 1875, ~155~, 631; ~156~, 466.

  [148] _Annalen_, 1902, ~320~, 231; see also Muthmann and Weiss,
  _ibid._ 1904, 331, 1.

  [149] _Met. Chem. Eng._ 1911, ~9~, 543.

By electrolytic reduction of the mixed chlorides of the cerium elements,
a mixture known as ‘Misch metal’ is obtained; this has powerful reducing
properties, and, like aluminium, reduces the oxides of iron, chromium,
etc., with great development of heat.[150] The yttrium metals have not
yet been obtained in the pure state, the electrolytic method giving
unsatisfactory results on account of the high melting-points of the
metals, and the volatile nature of their chlorides.

  [150] A full account of the properties and preparation of the cerium
  metals and their alloys will be found in the monograph of Kellermann,
  ‘_Die Ceritmetalle und ihre pyrophoren Legierungen_, Wilhelm Knapp,
  Halle, 1912.

The cerium metals are white or slightly yellowish in colour, and are
moderately stable in dry air. In moist air they tarnish slowly,
lanthanum, as the most positive, being most readily oxidised. The
melting-points and specific gravities are as follows:

  Element       Melting-point  Specific Gravity

  Cerium             623°           7·0242
  Lanthanum          810°           6·1545
  Praseodymium       940°           6·4754
  Neodymium          840°           6·9563
  Samarium       1300°-1400°       7·7-7·8

The metals decompose water slowly in the cold, but rapidly at the
boiling-point, with evolution of hydrogen. They have a great affinity
for oxygen, the heats of formation of the oxides being of the order of
those of alumina and magnesia:

                 Heat of Formation per Equivalent
                        Weight of Oxide[151]

  ¹⁄₃La₂O₃                    74·1 K
  ¹⁄₃Nd₂O₃                    72·5 „
  ¹⁄₃Pr₂O₃                    68·7 „
  ¹⁄₄CeO₂                     56·1 „
  ¹⁄₃Al₂O₃                    64·3 „
  ¹⁄₂MgO                      71·9 „

  [151] Muthmann and Weiss, _loc. cit._; K = 1 kilogram-calorie, or 1000
  cal.

In consequence of the high values of the heats of combustion, the metals
have powerful reducing properties.

The cerium metals form alloys with magnesium, zinc, aluminium, and iron,
and combine with boron and silicon. The alloys of cerium, and the metal
itself, are remarkable for their property of emitting brilliant sparks
when scratched (see Chapter XXI). Cerium also forms an amalgam with
mercury.

The metals burn brilliantly when heated in oxygen, and dissolve readily
in dilute mineral acids. When heated to a temperature of 200°-300° in a
current of hydrogen, they absorb the gas very readily, forming the
_hydrides_. These compounds are also obtained by heating the oxides with
magnesium in a current of hydrogen. They were first prepared by
Winkler,[152] who deduced from his analyses the general formula RH₂; the
more recent work of Muthmann and Beck,[153] however, points to the
formula RH₃.

  [152] _Ber._ 1890, ~23~, 2642; 1891, ~24~, 873.

  [153] _Annalen_, 1904, ~331~, 58.

If nitrogen be substituted for hydrogen in either of the above methods
of preparation, _nitrides_ of the general formula RN are obtained;
cerium nitride, however, cannot be obtained by heating the element in
the gas.[154] These compounds are also obtained when the carbides are
heated in ammonia. They are amorphous solids, which yield ammonia when
acted upon by water.

  [154] Dafert and Miklanz, _Monats._ 1912, ~33~, 911.


~Hydroxides.~--The hydroxides are thrown down as gelatinous precipitates
on the addition of alkalies to hot dilute solutions of the salts;
precipitation in the cold, or in strong solution, usually gives a basic
salt, or an hydroxide mixed with a large quantity of basic salt. The
hydroxides are insoluble in excess of precipitant, but the precipitation
is inhibited by the presence of some organic hydroxy-acids.[155]

  [155] For effect of tartaric acid, see p. 133.

The hydroxides are insoluble in water, but dissolve very readily in
acids. The most basic of them absorb carbon dioxide from the air;
lanthanum hydroxide is exceptional in that it colours litmus blue.

Whilst hydrogen peroxide in neutral solution does not react with rare
earth salts,[156] alkalies in presence of this reagent precipitate
gelatinous hydrated peroxides, which are very unstable, decomposing on
standing, or on treatment with acids, with evolution of oxygen. The
general formula R₄O₉ + _x_H₂O was proposed for these compounds by Cleve,
but more recently the formula R(OOH)(OH)₂ has been advanced.[157]

  [156] Compare behaviour of thorium and zirconium, Ch. XVI.

  [157] Melikoff and Pissarjewski, _Zeitsch. anorg. Chem._ 1899, ~21~,
  70; Melikoff and Klimento, _Chem. Zentr._ 1902, ~1~, 172.


~Oxides.~--In their most stable state of oxidation, the rare earth
elements are generally trivalent. In the case of cerium, the dioxide,
CeO₂, is more stable than the sesquioxide Ce₂O₃, but the ceric salts are
unstable, and are very readily reduced to cerous compounds,
corresponding to the oxide Ce₂O₃. Higher oxides are known with certainty
among the other elements only in the cases of praseodymium and terbium,
but these do not give rise to salts.

The oxides R₂O₃ are fairly strong bases, being comparable in strength to
the alkaline earths, and far more strongly basic than alumina and oxides
of other trivalent elements; thus they liberate ammonia from ammonium
compounds, whilst the salts they form with strong acids are not easily
hydrolysed. Their relative strengths as bases are expressed in the
following series, in which the elements are placed in order of
diminishing electropositive character:[158]

  La, Ce´´, Pr, Nd, Yt, Eu, Gd, Sa, Tb, Dy, Ho, Er, Tm, Yb, Lu, Sc,
  Ce^{iv}.

  [158] The position of yttrium in this series is not known with
  certainty; it is probably as positive as neodymium. It is usually
  stated (see Meyer and Hauser, pp. 32-33) that the terbia oxides are
  intermediate in basic strength between the ceria and yttria earths,
  though the arrangement into two series, consisting of the cerium and
  yttrium groups respectively, is generally adopted; the electropositive
  character of the elements in each series then weakens as the atomic
  weight rises, scandium being of course exceptional.

It will be seen that, with the exception of scandium and yttrium, the
metals of the cerium and yttrium groups become less electropositive as
the atomic weight increases.

This arrangement is obtained by ascertaining the order in which the
various hydroxides are precipitated from a solution by gradual addition
of a dilute solution of a strong base. The weakest base is precipitated
first, and the strongest last; those intermediate in strength are thrown
down in ascending order of strength. Similar results may be obtained by
the fractional decomposition of the nitrates by heat; in this case the
nitrate of the weakest base is decomposed at the lowest temperature.
This order is also confirmed, as far as the data are available, by
measurements of the equivalent conductivities of solutions of the salts
(see, for example, p. 122).

Quite recently, a very different order has been obtained from a
consideration of the dissociation tensions, and of the heats of
dissociation of the anhydrous sulphates.[159] In the following table the
elements are arranged in the order of the increase of the dissociation
tension (T) measured at 900°, which is the same as the order of decrease
of the heats of dissociation (Q):

  Element   At. Wt.   T. (Mm. Hg.)   Q.
    La       139·0         2        59·8
    Yt        89·0         3        58·9
    Lu       174·0         3·5      58·5
    Yb       172·0         4        58·2
    Er       167·7         5        57·6
    Pr       140·6         5·5      57·4
    Nd       144·3         6        57·2
    Gd       157·3         7        56·9
    Sa       150·4         8        56·5
    Sc        44·1        11        54·5
    Ce       140·25                 52·4

  [159] Wöhler and Grünzweig, _Ber._ 1913, ~46~, 1726.

It will be observed that the order is very different from the order of
increase of atomic weight, the positions of lutecium and ytterbium being
especially surprising; these elements are generally considered to be
among the least electropositive of the whole series. The anomalous
position of cerium is probably due to the fact that the sulphate on
decomposition leaves the dioxide, and not the sesquioxide, as with the
other elements; this would undoubtedly affect the values. The heats of
dissociation are the greatest yet observed for the sulphates of
trivalent metals, a further evidence of the strongly basic nature of the
oxides.

Ignited lanthana resembles quicklime in that it readily absorbs carbon
dioxide from the air, and hisses when slaked with water; as the basicity
becomes weaker, the affinity for water and carbon dioxide becomes less
marked. All the oxides are soluble in dilute acids, even after prolonged
ignition; but the ease with which solution occurs is naturally much
influenced by the treatment to which the oxide has been subjected, as
well as by its strength as a base.

The rare earth oxides are capable of existing in more than one
modification, the compounds obtained by ignition of the hydroxides
differing in appearance and reactivity from those prepared by ignition
of the oxalates or nitrates, and so on; they are probably highly
polymerised. Cerium dioxide, CeO₂, is remarkable for its power of
combining with the other oxides, R₂O₃, of the rare earth metals. The
pure dioxide is insoluble in nitric acid, but mixtures of earths
containing up to 50 per cent. of the dioxide dissolve readily. The
various colours of mixtures of the ceria earths may sometimes be
attributed to a similar combination,[160] and there can be little doubt
that the dioxide sometimes functions as an acid in the rare earth
minerals.

  [160] The brown colour of a mixture of ceria oxides containing
  praseodymium is generally attributed to the presence of the strongly
  coloured peroxide of that element.


~Sulphides.~--These compounds cannot be prepared in the wet way, that
is, by the action of hydrogen sulphide or ammonium sulphide on the salts
in solution; the former reagent gives no precipitate, the latter throws
down the hydroxides. In this behaviour, the rare earth elements resemble
aluminium and chromium.

The normal sulphides, R₂S₃, are obtained by reduction of the anhydrous
sulphates, or from the oxides at high temperatures, by treatment with
hydrogen sulphide. They are strongly coloured compounds, fairly stable
towards cold water, but readily hydrolysed on boiling.

Disulphides, RS₂, are known in the cases of cerium, lanthanum, and
praseodymium; these are to be regarded as polysulphides, since on
treatment with dilute acids they yield hydrogen persulphide, H₂S₂.


~Carbides.~--By reduction of the oxides with carbon in the electric
furnace, Moissan obtained the carbides in the form of microscopic yellow
crystals. They have the general formula RC₂, and are attacked by water
and dilute acids, with evolution of very complex mixtures of gases.[161]
The principal product is acetylene, with various higher homologues, and
in smaller quantities ethylene and ethane and their homologues. No
methane is formed,[162] but hydrogen is always present, the olefines and
paraffins probably arising from its action on the acetylenic
hydrocarbons. The relation of the rare earth elements to the calcium
group is here very close; calcium carbide when attacked by water yields
pure acetylene, whereas aluminium carbide gives pure methane.

  [161] Damiens, _Compt. rend._ 1913, ~157~, 214.

  [162] Moissan stated that 24-30 per cent. of methane was formed in
  this action; compare _Compt. rend._ 1900, ~131~, 595.


~Halogen Salts.~--The halides of the rare earth elements show a close
analogy with the corresponding compounds of the alkaline earth elements.
The _fluorides_ are insoluble in water and dilute mineral acids, and are
obtained as gelatinous precipitates by the addition of hydrofluoric
acid, or a soluble fluoride, to solutions of the salts. They may be
prepared in the crystalline condition by heating the carbides in a
stream of fluorine, or by the action of hydrofluoric acid upon the
hydroxides in aqueous suspension. The rare earth elements, as well as
thorium, may be separated from zirconium by taking advantage of the
insolubility of their fluorides in excess of hydrofluoric acid or alkali
fluorides, since zirconium fluoride is readily soluble in excess of the
precipitant. The solubility of the fluorides in a large excess of
concentrated acid increases with the electropositive character of the
metal, the fluorides of the more negative elements being the least
soluble. Thorium and scandium may, therefore, be concentrated to a large
extent by repeated precipitation with hydrofluoric acid in acid
solution.

The _silicofluorides_ of the rare earth elements have been used by R. J.
Meyer in the extraction of scandium from wolframite (see Chapter I and
under Scandium, p. 215). They are thrown down as gelatinous precipitates
on addition of potassium or sodium silicofluoride to boiling, neutral
solutions of rare earth salts. In presence of mineral acids, however,
they are not thrown down in the cold; on boiling, the cerium metals are
precipitated as fluorides, by hydrolysis of the silicofluorides--the
yttrium elements, with the exception of scandium, being held in solution
by the mineral acid.

With the exception of the fluorides, the halogen salts of the rare earth
metals are readily soluble in water, and crystallise from the
concentrated solutions in the hydrated form. The bromides and iodides
have not been so fully studied as the chlorides; they are hygroscopic
salts, and decompose rather easily. The iodides have been obtained by
Moissan in the anhydrous state, by the action of iodine vapour on the
carbides at high temperature.

The anhydrous _chlorides_ may be obtained by the application of any of
the ordinary methods, _e.g._ by heating the oxides with carbon in a
stream of chlorine, by heating the carbides in the same gas, by heating
the sulphides or hydrated chlorides in hydrogen chloride, or by
evaporating the solutions of the hydrated salts to dryness in presence
of ammonium chloride, and then igniting till the latter has all been
removed. As obtained by any of these methods, they are fusible at a red
heat, but only slightly volatile; they are easily soluble in water or
alcohol, with disengagement of heat. They are insoluble in most organic
solvents, but dissolve to some extent in some bases; the chlorides of
the yttrium elements, for example, are readily soluble in pyridine. With
such solvents, the chlorides form compounds which may be considered as
derived from the hydrated forms, by replacement of the so-called water
of crystallisation by the organic base.

Conductivity measurements show that the salts are not perceptibly
hydrolysed in moderately dilute aqueous solutions, though the values for
the equivalent conductivities vary somewhat with the variations in the
electropositive character of the elements. In the following table, the
equivalent conductivities of the chlorides in solutions of dilution 32
and 1024 at 25°C. are given. It will be seen that the value (λ₁₀₂₄ -
λ₃₂) ÷ 10 is in all cases (except for the highly hydrolysed scandium
salt) very close to 3, an experimental proof of the trivalent nature of
the elements. The values for the chlorides of iron, aluminium and
chromium are included; it will be seen that these elements are
considerably less positive than the rare earth metals (with the
exception, of course, of scandium).

  Salt      λ₃₂     λ₁₀₂₄  λ₁₀₂₄ - λ₃₂
  LaCl₃    105·8    131·5      25·7
  CeCl₃    107·8    135·2      27·6
  PrCl₃    105·5    135·9      30·4
  NdCl₃    103·8    134·3      30·5
  YtCl₃     98·8    123·4      24·6
  YbCl₃    107·4    140·4      33·0
  ScCl₃    116·9    257·9     141·0

  AlCl₃     99·9    138·0      38·1
  CrCl₃     98·4    152·6      54·2
  FeCl₃    117·2    200·7      83·5

From aqueous solutions the chlorides crystallise with six molecules of
water, except praseodymium chloride, which has seven. The hydrated
salts, when heated to 120° in the air, form insoluble oxychlorides of
the general formula ROCl.

The chlorides do not show a great tendency to form double salts with
other metallic chlorides; on the other hand, they readily form complex
compounds with the chlorides of the less electropositive metals, e.g.
tin, bismuth, gold, and platinum.

Subchlorides of samarium and europium have recently been obtained; in
these compounds, for the first time, rare earth metals have been shown
to be capable of functioning as divalent elements.

_Cyanides_ of the rare earth elements are not known; addition of
potassium cyanide to solutions of the salts throws down the hydroxides.
The _platinocyanides_ may be obtained by double decomposition of the
sulphates with barium platinocyanide. They are very stable and
characteristic bodies, of the general formula R₂[Pt(CN)₄]₃, with 18 or
21 molecules of water. The compounds of the cerium elements are yellow,
with a strong blue fluorescence; they crystallise in the monoclinic
system. The platinocyanides of the yttrium metals are red or crimson,
with a splendid green fluorescence, and crystallise in the rhombic
system. Scandium platinocyanide is of great interest from the fact that
it exists in two modifications, which show the characteristic appearance
of the two groups of compounds respectively.

Potassium ferrocyanide precipitates _potassium earth ferrocyanides_ of
the general formula KR(FeC₆N₆),3H₂O, from neutral solutions;[163] the
precipitate is somewhat soluble in excess. The ferrocyanides have been
proposed for the purification of yttrium; the method is useful where
rapid concentration of the element is required, yttrium ferrocyanide
being far more soluble than the analogous compounds of the erbium and
ytterbium metals, but the precipitates are gelatinous, and very
difficult to handle.

  [163] Compare Astrid Cleve, _Zeitsch. anorg. Chem._ 1902, ~32~, 129.


~Halogen Oxy-salts.~--_Perchlorates_ and _periodates_ of the rare earth
elements, of the general formula R(XO₄)₃,_x_H₂O, have been obtained. The
existence of _chlorates_ has been observed only in the yttrium group;
yttrium chlorate, Yt(ClO₃)₃,8H₂O, has been prepared by double
decomposition of the sulphate with barium chlorate. The _bromates_ are
also prepared in this way. They are readily soluble compounds, of which
several hydrated forms are known. They are of considerable importance
for purposes of separation in the yttrium group.

The _iodates_ are sparingly soluble bodies, precipitated by addition of
the alkali compound to solutions of the rare earth salts. The rare earth
iodates are soluble in nitric acid, the solubility increasing as the
electropositive character of the element becomes stronger. A method for
the purification of yttrium has recently been based upon this property
of the iodates, whilst the fact that thorium iodate is completely
insoluble in nitric acid allows of the easy separation and estimation of
thorium in minerals or mixtures containing rare earth elements.


~Sulphates.~--The sulphates of the rare earth elements are obtained by
dissolving the oxides or hydroxides in sulphuric acid. From the
solutions so obtained, various hydrated salts separate according to the
temperature of crystallisation. By heating the hydrated salts to a
temperature of 300°-400°, the anhydrous salts are prepared. These are
extremely soluble in water at 0°, having a great tendency, which is
indeed to be observed in the hydrated forms also, to form supersaturated
solutions. When the temperature of such a solution is allowed to rise,
larger or smaller quantities of an hydrated form separate out, the
differences of solubility among the sulphate hydrates of the various
elements being sometimes considerable.

The hydrated sulphates of the cerium elements have been very closely
studied in connection with the purification of thorium. Cerium sulphate
itself forms hydrates with 12, 9, 8, 5, and 4 molecules of water, but
sulphates of the other elements generally form fewer hydrates; the
commonest have 12, 8, or 4 molecules of water, and numerous cases of
isomorphism are known among them. The solubility curve of the cerium
sulphate hydrates is shown in the diagram. Fig. 3. The sulphates of the
yttrium elements have not yet been systematically investigated, and in
most cases only the octohydrates are known. Scandium sulphate is notably
different from the other sulphates, in that it is considerably more
soluble, and crystallises with six molecules of water.

[Illustration: FIG. 3.]

It is an important characteristic of the rare earth elements that the
solubility of the sulphates diminishes rapidly as the temperature
rises. The study of the various equilibrium conditions is greatly
complicated by the tendency to form supersaturated solutions, and the
fact that many hydrates can exist throughout considerable ranges of
temperature in the metastable condition; in consequence of this, also,
the solubilities of many hydrates are known for temperatures far beyond
the transition points. Foreign elements may be separated by taking
advantage of the very great solubility of the anhydrous sulphates at 0°,
and the rapid decrease in solubility with rise of temperature. For this
purpose, a solution of the anhydrous sulphates saturated at 0° is
prepared, and after filtration is slowly allowed to come to room
temperature; the hydrated rare earth sulphates then separate, leaving in
solution the foreign sulphates. This method may indeed be used instead
of the oxalate separation (see p. 147).

In presence of excess of sulphuric acid, _acid sulphates_ of the general
formula R(HSO₄)₃ are formed. These are fairly stable, and must be heated
to a temperature of 400°-500° to decompose them completely to the normal
salts; even at that temperature, traces of acid are tenaciously
retained, a fact which renders the determination of the equivalents by
the sulphate method unreliable, unless special precautions are taken. On
further heating, the normal sulphates pass into _basic salts_, R₂O₃,SO₃,
and finally, at the temperature of the blowpipe flame, into the oxides.
The temperatures at which these decompositions occur vary with the
positive character of the elements; the most basic oxide clings most
tenaciously to sulphuric anhydride, and forms the most stable acid salt.
Lanthanum sulphate, for example, requires to be heated for a
considerable time at a white heat if the pure oxide is required, whilst
the sulphates of the less positive elements are easily decomposed at a
red heat. The order of basic strength of the oxides, as determined by
the ease with which the sulphates are decomposed, seems, however, to be
very different from the order determined by decomposition of the
nitrates (see p. 118).

With the alkali sulphates, the sulphates of the rare earth elements
readily form _double salts_, which are of great importance in
separation, on account of the great differences in solubility. The
double sulphates of the cerium group are almost insoluble in excess of
alkali sulphate, whereas the yttrium double sulphates, with the
exception of those of the terbium metals, which occupy an intermediate
position, are very easily soluble. This method of separating the
elements into the two main groups was first employed by Berzelius, and
though a century has elapsed, it remains to-day the most efficient
method of effecting the separation.

The _ethylsulphates_ have been employed by Urbain and others in
effecting separations, especially in the erbium and terbium groups. The
solubilities of these salts are in the same general order as those of
the alkali double sulphates, and they are especially convenient for
separating the metals into the three groups of the cerium, terbium, and
yttrium elements respectively. They may be prepared by double
decomposition of the rare earth sulphates with barium ethylsulphate, but
on account of the ease with which the alkylsulphates are hydrolysed by
acids, it is essential that the solutions should be quite neutral. A
more convenient method, according to James, is the treatment of the
anhydrous chlorides in alcohol solution with sodium ethylsulphate
dissolved in the same medium; sodium chloride is precipitated, whilst
the ethylsulphates of the rare earth elements remain in solution.

The _sulphites_ of the rare earth elements are sparingly soluble
crystalline salts, of the general formula R₂(SO₃)₃,_x_H₂O. They are
obtained by passing sulphur dioxide into a suspension of the hydroxides
in water, or by double decomposition of soluble salts with alkali
sulphite. They dissolve in excess of sulphurous acid, and on evaporation
of the solution are deposited unchanged. They are distinguished from
thorium sulphite by the fact that they form no alkali double salts. The
strongly electropositive character of the rare earth metals is shown by
the fact that they form normal and not basic sulphites.

The _thiosulphates_ are readily soluble, crystalline bodies. With the
exception of the ceric and scandium salts, they are not hydrolysed in
boiling solution, a fact which allows of a complete separation from the
readily hydrolysed thiosulphates of zirconium and thorium.

_Dithionates_ of the commoner rare earth elements, of the general
formula R₂(S₂O₆)₃,_x_H₂O, have been prepared by double decomposition of
the sulphates with barium dithionate. They are readily soluble,
crystalline salts.

The _selenates_ are soluble, crystalline salts, which separate from
aqueous solutions in various hydrated forms. They resemble the sulphates
in being less soluble in hot than in cold water, and numerous cases of
isomorphism have been observed among the corresponding sulphate and
selenate hydrates. Several alkali double selenates have been described;
they show a close resemblance to the analogous double sulphates.

The _selenites_ are amorphous, insoluble compounds, obtained by the
action of selenious acid on the carbonates, or on solutions of neutral
salts. Basic and acid selenites are also known.


~Nitrates.~--The nitrates are crystalline, deliquescent compounds,
readily soluble in water and alcohol, but less easily in nitric acid, a
fact which has been of considerable importance for purposes of
separation. The solubility is greatest in the case of lanthanum nitrate,
diminishing through the cerium group to a minimum in gadolinium nitrate,
and then increasing again. They separate from aqueous solution in the
form of crystalline hydrates; in the cerium group, these have commonly
the formula R(NO₃)₃,6H₂O, whilst the nitrates of the yttrium elements
usually crystallise with 3 or 5 molecules of water. By carefully heating
the hydrated salts, basic nitrates may be obtained, which in the yttrium
group are soluble in water, and may be obtained crystalline; in the
cerium group, the basic nitrates are insoluble. By further heating,
insoluble ‘superbasic salts,’ and finally the oxides, are obtained in
all cases. The temperatures at which these basic and superbasic
compounds are formed vary with the electropositive character of the
element; this fact affords a method of separation which has been very
frequently employed.

An interesting series of addition compounds of the rare earth nitrates
with antipyrine (dimethylphenylpyrazolone, C₁₁H₁₂ON₂) has been described
recently by Kolbe.[164] Those of the cerium metals have the general
formula R(NO₃)₃,3C₁₁H₁₂ON₂; the yttrium nitrates appear to combine with
four molecules of the base.

  [164] _Zeitsch. anorg. Chem._ 1913, ~83~, 143

The tendency to form double nitrates with nitrates of the metals of
Group IA and Group IIA also varies with the basic strength of the
hydroxides. In the most positive elements of the cerium group, the
tendency is very pronounced, and there are a large number of stable,
crystalline double salts; but the stability decreases rapidly as the
atomic weight of the element rises, and in the terbium and yttrium
groups crystallised double nitrates cannot be obtained. The solubility
of these double salts increases rapidly in the same direction, the
lanthanum double nitrates being the least soluble. For this reason,
these compounds are of great importance for the purpose of separation,
especially in the cerium group. Bismuth nitrate and the various bismuth
double nitrates are isomorphous with the corresponding compounds of the
cerium group, and the double bismuth ammonium and bismuth magnesium
salts have been largely used by Urbain in the separation of samarium and
the elements of the terbium group.


~Phosphates.~--Addition of phosphoric acid, or an alkali phosphate to
solutions of rare earth salts throws down the phosphates as gelatinous
precipitates, which slowly become crystalline on standing. The
precipitate is soluble in excess of phosphoric acid, and in other
mineral acids, a fact of great importance in the commercial treatment of
monazite. The composition of the precipitate is not known with
certainty; both neutral and acid phosphates can probably be obtained
according to the conditions. Double salts with the alkali phosphates can
be prepared by fusion methods. The naturally occurring phosphates,
monazite and xenotime, are mixtures of the orthophosphates of the cerium
and yttrium elements respectively.

_Phosphites_ are known in a few cases only; _arsenates_ and _arsenites_
of lanthanum have been prepared. _Vanadates_ of some of the rare earth
elements have been described.


~Chromates.~--The rare earth chromates are, as a rule, sparingly soluble
in water, and show considerable differences of solubility amongst
themselves; for this reason, they have been of some use in the
separation of the cerium elements.[165] They are obtained by addition of
potassium chromate to neutral solutions of rare earth salts as
crystalline precipitates, of the general formula R₂(CrO₄)₃,8H₂O; with a
large excess of alkali chromate, double chromates are obtained, which
are more readily formed, and more soluble, in the yttrium series than in
the cerium group. Addition of chromic acid or alkali bichromate to
solutions of the soluble salts gives no precipitate, a fact which allows
of the separation of zirconium and thorium, and of cerium in the
tetravalent state, since the tetravalent elements are precipitated by
both these reagents.

  [165] Muthmann and Böhm, _Ber._ 1900, ~33~, 42; Böhm, _Zeitsch. angew.
  Chem._ 1904, ~15~, 372 and 1282.

Ammonium molybdate throws down from neutral solution of rare earth salts
gelatinous precipitates of the _molybdates_; the formula La₂2(HMoO₄)₆ is
assigned to the lanthanum compound obtained in this way. No
precipitation occurs if the solution be strongly acid; on this fact a
process has recently been based for the volumetric estimation of
thorium, in presence of rare earth salts, by means of ammonium molybdate
(see p. 289).

Various _silicotungstates_ and _double tungstates_ have been described.


~Carbonates.~--The more pronounced electropositive character of the rare
earth elements, as contrasted with other trivalent metals, is well
illustrated by the fact that they form stable neutral carbonates of the
formula R₂(CO₃)₃,_x_H₂O. These may be obtained by passing a current of
carbon dioxide through an aqueous suspension of the hydroxides, or by
addition of an alkali carbonate to neutral solutions of the salts. Basic
carbonates are known in the case of the less positive yttrium elements
only; both these and the neutral carbonates are insoluble in water.

In presence of a large excess of alkali carbonate, double carbonates are
formed. The stability as well as the solubility of these compounds
increases in passing from the cerium to the yttrium group, _i.e._ as
the electropositive character becomes weaker. The double carbonates of
the cerium elements are sparingly soluble, and are decomposed by water,
especially on warming; they may, however, be recrystallised from alkali
carbonate solution. The sodium and ammonium double salts are less
soluble than the potassium compounds. The latter have the general
formula R₂(CO₃)₃,K₂CO₃,12H₂O, and are of considerable importance in many
processes of separation. The yttrium elements can be separated from the
cerium metals, and the latter from one another, by taking advantage of
the differences of solubility shown by the potassium double carbonates.
If a concentrated solution of the salts in potassium carbonate solution
be fractionally diluted with water, the cerium elements separate in the
order: lanthanum, praseodymium, cerium, neodymium, and samarium; the
more soluble yttrium compounds remain in the solution. Thorium forms
double alkali carbonates which are very readily soluble in excess of
alkali carbonate; this property is of great importance for the technical
separation of the element.


~Oxalates.~--The oxalates of the rare earth elements are of the greatest
importance, on account of the fact that they are not only insoluble in
water, but are also very sparingly soluble in dilute mineral acids, and
in excess of oxalic acid. They can be completely precipitated even from
strongly acid solutions by addition of sufficient excess of oxalic acid,
or alkali oxalate, and thus afford a means of easily and completely
separating the rare earth group from the commoner elements.

They are thrown down by addition of oxalic acid, or alkali oxalate, as
amorphous precipitates, which rapidly become crystalline, especially if
the solution is warmed. From water at normal temperatures they usually
separate as the decahydrates, R₂(C₂O₄)₃,10H₂O, but hydrates with 7, 9,
and 11 molecules of water of crystallisation are also known. From
strongly acid solutions, mixed oxalo-salts of the general formula
R(C₂O₄)X, where X = Cl, NO₃, HSO₄, etc., may be obtained. These mixed
salts may also be prepared by dissolving the oxalates in concentrated
solutions of the chlorides, nitrates, etc., whilst nitro-sulphates,
R(SO₄)NO₃, have been obtained by recrystallising the sulphates from
strong nitric acid. The tendency to form salts with mixed acid radicles
appears to be general.[166]

  [166] See Meyer and Marckwald, _Ber._ 1900, ~33~, 1003; also Matignon,
  _Ann. Chim. Phys._ 1906, [viii.], ~8~, 243.

The solubilities of the oxalates in mineral acids of various
concentrations have been examined by Hauser and Wirth.[167] Whilst the
solubilities in water are exceedingly slight, and increase with
increasing atomic weight of the elements, _i.e._ from the cerium to the
yttrium group, in mineral acids of concentration 3-4N the solubility
becomes noticeable, and is greatest for the oxalates of the most
positive elements. The solubility is greatly lessened, however, if
considerable excess of oxalic acid be present.

  [167] _Zeitsch. anal. Chem._ 1908, ~47~, 389.

Double oxalates with the alkali oxalates can be obtained with the salts
of the yttrium elements only, the oxalates of the cerium elements being
almost insoluble in excess of alkali oxalate in the cold. Of the alkali
double oxalates, the potassium compounds are the most soluble, but the
ammonium compounds show the greatest differences in solubility; von
Welsbach has employed the method of fractional crystallisation of these
salts from a saturated solution of ammonium oxalate for separations in
the yttrium group. The sodium double oxalates are the least soluble of
these double salts.

Since the rare earth elements are almost always separated in the form of
the oxalates, the methods for transforming these into soluble compounds
become important. They may be ignited to oxides, and these dissolved in
nitric acid; if the content of ceria is very high, the oxide mixture may
become insoluble, but this difficulty may be overcome by addition of a
reducing agent--hydrogen peroxide is very convenient for this purpose.
The oxalates may also be dissolved directly in fuming nitric acid, care
being taken to avoid loss; if the mixture contains cerium, the oxidation
is hastened, ceric salts having the property of acting as oxygen
carriers. By boiling for a short time with potash, the oxalates may be
easily transformed into the hydroxides, which can be dissolved in dilute
acids.


~Formates.~--On account of the considerable differences in solubility by
which they are characterised, these salts have been employed for
separations. The formates of the cerium group are considerably less
soluble than those of the yttrium group. They may be partly precipitated
from solutions of rare earth salts by addition of alkali formate--formic
acid itself causes precipitation only with salts of weak acids, _e.g._
the acetates--but are best prepared by dissolving the oxides in formic
acid; on concentration of the solution, the formates of the cerium and
terbium elements successively separate, the salts of the yttrium group
remaining in solution. The separation of the terbium earths by this
method was attempted by Delafontaine; his ‘new’ element, Philippium,
obtained from the mother-liquors, was in reality a mixture of the
terbium and yttrium elements, which cannot be completely separated by
the formate method.[168]

  [168] See Urbain, _Ann. Chim. Phys._ 1900, [vii.], ~19~, 184.

The _acetates_ are readily soluble in water, the yttrium salts being
rather less easily soluble than those of the cerium group. They are
therefore obtained by dissolving the oxides in acetic acid; addition of
alkali acetate to a solution of a rare earth salt gives no precipitate,
even on boiling, behaviour which is in marked contrast to the ease with
which the salts of other trivalent metals are hydrolysed under these
conditions. In this respect the rare earth elements differ also from the
tetravalent elements zirconium and thorium (and from cerium in the
tetravalent state); soluble salts of the latter, on boiling with sodium
acetate, give insoluble basic acetates. Even sparingly soluble compounds
of the rare earth elements are as a rule taken into solution by
digestion with ammonium acetate.

_Tartrates._--Addition of ammonium tartrate to a neutral solution of
rare earth salts throws down an amorphous precipitate, which dissolves
easily in acids, and in excess of the precipitant. In the presence of
tartaric acid, precipitation of the earths by addition of sodium
hydroxide is completely inhibited. Potassium hydroxide under these
conditions gives a precipitate in the case of the yttrium elements,
though only on boiling; ammonia gives a crystalline precipitate even in
the cold with this group. These precipitates are alkali double tartrates
of the yttrium metals; the cerium elements give no precipitate at all.
In all cases, therefore, the precipitation of the hydroxides is
inhibited by the presence of tartaric acid.

A very large number of organic salts of the rare earth elements has been
prepared and examined during the past two decades, in the endeavour to
find some class of compounds which will allow of an easy separation of
the group. The _benzoates_, _succinates_, _hippurates_, _citrates_ and
similar relatively simple salts first received attention, but less
common acids, as _e.g._ the hydroxynaphthalenesulphonic acids, have also
been employed.[169] The use of various organic acids for the separation
and estimation of thorium in presence of the rare earths is outlined in
that connection (see p. 288). More recently, the glycollates and
cacodylates have been prepared. The _glycollates_[170] of the cerium
elements have the general formula R(C₂H₃O₃)₃, and crystallise in crusts;
they are more soluble than the yttrium compounds, which have the formula
R(C₂H₃O₃)₃,2H₂O, and crystallise in needles. The _cacodylates_,[171]
R₂[As(CH₃)₂O₂]₆, crystallise with 16 or 18 molecules of water, and have
similar solubility relations.

  [169] Erdmann and Wirth, _Annalen_, 1908, ~361~, 190; see also Pratt
  and James, _J. Amer. Chem. Soc._ 1911, ~33~, 1330; Baskerville and
  Turrentine, _ibid._, 1904, ~26~, 46; James, Hoben and Robinson,
  _ibid._, 1912, ~34~, 276, etc.

  [170] Jantsch and Grünkraut, _Zeitsch. anorg. Chem._ 1913, ~79~, 305.

  [171] Whittlemore and James, J. _Amer. Chem. Soc._ 1913, ~35~, 627.

The _phthalates_ of the yttrium group have been found to be very
valuable for purposes of separation by Meyer and Wuorinen.[172] The
salts are readily obtained in solution by shaking together cold aqueous
suspensions of the rare earth hydroxides, and phthalic acid; the clear
solutions when warmed become cloudy, the organic salts hydrolysing very
easily, with separation of the hydroxides. The most positive elements
naturally remain longest in the solution, the weakly basic oxides
accumulating in the first precipitates.

  [172] _Zeitsch. anorg. Chem._ 1913, ~80~, 7.

An organic compound which has proved very useful in the treatment of
the rare earths is acetylacetone, CH₃.CO.CH₂.CO.CH₃.[173] In its enolic
form, this substance forms salts with metals, which in the case of the
rare earth elements are especially characterised by the ease with which
they may be obtained, and their high crystallising power. They may be
prepared by double decomposition of neutral solutions of rare earth
salts with ammonium acetylacetone, and crystallise readily from dilute
alcohol. They have been used by Urbain in the fractionation of the
yttrium group, and for determination of molecular weights by the boiling
point method; Biltz[174] has shown that in solution they generally have
the double formula R₂(C₅H₇O₂)₆.

  [173] Urbain, _Bull. Soc. chim._ 1897, [iii.], ~17~, 98; Urbain and
  Budischofsky, _Compt. rend._ 1897, ~124~, 618; Biltz and Clinch,
  _Zeitsch. anorg. Chem._ 1904, ~40~, 218.

  [174] _Annalen_, 1904, ~331~, 334.


THE RARE EARTH ELEMENTS, AND THE PERIODIC CLASSIFICATION

At the time of the introduction of the periodic classification the rare
earth elements were generally believed to be divalent. This belief,
which has persisted until quite recently,[175] was based chiefly on the
electropositive character of the metals, and their general chemical
resemblance to the elements of the alkaline earths; the isomorphism of
the tungstates of calcium and the cerium elements, and of the molybdates
of lead and the cerium elements, also supports this view. The physical
evidence in favour of Mendelejeff’s view, however, is quite
overwhelming; the specific heats of the metals, the equivalent
conductivities of the chlorides, and molecular weight determinations by
means of vapour densities and the boiling point method, prove beyond
doubt that the elements are in fact trivalent.

  [175] See Wyrouboff, _Bull. Soc. franc. Min._ 1896, ~19~, 219;
  Wyrouboff and Verneuil, _Compt. rend._ 1897, ~124~, 1230 and 1300;
  _ibid._, 1899, ~128~, 1573; etc.

In deciding in favour of the trivalent nature of the rare earth metals,
Mendelejeff was influenced chiefly by the fact that there was no room in
the table for divalent elements with the equivalent weights then
assigned to the cerium and yttrium elements. At that time, only the six
oxides obtained by Mosander were known; of these the accepted
equivalents and atomic weights were as follows:

   Element.  Equivalent.  Atomic Weight.
  Lanthanum      46             92
  Cerium         46             92
  Didymium       48             96
  Yttrium        31             62
  Erbium         56            112

the values for terbium being uncertain. If cerium be considered
trivalent in the cerous salts, its atomic weight becomes 138, that of
barium being 136. Mendelejeff placed cerium in Group IV, series 8, in
the position which it still occupies; he pointed out that the accepted
equivalent must be too low, and suggested that the atomic weight should
be at least 140, almost exactly the value accepted to-day.

This choice left the positions in Group III, series 8, horizontally
before cerium, and in Group IV, series 10, vertically below it (see
figure), to be filled by the two elements, lanthanum and didymium. No
chemical evidence being available to decide the choice, he provisionally
assigned didymium to the first (Group III, series 8), and lanthanum to
the second (Group IV, series 10) position, at the same time expressing
the opinion that didymium was probably a mixture of closely related
elements. Yttrium then fell into place in Group III, series 6, above
didymium, and erbium in Group III, series 10, below it. To the vacant
space above yttrium in Group III, series 4, he assigned the hypothetical
element Eka-boron, with atomic weight 44; this space is now occupied by
scandium, which corresponds almost exactly in properties to the metal
described by the Russian chemist. A part of the table illustrating these
positions is shown in Fig. 4.

The determination of the specific heats of the metals by Hillebrand and
Norton in 1875, whilst confirming the trivalency of the elements,
rendered it necessary to alter the position of lanthanum, which was
placed in Group III, series 8, instead of didymium, which was thus left
without a place. This first indication that all the rare earth elements
could not be fitted into the table without difficulties was soon
followed by the discovery of several other members of the group, for
which places could not easily be found.

[Illustration:

  +---------+--------+--------+--------+--------+--------+--------+
  |  Group  |    0   |    I   |   II   |   III  |   IV   |    V   |
  |         |        |A      B|A      B|A      B|A      B|A       |
  +---------+--------+--------+--------+--------+--------+--------+
  |Series  1|        |    H   |        |        |        |        |
  +---------+--------+--------+--------+--------+--------+--------+
  |  „     2|        |   Li   |   Be   |    B   |    C   |        |
  +---------+--------+--------+--------+--------+--------+--------+
  |  „     3|        |   Na   |   Mg   |   Al   |   Si   |        |
  +---------+--------+--------+--------+--------+--------+--------+
  |  „     4|        |K       |Ca      |_Eka-   |Ti      |        |
  |         |        |        |        |boron_  |        |        |
  |  „     5|        |      Cu|      Zn|        |        |        |
  +---------+--------+--------+--------+--------+--------+--------+
  |  „     6|        |        |Sr      |~Yt~    |Zr      |        |
  |         |        |        |        |        |        |        |
  |  „     7|        |      Ag|      Cd|        |      Sn|        |
  +---------+--------+--------+--------+--------+--------+--------+
  |  „     8|        |        |Ba      |~Di?~   |~Ce~    |        |
  |         |        |        |        |        |        |        |
  |  „     9|        |        |        |        |        |        |
  +---------+--------+--------+--------+--------+--------+--------+
  |  „    10|        |        |        |~Er~    |~La?~   |        |
  |         |        |        |        |        |        |        |
  |  „    11|        |      Au|      Hg|        |      Pb|        |
  +---------+--------+--------+--------+--------+--------+--------+
  |  „    12|        |        |        |        |Th      |        |
  +---------+--------+--------+--------+--------+--------+--------+

FIG. 4.--PART OF THE PERIODIC TABLE, SHOWING THE POSITIONS ORIGINALLY
ASSIGNED TO THE RARE EARTH ELEMENTS BY MENDELEJEFF]

It was first pointed out by Brauner in 1881 that, with the exception of
scandium (44·1) and yttrium (89·0), the rare earth elements form a zone
of increasing atomic weight between barium (137·37) and tantalum
(181·5). In 1902 he proposed[176] to consider the rare earth metals as a
kind of zone or belt among the elements, comparable to the asteroids in
the solar system, extending from cerium in Group IV to tantalum in Group
V in a continuous series. The suggestion seems at first sight contrary
to the whole principle of periodic classification, but it accords very
well with the anomalous position of the rare earth group among the other
elements; it is very well illustrated in the accompanying Fig. 5, which
shows an helical or space representation of the table.

  [176] _Zeitsch. anorg. Chem._ 1902, ~32~, 1.

[Illustration: FIG. 5.--HELICAL REPRESENTATION OF THE PERIODIC LAW

Electropositive Elements, above plane of paper, black letters on white
ground. Electronegative Elements, below plane of paper, white letters on
black ground. Intermediate Elements, in plane of paper, black letters on
sectioned ground.]

[Illustration:

  +---------+--------+--------+--------+--------+--------+--------+
  |  Group  |  III.  |   IV.  |   V.   |   VI.  |  VII.  | VIII.  |
  |         |A      B|A      B|A      B|A      B|A      B|        |
  +---------+--------+--------+--------+--------+--------+--------+
  |Series  6|~Yt~    |~Zr~    |Cb      |Mo      |        |Ru Rh Pd|
  |         |        |        |        |        |        |        |
  |  „     7|      In|      Sn|      Sb|      Te|       I|        |
  +---------+--------+--------+--------+--------+--------+--------+
  |  „     8|~La~    |~Ce~    |~Pr~    |~Nd~    |~Sa~    |~Eu~    |
  |         |        |        |        |        |        |        |
  |  „     9|    ~Gd~|    ~Tb~|    ~Dy~|    ~Ho~|    ~Er~|~Tm~~Yb~|
  +---------+--------+--------+--------+--------+--------+--------+
  |  „    10|~Lu~    |        |Ta      |W       |        |Os Ir Pt|
  |         |        |        |        |        |        |        |
  |  „    11|        |        |        |        |        |        |
  +---------+--------+--------+--------+--------+--------+--------+

FIG. 6.--PART OF THE PERIODIC TABLE, SHOWING THE POSITIONS ASSIGNED TO
THE RARE EARTH ELEMENTS BY BRAUNER IN 1908]

Brauner’s conception is also in accord with the physical properties of
the elements and their compounds. These vary continuously throughout the
group, and show nowhere the sudden transitions which are characteristic
of other series in the table. Benedicts[177] has collected all the data
bearing on the atomic volumes, and finds that those also vary
continuously, with rise in the atomic weights, within quite small
limits, all lying between the values for barium and tantalum. In face of
all the evidence furnished by physical and chemical properties, however,
Brauner[178] has recently reverted to an idea which he put forward in
1881, according to which lanthanum and cerium are placed as usual in
Groups III and IV, series 8, whilst the other elements are distributed
in order throughout the remaining groups, as shown in Fig. 6.

  [177] _Zeitsch. anorg. Chem._ 1904, ~39~, 41.

  [178] _Monats._ 1881, ~3~, 1; _Zeitsch. Elektrochem._ 1908, ~14~, 525.

In support of this arrangement, he quotes the fact that some of the
elements appear to be able to form higher oxides in the presence of
other oxides, which act as oxygen carriers (see pp. 174, 177-8), though
these higher oxides are certainly not salt-forming. He also deduces,
from the rates of hydrolysis of the sulphates, that the elements fall
into two parallel series, according to the strengths of the hydroxides
as bases, on which ground he justifies the distribution throughout
series 8 and 9. There can be no doubt, however, that this disposition is
far less in accordance with the behaviour and properties of the rare
earth elements than is the first arrangement, which places them in a
transition zone between barium and tantalum; it is impossible, for
example, to reconcile the properties of praseodymium with those of
columbium and tantalum, or to find the slightest analogy between
neodymium and molybdenum or tungsten, as the second arrangement
requires.

The analogy of the rare earth group to the elements of Group VIII has
been pointed out by many authors.[179] On the ground that the rare earth
elements cannot be spread over the table in series 8-10, Steele[180]
favours the early classification of Thomsen, according to which the
elements are divided into three groups. The first, corresponding to
Groups I and II of Mendelejeff’s table, consists of two sub-groups, each
containing seven elements[181]; the second, corresponding to the first
two long series of the periodic table, has two sub-groups, each of
seventeen elements, of which the first and last seven are
analogous--these elements fall into the same groups in the periodic
table--whilst the middle three are interperiodic. These interperiodic
elements are those which Mendelejeff places in Group VIII. The third
division consists of one (or two) group(s) of thirty-one elements; here
again, the first and last seven are analogous, whilst the interperiodic
elements, which are seventeen in number, include the rare earth metals.

  [179] Compare Biltz, _Ber._ 1902, ~35~, 562.

  [180] _Chem. News_, 1901, ~84~, 345.

  [181] The inert gases are not included.

Steele’s idea has been extended by Werner,[182] who has drawn up a table
to illustrate it. In this classification, the elements are arranged in
order of atomic weight, but arbitrary gaps are left in such a way that
similar elements may fall into the same vertical columns, as in the
periodic table. The arrangement has the advantage that the interperiodic
elements, consisting of the rare earth elements and the elements placed
in Group VIII of the periodic table, here do fall in the middle of their
respective periods, but it has several drawbacks, and does not represent
the transition of properties from element to element so well as the
helical representation of the periodic table, which brings out most
clearly the true relations between the elements, and the anomalous
position of the rare earth metals.

  [182] _Ber._ 1905, ~38~, 914.

Mention must be made at this point of the theory of ‘Meta-elements’ put
forward in 1888 by Sir William Crookes.[183] From his work on the
cathode luminescence of some of the oxides (see next chapter), that
author was led to the conclusion that several of the then-accepted rare
earth elements, notably samarium and yttrium, were in reality
heterogeneous, consisting of large numbers of very closely related
bodies, differing so very slightly in properties that only the most
refined methods could perceive the variations; for these he proposed the
name Meta-elements. Though it has been proved that the differences
observed by Crookes in the luminescence spectra were really due to the
presence of very small quantities of impurities, his paper is of great
interest, in that it contains a theory of evolution of the elements, and
postulates the possibility of their decay. Modern developments in
radioactivity have not only lent a curious force to these speculations,
but even support his contention that a chemical element, in the ordinary
sense of the word, is not necessarily homogeneous.[184] In the field of
the rare earths, also, the homogeneity of elements is even now
continually being called into question (see Thulium, p. 204). In any
case, we have in the rare earth elements a series of bodies in which the
change of properties from one member to another--and the consequent
possibility of easy separation--is so very slight, and so far without
parallel in the whole field of chemistry, that we are at least justified
in asking whether some extension of our ordinary conception of an
element is not required.

  [183] _Trans. Chem. Soc._ 1888, ~53~, 487.

  [184] See Soddy, _The Chemistry of the Radio-Elements_, Part II.,
  Introduction.


CHAPTER X

GENERAL METHODS OF SEPARATION

The chemist who sets out to prepare a pure compound of a rare earth
element is faced by a great difficulty. The rare earth compounds occur
in nature, as one might expect from their great similarity, as mixtures
of very complex composition. After the relatively simple separation from
foreign elements has been accomplished, the enormously greater
difficulty of separating the elements from one another has to be
encountered. So great is this difficulty, by reason of the fact that,
with the sole exception of cerium, the elements show no variation in
property sufficient to allow of the use of ordinary analytical methods,
that even at the present day it is extremely doubtful if all the
elements in the yttrium group are known to us.

The methods which can be adopted in attempting a separation are of two
kinds. The first includes those processes which take advantage of the
gradual variation in basic strength of the hydroxides as the atomic
weight changes; the most important of these are fractional precipitation
of the hydroxides, and fractional decomposition of the nitrates.
Fractional precipitation of the hydroxides is generally effected by
gradual addition of ammonia, soda, magnesia, or other base, to a
solution of the mixed salts; such a solution may also be digested with
the oxides obtained by ignition of another fraction of the rare earth
compounds. If the digestion be sufficiently complete, the precipitate in
each case will be richer in the less basic hydroxides, whilst the
solution will be richer in the salts of the more electropositive
elements.

The fractional decomposition of the nitrates is based on the fact that
when a mixture of the salts is heated gradually, the nitrate of the
least positive element begins to decompose first. The temperature is
maintained for some time at the point at which decomposition begins;
when nitrous fumes cease to be evolved the mixture is cooled, and
extracted with water or dilute acids. The insoluble portion--basic or
superbasic nitrate (see p. 128)--will then be richer in the less
electropositive elements; the solution is evaporated, and the solid so
obtained subjected to a somewhat higher temperature, and the process
repeated several times. In this way, a series of fractions is obtained,
in which the elements tend to distribute themselves in order of
electropositive character. By a sufficient number of systematic
repetitions of such steps, the elements may eventually be obtained in
the form of compounds of approximate purity, which may then be refined
by one of the methods of the second kind described below. Experience has
shown, however, that a quicker and more complete separation may
generally be effected by combining two or more methods of separation;
one method will give the best separation up to certain limits, but then
becomes much less valuable; the separation at this point is therefore
taken up by another process. A process depending on differences of basic
strength of the hydroxides is generally supplemented by a method of the
second class, i.e. a process of fractional crystallisation; where the
basicity method is not used (as, for example, in most of the recent
processes for separation of the cerium elements), two or more different
methods of fractional crystallisation will supplement one another.

The methods of the second class, which are processes of fractional
crystallisation, depend on the differences in solubility which are
observed in analogous compounds in passing from one member of the group
to another. The value of these methods, as opposed to the methods
depending on differences in basic strength, was clearly shown by Auer
von Welsbach, who in 1885 succeeded in resolving Mosander’s ‘Didymium’
into two new elements, praseodymium and neodymium, by fractional
crystallisation of the ammonium double nitrates; since that date, much
attention has been devoted to the task of finding rare earth compounds
which will lend themselves to such processes. The method is extremely
laborious, and may involve several thousand recrystallisations, in
consequence of the generally very slight differences of solubility, and
the ease with which the rare earth compounds, being almost always
isomorphous with one another, form mixed crystals.

Whilst the method of fractional crystallisation has come into general
use for the separation of one element from another only within the last
thirty years, processes for the separation of the cerium group from the
yttrium elements, depending on differences of solubility, have long been
known and used. The most important of these, the double sulphate method,
depends on the fact that the potassium double sulphates of the cerium
metals are almost insoluble, whilst those of the terbium group are
sparingly, and of the yttrium group readily soluble in a concentrated
solution of potassium sulphate. The cerium elements may be thus
completely removed from a solution of mixed salts by addition of a crust
of potassium sulphate crystals, or of an hot concentrated solution of
the same reagent. In other cases, _e.g._ in the double carbonate and
double oxalate processes, separation is effected by taking advantage of
the greater tendency to the formation of double salts possessed by the
yttrium metals.

In effecting a separation of closely related bodies by fractional
processes, in which a large number of repetitions of the same operation
are necessary, only the most careful and systematic procedure can avoid
much waste of valuable material; in these processes, the object of the
chemist is to obtain pure end fractions, whilst keeping the middle
fractions as small as possible. One method of procedure generally
adopted is illustrated in Fig. 7, which represents a fractional
crystallisation of a mixture of four or five substances, α, β, ... φ;
the separations being usually conducted in such a way that subgroups of
three, four or five elements are first obtained, these being then
further fractionated to obtain the pure elements. In the diagram, crops
of crystals are represented by crosses, the mother-liquors by circles;
for the sake of illustration, the process is made to appear as simple
as possible.

[Illustration: FIG. 7]

The mixture is dissolved up, and allowed to crystallise; the crystals
are filtered off, the filtrate concentrated, and a second crop obtained;
this is repeated until five or six crops of crystals have been obtained.
These, with the mother-liquor, constitute series A. The first fraction
is now recrystallised; it yields a crop of crystals, fraction 1 of
series B, and a mother-liquor, which is added to fraction 2 of series A,
as indicated by the dotted arrow and circle; on recrystallisation of
this mixture, a crop of crystals, fraction 2 of series B, is obtained,
together with a mother-liquor, which is recrystallised with fraction 3
of series A. In this way, by continued repetition, series are obtained,
of which each contains one fraction more than its predecessor; the least
soluble constituent is thus concentrated in the fractions represented on
the left of the diagram, whilst the most soluble accumulates in the
mother-liquors. After a greater or smaller number of series have been
traversed, according to the differences in solubility, the end fractions
in each series will be pure. These are no longer fractionated, and the
number of fractions in each series begins to diminish, as shown on the
diagram. The middle fractions will contain the compounds of intermediate
solubility; these may be separated by further fractionation on the same
lines, or may perhaps be better treated by a different or modified
process.

In a modification of the method, each fraction of series A is
recrystallised separately, yielding a crop of crystals, and a
mother-liquor; series B is then built up by adding to the crystals from
fraction 2 the mother-liquor from fraction 1, to the crystals from
fraction 3 the mother-liquor from fraction 2, and so on; the fractions
in this series are then recrystallised separately, and the third series
built up by the similar combination of the crystals and mother-liquors.

Similar systematic methods of procedure must be adopted in working out
any method of fractional separation; it can at once be seen that where,
as in the rare earth group, only small variations in properties exist,
much time and care must be expended, if pure products are required.

Since the development of the methods of spectrum analysis, the
difficulty of testing the efficiency of a method of separation, and of
examining the purity of the products obtained, has been greatly
lessened. The only reliable test at the disposal of the earlier chemists
was the determination of the equivalent weight, which still constitutes
an important check on the modern methods. Some account of the methods
available for the control of the methods of separation is essential in a
general account of the rare earths; but before describing these, it will
be convenient to give a short description of the methods used in the
extraction of the elements from the rare earth minerals.


EXTRACTION OF THE RARE EARTHS FROM MINERALS

With the exception of those containing large proportions of columbium,
tantalum, and titanium, the rare earth minerals are easily decomposed
by acids. The silicates, as a general rule, can be satisfactorily
treated with hydrochloric acid in the ordinary way, but for large
quantities, the use of sulphuric acid is more desirable. The more
refractory minerals are completely decomposed by fused alkali hydrogen
sulphate; sodium bisulphate is more suitable for this purpose than the
potassium compound, the sodium double sulphates of the rare earth
elements being more soluble than the potassium salts. Hydrofluoric acid
also attacks the refractory minerals very readily; the rare earths, in
this case, are left as the insoluble fluorides.

After decomposition with sulphuric acid or bisulphate, the cold residue
is extracted with water, the rare earth sulphates or double sulphates
being removed in solution. Digestion with nitric acid may be necessary
at this stage, if titanium, columbium, etc., are present; after
filtration, the solution is evaporated to dryness, and the residue
extracted with dilute hydrochloric acid. The solution is saturated with
sulphuretted hydrogen to remove lead, copper, bismuth, molybdenum, etc.,
and treated in the usual way with ammonium chloride and ammonia. The
precipitate is washed, and dissolved in hydrochloric acid, the solution
heated to about 60°, and the rare earths precipitated by addition of
excess of oxalic acid, which holds in solution any zirconium which may
be present. In the presence of phosphates, _e.g._ in the treatment of
monazite or xenotime, the precipitate of oxalates should be ignited to
the oxides, these dissolved in acid, and a second precipitation with
oxalic acid effected; this treatment is necessary to remove phosphoric
acid completely.


~Preliminary examination of the earth mixture.~--Before a method of
separation can be decided upon, some knowledge of the composition of the
mixture to be treated must be obtained. The nature of the mineral used
for the extraction will, as a rule, afford useful information. It is
known that in some minerals the cerium group, in others the yttrium
group, predominates more or less completely; certain minerals, also, are
known to be rich in elements of one or another subgroup. An approximate
knowledge of the relative proportions of the cerium, terbium, and
yttrium groups will be afforded by a rough double sulphate separation;
thorium, zirconium, and scandium come down with the cerium earths. For
approximate separation, Urbain[185] proposes the use of the
ethylsulphates. The yttrium elements can be quickly separated in an
approximate manner by fractional precipitation of the hydroxides with
magnesia. The successive fractions obtained by these methods are
examined spectroscopically; from the results, the composition of each,
and so of the original mixture, may be roughly deduced.

  [185] _Ann. Chim. Phys._ 1900, [vii.], ~19~, 184.


THE SPECTRUM EXAMINATION

In no department of chemistry have the methods of spectrum analysis
proved of more value than in the field of the rare earths. They provide
the chemist with a means of following and controlling his processes of
separation which is far more delicate and decisive than the older method
of determining the equivalent weight. Whilst the examination of emission
spectra, and especially of arc spectra, is of decisive value in every
case, it has the disadvantage of requiring delicate and complicated
apparatus and great experimental skill; wherever possible, therefore,
the examination of the absorption spectra is preferred, though this is
useful only for a few of the elements, and varies considerably with the
conditions employed.


~The Absorption Spectra.~--Absorption in the visible region of the
spectrum is observed only with those rare earth compounds which are
coloured, and is of value, therefore, chiefly for identification in the
case of praseodymium and neodymium among the cerium elements, and of
erbium among the yttrium metals; these give characteristic absorption
bands, even in dilute solution. The absorption spectra of the rare earth
compounds are highly characteristic, the bands being well defined and
sharply bounded, whereas coloured compounds of the common elements show
general absorption, or at best diffuse bands, under the same conditions.

In observing an absorption spectrum, the light from a Nernst lamp, or
incandescent burner, is passed through a layer of a suitable solution of
the coloured compound, of known concentration and thickness, and after
collimation is analysed by a suitable prism; the spectrum is observed by
a telescope in the ordinary way. Where accurate readings are not
required, as, for example, in testing for the presence or absence of a
particular element, the position of the bands may be read to a
sufficient degree of accuracy by means of a scale, the image of which is
adjusted to coincide with the spectrum as seen through the eyepiece; but
in mapping a spectrum accurately, more refined methods must of course be
used. The photographic method, in which a photograph of the spectrum is
taken on a plate which bears, for purposes of measurement, a comparison
spectrum of known lines, is very convenient for examining the absorption
in the violet and ultraviolet regions.

The intensity, and to some extent also the position, of bands in an
absorption spectrum may vary considerably, according to the conditions
employed. Of the various factors which must be considered, the
concentration of the solution, the thickness of the layer used, the
nature of the solvent, and of the acid radicle, and the presence of
other earths are the most important. The concentration of the solution,
and the thickness of the layer, which together constitute the Optical
Density, must be so adjusted that the absorption is neither too strong
nor too weak; in the first case the sharp bands tend to merge into broad
diffusion areas, and details are obscured, whilst in the second case the
presence of coloured compounds which do not show strong absorption bands
may be overlooked.

The nature of the acid radicle has considerable influence on the
position of the absorption maxima, the general rule being that the bands
are shifted towards the red end of the spectrum as the molecular weight
of the compound used increases. Naturally, also, the nature of the
solvent has an important effect, all the usual phenomena which must be
considered in the measurement of the physical properties of substances
in solution coming into play; electrolytic dissociation, hydration,
dissociation and the formation of complexes, for example, are all
important factors. The presence of colourless earths has also been found
to cause important differences. It follows, therefore, that for the
chemist, the absorption spectra can be considered as a valuable aid only
in detecting the presence or absence of the three elements which give
the strongest and most characteristic absorption bands, viz.
praseodymium, neodymium, and erbium, and that conclusions regarding the
quantitative composition of mixtures must be drawn with the utmost
caution.


~The Emission Spectra: Spark Spectra.~--The factors which tend to limit
the value of the absorption spectra for analytical purposes, for the
most part disappear when the emission spectra are employed. In the case
of the spark spectra, indeed, great differences are observed according
to the conditions and method of experiment; but the arc spectra are
practically invariable under all conditions, and hence they constitute
the ultimate test in all cases. The spark spectra are observed when one
terminal--the cathode--of an induction coil is embedded in the oxides to
be examined, and the discharge then passed. The discharge is also
frequently passed between platinum poles partly immersed in a strong
solution of a salt of the element under examination; a form of apparatus
very suitable for this method of observation has been described by Sir
W. Crookes.[186] The spectra so obtained are in a high degree
characteristic, but they vary very considerably with the form and
dimensions of the coil, the length and cross-section of the wires, the
potential difference employed, and so on. An entirely new spectrum also
is obtained in many cases by mere reversal of the current; under these
conditions, a phosphorescent appearance is observed, the spectrum of
which--reversed spark spectrum of de Boisbaudran--has been found in many
cases to resemble the cathode luminescence spectra of Crookes.

  [186] _Proc. Roy. Soc._, 1903, ~72~, 295.


~The Arc Spectra.~--The final criterion of purity in the examination of
a rare earth element is in almost all cases the arc spectrum. Since for
some of the elements, especially in the yttrium group, the entire
spectrum has not yet been accurately mapped out, spectra are generally
observed frequently throughout the course of a fractionation; by this
means, the separation can be followed by the disappearance of some
lines, and the appearing or strengthening of others, and such
examinations have led occasionally to the discovery of new elements
(see, for example, under Separation of ytterbium earths, p. 205). Such
determinations, however, require much time and extensive and complicated
apparatus.

Carbon electrodes are generally employed, and it is immaterial in this
case which is the anode, and which the cathode. The lower carbon is
hollowed out, and the space filled with the oxide or sulphate of the
element or mixture to be examined; or the electrode may be impregnated
with a concentrated solution of a salt. The light is examined by means
of a diffraction grating, and the spectrum photographed on a plate which
bears a comparison spectrum for measurement. The lines are most numerous
in the violet and ultraviolet regions, and the most characteristic
spectra are given by the colourless earths. The method is naturally more
delicate for some elements than for others; the great persistency of the
scandium line 3613·984, for example, was found very valuable by Crookes
and by Eberhard in the examination of various rocks and minerals for
that element, whilst other intense and persistent lines have served for
the detection of various rare earth elements in the sun and many stars.


~The Cathode Luminescence Spectra.~--The phenomenon of cathode
luminescence, which was observed and very fully investigated by Sir
William Crookes, and which led that author to his theory of
Meta-elements, is one of the greatest scientific interest. Crookes
observed that certain of the rare earths, when subjected to the action
of cathode rays in a vacuum tube, exhibit a brilliant phosphorescence,
which, when examined by the spectroscope, show characteristic spectra,
which differ greatly for fractions of apparently identical chemical
composition, and are otherwise distinguishable by physical properties.
The researches of Lecoq de Boisbaudran, and the more recent work of
Baur and Marc,[187] have shown that this luminescence is observed when a
small quantity of a coloured earth is present with a very large quantity
of a colourless earth, the maximum phosphorescence being produced by
about 1 per cent. of the coloured earth, or ‘phosphorogen.’ The question
has recently been very fully examined by Urbain.[188] He shows that the
sensitiveness of the phenomenon is so great that it cannot be employed
for the ordinary purposes of chemical analysis, one part in a million of
the phosphorogen being sufficient to cause a clearly perceptible
luminescence in a pure colourless oxide.

  [187] _Ber._ 1901, ~34~, 878.

  [188] _Ann. Chim. Phys._ 1909, [viii.], ~18~, 222; see also
  _Introduction à l’étude de la Spectrochimie_, pp. 145 _et seq._


~The Magnetic Susceptibility.~--The fact that the rare earths differ
very considerably from one another in their magnetic properties has been
known for several years,[189] and has recently been employed by Urbain
and Jantsch[190] as a means of identification, and a test of purity, and
for following processes of fractionation. The magnetic susceptibility
reaches a minimum at samarium, and rises very sharply on either side of
that element, so that the presence of the closely related elements,
neodymium on the one side, and europium and gadolinium on the other,
which differ only very slightly from samarium in atomic weight and
solubility, can easily be detected by this means. The property is highly
additive, and can be used, therefore, to estimate the relative
proportions of two oxides in a mixture; the determinations are said to
be easily and quickly carried out.

  [189] See Meyer, _Monats._ 1898, ~20~, 369 and 793.

  [190] _Compt. rend._ 1908, ~147~, 1286; see also Urbain, _ibid._,
  1910, ~150~, 913.

When the elements are considered in order of atomic weight, the
coefficient reaches a maximum at neodymium in the cerium group, and
again at dysprosium (or holmium) in the yttrium group:--[191]

                               Coefficient of magnetisation
   Element.    Atomic Weight.         for the oxide.
                                        _x_ × 10⁻⁶

  Scandium          44·1                   -0·05
  Yttrium           89·0                   -0·14
  Lanthanum        139·0                   -0·18
  Neodymium        144·3                   33·5
  Samarium         150·4                    6·5
  Europium         152·0                   33·5
  Gadolinium       157·3                  161
  Terbium          159·2                  237
  Dysprosium       162·5                  290

  [191] See Urbain and Jantsch, _loc. cit._; the values for lanthana,
  scandia, and yttria were determined by Wedekind (see Meyer and
  Wuorinen, _Zeitsch. anorg. Chem._ 1913, ~80~, 7).

Erbium, thulium, ytterbium, and lutecium appear in descending order at
the end of the series, but no figures are given.

The most interesting application of the property has been Urbain’s
discovery of the new element Celtium (see p. 207).


THE EQUIVALENT WEIGHT DETERMINATION

The determination of the mean equivalent weight, which was for the
earlier chemists the only reliable method of controlling their
fractionations, is still of considerable importance for this purpose,
especially in the yttrium group, in which the differences in atomic
weights are more considerable than among the cerium metals. Great
importance, moreover, still attaches to these determinations, since they
serve to fix the atomic weights; save that the methods used in an atomic
weight determination are somewhat more elaborate and refined than those
used when it is desired merely to test a fractionation, the same
processes apply in both cases.

The methods which have been most commonly used are those based on a
determination of the ratio R₂O₃ : R₂(SO₄)₃, and these are of two kinds,
the synthetic and the analytical. The first, in which a known weight of
the oxide is converted into the sulphate, has been most used for the
most strongly basic oxides, since with these it is difficult to remove
the last traces of sulphuric anhydride from the oxide by heat. The
oxides are best obtained from the oxalates, which are precipitated from
an acid solution of the nitrates, washed thoroughly with water, alcohol
and ether in succession, dried, and ignited in a tarred platinum
crucible. The oxide is best dissolved in dilute hydrochloric or nitric
acid on the waterbath, a slight excess of sulphuric acid being added
only when a clear solution has been obtained; the liquid is then heated
gradually to 300°, and finally in the electric furnace at 450°-550°
until constant in weight. If sulphuric acid be added directly to the
weighed oxide, particles of the latter may become completely coated with
the insoluble sulphate, and so escape the action of the acid.

In the analytical method, a known weight of sulphate is ignited to the
oxide, and weighed as such. This method is most suitable for the less
basic members of the yttria earths, of which the sulphates can be
completely decomposed without difficulty at a red heat. By the use of
the microbalance, a sufficiently accurate determination can be carried
out by either of these methods in little more than half an hour, as the
chemical changes are exceedingly rapid where only small quantities are
employed, and no time is required to allow the vessels and solids to
cool. Using the microbalance, Brill[192] has carried out a series of
experiments to determine the limits of temperature within which the
various steps of the process should be carried out. He finds that a
temperature of 400°-550° is required to decompose the last traces of
acid sulphate, and give the pure neutral sulphate. Between the
temperatures of 850° and 950°, basic salts are formed, from which the
last trace of sulphuric anhydride is expelled at 900°-1150°; the precise
temperature required in each case depends, of course, on the basic
strength of the oxide in question.

  [192] _Zeitsch. anorg. Chem._ 1905, ~47~, 464.

The determination of equivalents by means of the ratio R₂O₃ : R₂(C₂O₄)₃,
has been brought to a high degree of accuracy by Brauner.[193] A weighed
quantity of the carefully prepared oxalate is ignited, with suitable
precautions, to the oxide, in a tarred platinum crucible. A second
weighed specimen of the same oxalate preparation is dissolved in dilute
sulphuric acid, and titrated at 60° with permanganate, which is
standardised against pure ammonium oxalate.

  [193] _Ibid._ 1903, ~34~, 103, 207.

Of the methods of volumetric analysis which have been proposed, that put
forward by Feit and Przibylla appears to be the most suitable. A
convenient quantity of oxide, which has been ignited until constant in
weight, is dissolved by gently heating with a known excess of N/2
sulphuric acid, in a conical flask of Jena glass. The excess of acid is
titrated with N/10 sodium hydroxide, using methyl orange as indicator.
This method, which has the advantages of ease and quickness, is very
reliable, if suitable precautions are taken, in the case of the more
strongly basic oxides; but with the least strongly basic members of the
yttria group, the erbia and ytterbia oxides, the end point is not very
sharp, whilst with the weakly basic scandia, the method breaks down
entirely.[194]

  [194] _Zeitsch. anorg. Chem._ 1905, ~43~, 202; 1906, ~50~, 249.


CHAPTER XI

THE CERIUM GROUP--CERIUM

The extraction of the rare earth elements from minerals, by which they
are obtained in the form of the oxalates, and the methods of bringing
these into solution, have already been described. From the solution,
before any separation of the rare earths is attempted, thorium should be
removed; for this purpose, any of the methods described under estimation
of thorium (see p. 286) may be used, the most convenient being the
peroxide precipitation of Wyrouboff and Verneuil.

The solution is then treated with potassium sulphate until the
absorption bands of didymium (praseodymium and neodymium) can no longer
be observed, or appear only very faintly, when a layer of the solution
is examined with a spectroscope; the precipitate then consists of the
potassium double sulphates of the cerium with some of the terbium
elements. If the mixture is very rich in the cerium elements, and
correspondingly poor in the yttrium elements--as, for example, the
mixture of earths obtained from monazite--Drossbach[195] recommends a
preliminary separation by means of the double carbonates; the double
sulphate method may then be employed to remove the last of the yttrium
and most of the terbium elements. The sparingly soluble double sulphates
of the cerium metals may be transformed into the hydroxides by digestion
with potassium hydroxide, and these taken into solution, after washing,
by hydrochloric or nitric acid.

  [195] _Ber._ 1900, ~33~, 3506.


~Cerium~, Ce = 140·25

Of all the rare earth elements, cerium, by virtue of its property of
forming ceric salts corresponding to the dioxide CeO₂, is the one most
easily separated and obtained in the pure state. In those compounds in
which it is tetravalent, cerium functions as a much less strongly
electropositive element than in the cerous compounds, and all the
methods of separation are based on this fact. Mosander, who first
demonstrated that the old ‘ceria’ was a mixture, separated the element
by treating a suspension of the hydroxides in potassium hydroxide with
chlorine; yellow ceric hydroxide remains undissolved, whilst the other
elements go into solution as the chlorides and hypochlorites. This
method was extensively used until quite recently; it has the advantage
of separating the cerium completely, but the product is very impure, and
several repetitions are required to give good results. The basic nitrate
method, which is now used on the commercial scale in extracting cerium
from monazite (see p. 284), is also due to Mosander, though it has been
employed subsequently by many workers.

Several methods take advantage of the ease with which the ceric salts,
as compared with salts of the trivalent elements, may be hydrolysed.
Brauner[196] dissolves the oxides in nitric acid, and after removal of
excess of acid, boils with a large volume of water--basic ceric nitrate
is thrown down, the other elements remaining in solution as nitrates.
The precipitate is redissolved, and the process repeated until the
cerium is found spectroscopically to be free from didymium. The
hydrolysis of the ceric salt may be effected more quickly and completely
by the addition of ammonium sulphate or magnesium acetate.[197]
James[198] boils the solution of the nitrates with potassium bromate,
keeping the whole neutral by addition of powdered marble; the cerium is
completely and very quickly precipitated as basic nitrate.

  [196] _Trans. Chem. Soc._ 1885, ~47~, 879.

  [197] Meyer and Koss, _Ber._ 1902, ~35~, 672.

  [198] _J. Amer. Chem. Soc._ 1912, ~34~, 757; this paper gives a
  complete scheme for a full separation of all the elements.

An interesting method is due to Koppel[199]; the oxides are dissolved in
a solution of hydrogen chloride in methyl alcohol, and treated with
pyridine, when the sparingly soluble double chloride, (C₅H₅NH)₂CeCl₆,
separates, and may be obtained pure by recrystallisation from alcohol
and ether. The permanganate method of Drossbach, which is used on the
commercial scale, is described on p. 285.

  [199] _Zeitsch. anorg. Chem._ 1898, ~18~, 305.

The cerium compounds obtained by these methods are purified by
transformation into the anhydrous sulphate, which is dissolved in
ice-water; when this solution is allowed to come slowly to room
temperature, the pure octohydrate separates. Pure cerium salts should
show no trace of absorption when concentrated solutions are examined
spectroscopically; on ignition, the oxide obtained should be almost
colourless, having at most a faint yellow tinge. A reddish or
brownish-red shade indicates the presence of praseodymium. An arc
spectrum examination will generally show the presence of lanthanum,
which occurs in traces even in the most carefully purified cerium
preparations.

The preparation and properties of metallic cerium have already been
described (see p. 114); for an account of the pyrophoric alloys, see p.
314.


THE CEROUS COMPOUNDS

The salts of trivalent cerium are very similar to those of the other
rare earth elements, and a detailed description of them is therefore
unnecessary. The _sesquioxide_, Ce₂O₃, cannot be obtained by ignition of
the oxalate, nitrate, or other similar salt, since these decompose at
high temperatures with formation of the dioxide, CeO₂. It has been
prepared by the reduction of the dioxide with calcium;[200] it has a
great affinity for oxygen, and readily absorbs the gas when exposed to
moist air. _Cerous hydroxide_, Ce(OH)₃, obtained by addition of alkali
to solutions of cerous salts, has also strong reducing properties,[201]
and can only be prepared and preserved when oxygen is carefully
excluded. It has been obtained as a perfectly white solid by the action
of water on the carbide;[202] when dried in an inert atmosphere, it
yields a perfectly white oxide. In presence of air, it darkens,
assuming a reddish-violet colour, which passes into yellow as the
oxidation becomes complete. The oxidation proceeds more quickly in
presence of potash or soda, ceric hydroxide, Ce(OH)₄, being formed; in
presence of potassium carbonate, however, a dark-coloured peroxyhydrate
is formed by autoxidation. The colour so produced disappears on shaking
if an ‘acceptor’ is present, ceric hydroxide being left; if the acceptor
cannot reduce this, the solution after shaking loses the power of
re-forming the dark peroxide, but if the acceptor can reduce the ceric
compound to cerous hydroxide, the solution after shaking regains the
power of forming the peroxide which is a property of the lower
hydroxide.

  [200] Burger, _Ber._ 1907, ~40~, 1652.

  [201] Dennis and Magee, _J. Amer. Chem. Soc._ 1894, ~16~, 649; also
  Biltz and Zimmerman, _Ber._ 1907, ~40~, 4979.

  [202] Damiens, _Compt. rend._ 1913, ~157~, 214.

_Cerous nitride_, CeN, has been prepared by Moissan[203] by the action
of ammonia on the heated carbide; it can also be obtained by heating the
hydride in a stream of nitrogen.[204] Muthmann and Kraft also state[205]
that it can be prepared by heating metallic cerium in the gas, the metal
burning with the liberation of much energy in the form of heat and
light; but Dafert and Miklanz[206] deny that it can be obtained in this
way. Cerium nitride is a lustrous, brass yellow to bronze coloured
solid, stable in dry air, but at once attacked by moist air, with
evolution of ammonia, and formation of the dioxide. When moistened in
air with a few drops of water, the substance reacts violently, becoming
heated to redness. Alkalies and acids decompose it, with formation of
cerous compounds.

  [203] _Compt. rend._ 1900, ~131~, 865.

  [204] Dafert and Miklanz, _Monats._ 1912, ~33~, 911.

  [205] _Annalen_, 1902, ~325~, 261.

  [206] _Loc. cit._

The _sulphide_, Ce₂S₃, has been prepared by Biltz[207] by heating the
sulphate to a red heat in a current of sulphuretted hydrogen; he
describes it as a red powder. The _chloride_, CeCl₃, combines with
ammonia with evolution of heat even at a temperature of -80°. Five
additive compounds are described;[208] they are white powders,
decomposed by water.

  [207] _Ber._ 1908, ~41~, 3341.

  [208] Barre, _Compt. rend._ 1913, ~156~, 1017.

The solubility curve of the various _sulphate hydrates_ has already been
given (see p. 125). Various _double sulphates_ with ammonium sulphate,
and the sulphates of sodium, potassium, thallium and cadmium are known.
The cadmium double compound has the composition Ce₂(SO₄)₃,CdSO₄,6H₂O,
and is prepared by mixing solutions of the simple salts in presence of
sulphuric acid. Many _double nitrates_ have been prepared; these are for
the most part stable, highly crystalline compounds, easily soluble in
water and alcohol. With the nitrates of the common divalent metals,
cerous nitrate forms a series of double salts of the general formula
2Ce(NO₃)₃,3R(NO₃)₂,24H₂O, where R = Mg, Mn, Co, Ni, or Zn; these form an
isomorphous series, crystallising in the hexagonal system. The
_acetylacetone compound_ melts at 131°-132°.

In the presence of hydrogen peroxide in the cold, ammonia throws down
from solutions of cerous salts a reddish-brown peroxyhydrate,
Ce(OOH)(OH)₃,[209] which on heating loses oxygen, and yields ceric
hydroxide. The reaction is very delicate, and may be used as a test for
cerium. If the precipitate be treated with acids in the cold, ceric
salts are first obtained, but these are at once reduced, in the acid
solution, by the hydrogen peroxide formed, so that cerous salts remain;
ceric salts may be obtained by first boiling the suspension of the
peroxyhydrate and treating the ceric hydroxide so obtained with acids.

  [209] Pissarjewski, _Zeitsch. anorg. Chem._ 1902, ~31~, 359.


THE CERIC COMPOUNDS

The ceric salts are much more readily hydrolysed than the cerous salts,
and show a great tendency, in dilute solution, to pass over into the
latter. So great is this tendency that a solution of a ceric salt acts
as if it were supersaturated with oxygen; ceric sulphate, for example,
in dilute solution slowly evolves oxygen, whilst the chloride evolves
chlorine. In consequence of this behaviour, ceric compounds have a very
powerful oxidising action. The ceric salts are yellow to red in colour;
their solutions are strongly acid, owing to the ease with which the
salts hydrolyse, and on boiling deposit insoluble basic salts.

Beside the methods which have already been mentioned, ceric compounds
may be prepared from cerous by oxidation with sodium peroxide, bismuth
tetroxide, ammonium persulphate, etc. In electrolysis of cerous salts,
also, ceric compounds are obtained at the anode.

_Ceric hydroxide_, Ce(OH)₄, is obtained as a gelatinous yellow
precipitate on the addition of alkali to a solution of a ceric salt, or
by the oxidation of cerous hydroxide. The freshly prepared precipitate
dissolves in nitric acid with a reddish colour; hydrochloric acid
reduces it, with evolution of chlorine, and formation of cerous
chloride, whilst sulphuric acid dissolves it with partial reduction,
oxygen being evolved. If a solution of a ceric compound be dialysed for
some days, a clear neutral solution is obtained, which contains the
hydroxide in the colloidal condition; by evaporation of the solution, a
gummy mass is obtained, which dissolves again in water to a clear
solution. Electrolytes rapidly cause coagulation.

_Cerium dioxide_, CeO₂, is obtained by the ignition of any salt of
cerium with a volatile acid, or by burning the element in oxygen; the
latter reaction produces a very intense and blinding light, on account
of which cerium compounds are often suggested for use in flashlight
powders (see p. 319). The pure oxide should be almost white, or at most
a very faint yellow, but the exact shade and appearance vary according
to the method and temperature employed in preparation, doubtless by
reason of the possibility of different degrees of polymerisation.[210]
The oxide can act as an oxygen carrier towards other substances, notably
towards other oxides of the rare earth group,[211] but the phenomena
have not been fully elucidated. In virtue of this property, the dioxide
has been proposed as a substitute for platinised asbestos in Dennstedt’s
method for the combustion of organic bodies.[212]

  [210] See in this connection Wyrouboff and Verneuil, _Compt. rend._
  1898, ~127~, 863; _ibid._ 1899, ~128~, 501; and in _La chimie des
  terres rares_, ‘Conférences de la Société chimique de Paris,’ Paris,
  1903.

  [211] See Meyer and Koss, _Ber._ 1902, ~35~, 3740.

  [212] Bekk, _Ber._ 1913, ~46~, 2574.

The ignited oxide is soluble in nitric or hydrochloric acid only in
presence of a reducing agent. Concentrated sulphuric acid converts it
into ceric sulphate; fused bisulphate attacks it more readily. In the
crystalline form, obtained by fusing the amorphous form with borax, or
a suitable salt,[213] it is extremely resistant to acids and to
alkalies.

  [213] See, _e.g._ Sterba, _Ann. Chim. Phys._ 1904, [viii.], ~2~, 193.

By heating the dioxide in a stream of hydrogen, care being taken to
exclude air, a dark blue oxide, of which the composition corresponds
approximately to that required by the formula Ce₄O₇, is obtained.[214]
This substance has strong reducing properties; when warmed in air, it
glows, forming the dioxide, and reduces carbon dioxide when heated in a
current of that gas. This _intermediate oxide_ is said to correspond in
composition to the violet hydroxide which is obtained as an intermediate
product in the oxidation of cerous to ceric hydroxide, and which is said
to yield the blue oxide, Ce₄O₇, when dried _in vacuo_.

  [214] Sterba, _Compt. rend._ 1901, ~133~, 221; Meyer, _Zeitsch. anorg.
  Chem._ 1903, ~37~, 378.

The _disulphide_, CeS₂, has been obtained by Biltz[215] by prolonged
heating of anhydrous cerous sulphate in a current of sulphuretted
hydrogen at a dull red heat; it is a dark, yellowish-brown, crystalline
solid, which on treatment with hydrochloric acid yields hydrogen
persulphide.

  [215] _Ber._ 1908, ~41~, 3341.

_Halogen salts._--No halogen compounds are known in the free state,
except the _fluoride_, CeF₄,H₂O, which was obtained by Brauner as a
yellowish-brown mass, by the action of hydrofluoric acid on the
hydroxide. A _double fluoride_, 2CeF₄,3KF,2H₂O, was prepared by the same
author by dissolving the hydroxide in potassium hydrogen fluoride; it is
insoluble in water. By dissolving a ceric salt in concentrated
hydrochloric acid, a dark red solution is obtained, which is believed to
contain the unstable complex acid, H₂CeCl₆; this decomposes slowly in
the cold, more quickly on warming, with evolution of chlorine, and
formation of cerous chloride. Several double compounds of ceric chloride
with hydrochlorides of organic bases have, however, been obtained.

_Ceric sulphate_, Ce(SO₄)₂, is obtained by the action of concentrated
sulphuric acid on the dioxide. It is a deep yellow crystalline powder,
dissolving readily in water to a brown solution, which has a strongly
acid reaction; on warming or diluting, a basic sulphate separates. The
solution slowly evolves oxygen, and therefore always contains cerous
compounds. On evaporation, _a cero-ceric acid sulphate_ of the formula
HCe^{iii}Ce^{iv}(SO₄)₄,12(13 ?)H₂O first separates; the hydrated
sulphate Ce(SO₄)₂,4H₂O, being more soluble, separates on further
concentration.[216] The relative amounts of the two compounds obtained
depends on the temperature and the concentration of acid in the
solution; if both these factors are kept low, the almost pure hydrated
sulphate can be at once obtained. This separates in yellow crystals
belonging to the rhombic system; it is readily soluble in water. The
mixed acid salt is less soluble, and forms orange prisms and needles,
which cling tenaciously to sulphuric acid. Other complex and double
salts have also been obtained. When, for example, silver nitrate is
added to a warm solution of the sulphate in concentrated sulphuric acid,
a bright orange-yellow precipitate of the salt 10Ce(SO₄)₂,6Ag₂SO₄ is
obtained.[217]

  [216] See Meyer and Aufrecht, _Ber._ 1904, ~37~, 140; Brauner,
  _Zeitsch. anorg. Chem._ 1904, ~39~, 261.

  [217] Pozzi-Escot, _Compt. rend._ 1913, ~156~, 1074.

Neutral ceric nitrate is unknown. A _basic nitrate_, Ce(NO₃)₃OH,3H₂O, is
obtained in red crystals by evaporation of a solution of ceric hydroxide
in strong nitric acid. The solid is readily soluble in water, forming a
yellow, acid solution, which becomes paler by hydrolysis, on warming or
on standing. The course of the hydrolysis is also indicated by the
action towards acids, and towards hydrogen peroxide.[218] A freshly
prepared ceric salt, on addition of acid, becomes immediately much
darker in colour, whereas the colour change is very slow, if
considerable hydrolysis has occurred. Similarly, hydrogen peroxide at
once reduces a freshly prepared solution, forming colourless cerous
salts, whilst if much hydrolysis has occurred, deeply coloured higher
oxidation products are at first formed, and these lose their colour only
slowly.

  [218] Meyer and Jacoby, _Zeitsch. anorg. Chem._ 1901, ~27~, 359.

The _double ceric nitrates_[219] are a large and very important class of
compounds; they are the most stable of the ceric salts. With nitrates of
the monovalent metals, ceric nitrate forms double nitrates of the type
R₂Ce(NO₃)₆; these are deep red hygroscopic substances, crystallising in
the monoclinic system, readily soluble in water and alcohol, but
dissolving only sparingly in nitric acid. The ammonium salt is important
for the separation of cerium. A series of double nitrates with the
nitrates of manganese, magnesium, zinc, nickel, and cobalt has the
general formula RCe(NO₃)₆,8H₂O, but these are much less stable in
solution than the alkali double salts.

  [219] Meyer and Jacoby, _loc. cit._


ATOMIC WEIGHT OF CERIUM

No less than twenty-eight separate determinations of the atomic weight
of cerium have been carried out. The earlier determinations are rendered
unreliable by the almost certain presence of other elements, and
Brauner[220] has shown that some of the methods employed in later work
give erroneous results.

  [220] _Trans. Chem. Soc._ 1885, ~47~, 879; also _Zeitsch. anorg.
  Chem._ 1903, ~34~, 207.

A very careful determination was made by Robinson in 1884.[221] Cerium
oxalate was heated in a stream of dry hydrogen chloride, mixed with
carbon dioxide, and the anhydrous chloride freed from traces of acid in
a vacuum over chalk. The weighed chloride was then dissolved in water,
and titrated with silver nitrate. He obtained the value 140·26;
recalculation from his data with the modern values for silver and
chlorine give 140·19. Brauner points out that this result is too low,
since no account was taken of the solubility of silver chloride in
water. In the following year, Brauner[222] determined the ratio
Ce₂(SO₄)₃ : 2CeO₂, and obtained the atomic weight 140·22. Wyrouboff and
Verneuil[223] in 1897 disputed Brauner’s work, and as a result of
several determinations gave the values 139·21, 139·43, and 139·50; their
determinations, however, varied very considerably, and the work has been
severely criticised by Brauner. In 1903, the latter author and
Batěk[224] obtained the values 140·21 and 140·27 by the sulphate and
oxalate methods respectively; whilst in the same year, using the same
methods, Brauner[225] obtained from three independent series of
determinations the values 140·25, 140·24, and 140·25.

  [221] _Proc. Roy. Soc._ 1884, ~37~, 150.

  [222] _Loc. cit._

  [223] _Compt. rend._ 1897, ~124~, 1300.

  [224] _Zeitsch. anorg. Chem._ 1903, ~34~, 103.

  [225] _Zeitsch. anorg. Chem._ 1903, ~34~, 207.

The International Atomic Weight Committee have accepted the value 140·25
since 1904.


DETECTION AND ESTIMATION OF CERIUM

The detection of cerium in a mixture of earths is a comparatively simple
matter, as it has several distinctive reactions. The brown colour of the
peroxy-compounds has been suggested as a convenient test by several
authors. This may be observed when ammonia is added to a cerous salt in
presence of hydrogen peroxide. In the presence of a large excess of
foreign earths, very dilute ammonia should be added, drop by drop, with
continuous shaking, until a small permanent precipitate remains; this
will be rich in the weakly basic ceric hydroxide, and on addition of the
peroxide solution will show the colour clearly.[226] For very small
quantities of cerium, the neutral solution is added to warm concentrated
potassium carbonate solution, and one or two drops of dilute hydrogen
peroxide added to the clear liquid; the yellow colour is then very
characteristic.[227]

  [226] Marc, _Ber._ 1902, ~35~, 2370.

  [227] Meyer, _Zeitsch. anorg. Chem._ 1904, ~41~, 94.

Biltz and Zimmerman[228] employ the reducing powers of cerous hydroxide;
ammoniacal silver nitrate is added to the neutral solution of the cerous
salt, and the mixture warmed. Dilute solutions (1-2 mgms. per litre)
give a brown colour, concentrated solutions a black precipitate. The
oxidation of an ammoniacal solution of the tartrate by air or hydrogen
peroxide, by which an intense yellowish brown colour is developed, has
been recently suggested by Wirth[229] as a very delicate test for the
element.

  [228] _Ber._ 1907, ~40~, 4979.

  [229] _Abstr. Chem. Soc._ 1913, ~104~, ii. 712.

_Spectrum analysis._--Cerous salts show no absorption, ceric salts
general absorption of the violet end of the spectrum. Arc spectrum--see
Exner and Haschek,[230] Eder and Valenta,[231] and Cooper.[232] The
emission spectrum of cerium is especially rich in lines; for
identification, the following may be used:

  [230] _Die Spektren der Elemente, etc._, Leipzig and Vienna, 1911.

  [231] _Sitzungsber. kaiserl. Akad. Wiss. Wien_, 1910, ~119~, II_a_,
  531.

  [232] _Astrophys. J._ 1909, ~29~, 352.

  4150·11
  4186·78
  4222·78
  4296·88
  4337·96
  4382·32
  4386·95
  4460·40
  4479·52
  4487·06
  4527·51
  4528·64
  4539·90
  4562·52
  4572·45
  4594·11
  4628·33
  5512·72

The _estimation_ of cerium cannot be carried out accurately by
gravimetric methods in the presence of other earths; volumetric methods,
however, will give reasonably accurate results, if the necessary
precautions are taken. In Bunsen’s method the ignited oxides are treated
with hydrochloric acid in presence of potassium iodide, the iodine set
free from the hydriodic acid by reduction of the cerium dioxide being
estimated by means of sodium thiosulphate, in the usual way. This method
gives very inaccurate results, since in the presence of cerium dioxide,
other oxides of the group can be converted into higher oxides which will
also liberate iodine under these conditions.

The most reliable method is that of v. Knorre.[233] The solution to be
estimated is acidified with sulphuric acid, and oxidised by means of
ammonium persulphate. The excess of the oxidising agent having been
destroyed by boiling, the cooled solution is treated with a slight
excess of hydrogen peroxide, which reduces the ceric salt according to
the equation:

2Ce(SO₄)₂ + H₂O₂ = Ce₂(SO₄)₃ + H₂SO₄ + O₂

The excess of hydrogen peroxide is then estimated by means of a dilute
permanganate solution. Permanganate is itself reduced by the cerous salt
formed, but the action is so slow in acid solution at the ordinary
temperature that the excess of peroxide can be accurately determined
without unduly hurrying the titration. In this form the method is
generally employed for the estimation of cerium in monazite sands, and
in the incandescent mantle industry. The greatest difficulty is the
adjustment of the concentration of the sulphuric acid required. If this
be too low, basic ceric sulphate separates on boiling, and the
estimation fails; if it be too high, oxidation to the ceric salt is
hindered, and may even be inhibited. This difficulty disappears in the
modified method of Waegner and Muller,[234] in which the oxidation to
the ceric condition is effected by means of bismuth tetroxide in nitric
acid solution. A similar method, in which reduction to the cerous state
is effected by a ferrous salt, in place of hydrogen peroxide, has been
employed by Metzger.[235]

  [233] _Ber._ 1900, ~33~, 1924.

  [234] _Ber._ 1903, ~36~, 282 and 1732.

  [235] _J. Amer. Chem. Soc._ 1909, ~31~, 523; see also Metzger and
  Heideberger, _ibid._ 1910, ~32~, 642.

Many attempts have been made to estimate cerium compounds by means of
permanganate, which in alkaline solution oxidises cerous salts to the
ceric condition, but the autoxidation of cerous hydroxide in the air
introduces errors, unless suitable precautions are taken. Meyer and
Schweitzer[236] show that if the solution of the cerous salt be added,
with constant shaking, to a known volume of a standard permanganate
solution, in presence of excess of magnesia, the liquid being kept warm,
this difficulty is overcome; the results are usually a little high,
however, probably by reason of the oxidising action of the cerium
dioxide on the other oxides present.

  [236] _Zeitsch. anorg. Chem._ 1907, ~54~, 104; see also Roberts,
  _ibid._ 1911, ~71~, 305.

Good results have also been obtained by the use of potassium
ferricyanide in alkaline solution,[237] oxidation taking place according
to the equation:

Ce₂O₃ + 2K₃Fe(CN)₆ + 2KOH = 2K₄Fe(CN)₆ + 2CeO₂ + H₂O

The ceric hydroxide is filtered off, and the ferrocyanide formed
estimated by means of permanganate in acid solution.

  [237] Browning and Palmer, _Zeitsch. anorg. Chem._ 1908, ~59~, 71.


CHAPTER XII

CERIUM GROUP (_continued_)

LANTHANUM, PRASEODYMIUM, NEODYMIUM, AND SAMARIUM

In his examination of the ceria earths in 1839, Mosander discovered a
new constituent, which he called Lanthana; the new oxide was removed in
solution when the ignited mixture was extracted with dilute nitric acid,
which leaves cerium dioxide undissolved. On examination, the new oxide
was found to be heterogeneous; by fractional precipitation with ammonia,
and subsequent recrystallisation of the sulphates, he obtained two
oxides, which he called respectively Lanthana (λανθανειν, to be hidden),
from the absence of colour and specific reactions, and Didymia,
(διδυμοι, twins) from their similarity and the occurrence of the two
together.

Samaria was isolated by Lecoq de Boisbaudran, in 1879, from a specimen
of didymia extracted from the mineral samarskite. Two years previously,
Delafontaine had shown that the didymia separated from this mineral was
not spectroscopically identical with the oxide obtained from other
sources, and in 1878 had isolated an oxide which he called Decipia; this
was shown later, however, to be a mixture of which samaria was one
component. The samaria obtained by de Boisbaudran was by no means pure,
being associated with terbia earths; several investigators claimed to
have separated from it new oxides, most of these being proved afterwards
to have been more or less impure specimens of Europia.

In 1885, Auer von Welsbach[238] employed for the first time the method
which has now become of paramount importance for the separation of the
cerium group, viz. the fractional crystallisation of the double
nitrates. By this method he succeeded in resolving Mosander’s didymia
into two new oxides, for which he proposed the names Praseodidymia
(πρασινος, leek-green), from the colour of the salts, and Neodidymia
respectively; the shorter names praseodymia and neodymia are, however,
now generally adopted.

  [238] _Monats._ 1885, ~6~, 477; _Sitzungsber. kaiserl. Akad. Wiss.
  Wien_, 1885, ~92~, II, 317.

[Illustration: ~GROUP A~

MIXED DOUBLE NITRATES.

                                 2R(NO₃)₃,3Mg(NO₃)₂,24H₂O.
                                          |||
         +--------------------------------+||
         |                +---------------+|+---------------+
         |                |                |                |
        ~1~              ~2~              ~3~              ~4~
      La, Pr           Pr, Nd          Crude Nd       Mother-liquors.
    Compounds.       Compounds.       Compounds.
   Fractionate as   Fractionate as   Continue the     Sa, Eu, Gd, etc.
  R(NO₃)₃,2NH₄NO₃,     2R(NO₃)₃,      Separation.     Crystallise with
       4H₂O.        3Mn(NO₃)₂,24H₂O.       |         Bismuth magnesium
        |                 |                |              nitrate.
        |                 |                +-------+         |   | | | |
        +-------------+   +---------+-------------+|         |   Terbium
        |             |             |             ||         | elements.
        |             |             |             ||         |
        |             |             |             ||         +---+
        |             |             |             ||             |
       ~5~           ~6~           ~7~           ~8~            ~9~
    Pure ~La~    Pr with La.    Impure Pr     Pure ~Nd~      Pure ~Sa~
    Compound.                   Compound.     Compound.      Compound.
    Refine by      Continue.    Continue.     Refine by
    Sulphate          |            |          Sulphate
  crystallisa-        |            |        crystallisa-
      tion.           |            |            tion.
                      |            |
             +--------+-----+ +----+
             |              | |
         Mixture of        ~Pr.~
          Pr, La.    Refine by Sulphate
                      crystallisation.

FIG. 8.--SEPARATION OF THE CERIUM ELEMENTS]


SEPARATION

The modern methods for the separation of these elements are based almost
entirely on the differences in solubility of the various double
nitrates.[239] The mixed double sulphates separated by saturation of a
solution of the chlorides with sodium sulphate, which contain the cerium
and most of the terbium elements, are transformed into nitrates, and the
neutral solution boiled with potassium bromate, in presence of powdered
marble, till all the cerium is precipitated as basic ceric nitrate. From
the filtered solution the other elements are thrown down as oxalates,
transformed into the magnesium double nitrates (A in Fig. 8), and
fractionated from nitric acid solution[240] until a rough separation has
been effected (fractions 1, 2, 3, and 4). The separation, which is
somewhat long and tedious, is followed by means of the absorption
spectra, and by the colour changes of the fractions. Fraction 1,
containing lanthanum and some praseodymium, should be faint green to
colourless; fraction 2 is colourless by the complementary action of the
coloured salts of neodymium and praseodymium; fraction 3, which should
contain the crude neodymium salt, is amethyst; and fraction 4, the
mother-liquor, is yellow from the presence of the samarium compound.

  [239] The following scheme is largely from James, ‘The Separation of
  the Rare Earths,’ _J. Amer. Chem. Soc._ 1912, ~34~, 757.

  [240] See Demarçay, _Compt. rend._ 1900, ~130~, 1019 and 1186; also
  Drossbach, _Ber._ 1902, ~35~, 2826, and Muthmann and Weiss, _Annalen_,
  1904, ~331~, 1.

Fraction 1 is now converted to the double ammonium nitrates, which allow
of a readier separation at this stage; two fractions are obtained, of
which the less soluble, fraction 5, is the fairly pure lanthanum
compound, whilst the more soluble, fraction 6, contains the praseodymium
with a little lanthanum. The lanthanum ammonium nitrate, fraction 5, is
converted into the anhydrous sulphate, which is dissolved in ice-water;
when the solution is gradually warmed, the enneahydrate, La₂(SO₄)₃,9H₂O,
separates, and may be obtained perfectly pure by recrystallisation. It
is of interest that the radioactive element actinium is chemically very
similar to lanthanum, and follows it closely through the process of
separation.

The mixed praseodymium and neodymium magnesium nitrates which constitute
fraction 2 are transformed into the double manganese nitrates, and the
crystallisation from nitric acid continued.[241] The less soluble part,
fraction 7, is fairly free from neodymium, and the separation is
continued with that of fraction 6, until both lanthanum and neodymium
have been completely removed. The more soluble part, fraction 8, yields
the pure neodymium compound, as does also the crude neodymium magnesium
nitrate which constitutes fraction 3, if the crystallisation be
continued.

  [241] Cf. Lacombe, _Bull. Soc. Chim._ 1904, [iii.], ~31~, 570.

The mother-liquors, fraction 4, are treated with bismuth magnesium
nitrate,[242] which is intermediate in solubility between the analogous
compounds of samarium and europium, and the crystallisation continued.
The less soluble fraction contains the samarium compound, in which
bismuth is the only impurity; this is easily removed by treatment with
sulphuretted hydrogen. The remaining fractions are used as a source of
the terbium elements (see p. 186).

  [242] See Urbain and Lacombe, _Compt. rend._ 1903, ~137~, 792; _ibid._
  1904, ~138~, 84 and 1136.

The double carbonate method[243] is very suitable for the preparation of
pure lanthanum compounds after the removal of cerium. The mixture of
salts is added to a warm 50% solution of potassium carbonate, and to the
clear liquid, water is added gradually, with constant stirring. The
double carbonates of the most positive elements are the least soluble,
and are first thrown down, so that the precipitate is rich in lanthanum;
it is collected and washed with a 25% potassium carbonate solution, and
the process repeated. A few repetitions suffice to separate lanthanum
completely from the other members of the group. The method may also be
used for the purification of praseodymium salts.

  [243] Meyer, _Zeitsch. anorg. Chem._ 1904, ~41~, 94.


~Lanthanum~, La = 139·0

As the most electropositive element of the rare earth group, lanthanum
is the most similar in its chemical properties to the metals of the
alkaline earths. The _metal_ itself (see p. 115) oxidises even in dry
air, and in moist air rapidly becomes coated with a white layer of
hydroxide; it attacks water, and burns vigorously when heated in the
air. An alloy with aluminium, of the formula LaAl₄, has been prepared by
Muthmann and Beck[244]; it forms lustrous white crystals, very stable in
the air and very resistant towards acids.

  [244] _Annalen_, 1904, ~331~, 46.

The _hydroxide_ is of interest from the fact that, if precipitated under
suitable conditions, it has the power of taking up solid iodine to form
a deep blue adsorption compound[245]; colloidal solutions of basic
lanthanum acetate are also coloured blue by addition of a few drops of
iodine solution. If precipitation with alkali be carried out in presence
of hydrogen peroxide, an hydrated _peroxide_ of the composition
La₂O₅,_n_H₂O is obtained.[246] This compound partially decomposes with
evolution of oxygen at ordinary temperatures; towards carbon dioxide and
acids it acts as a true peroxide, with formation of hydrogen peroxide.

  [245] Damour, _Compt. rend._ 1857, ~43~, 976; see also Biltz, _Ber._
  1904, ~37~, 719

  [246] Melikoff and Pissarjewski, _Zeitsch. anorg. Chem._ 1899, ~21~,
  70.

The _oxide_ is colourless, and forms colourless salts with those acids
in which the anion is not coloured. The oxide is distinguished from the
other rare earth oxides in that it turns moistened litmus paper blue; it
resembles lime, in hissing when slaked, absorbing carbon dioxide from
the air, and liberating ammonia from ammonium salts. By fusion with
alkali carbonates, and by digestion with concentrated alkali hydroxides,
Baskerville and Catlett[247] claim to have obtained lanthanates and
metalanthanates, but their work has not yet been confirmed.

  [247] _J. Amer. Chem. Soc._ 1904, ~26~, 75.

The _sulphate_, La₂(SO₄)₃,9H₂O, is the least soluble of all the rare
earth sulphates. The enneahydrate is the only form stable at ordinary
temperatures,[248] though under special conditions, hydrates with 6 and
with 16 molecules of water of crystallisation have been obtained. It
separates in needles belonging to the hexagonal system; 100 parts of
water dissolve at 0°, 3·01, and at 100°, 0·69 parts of the salt. The
_acetylacetone compound_ melts at 185°.

  [248] Muthmann and Rölig, _Ber._ 1898, ~31~, 1718.

A large number of other lanthanum compounds have been prepared, but
these are so typical of the rare earth salts generally that no detailed
treatment is required; for a full account of them, the reader is
referred to Abegg’s classical handbook.


~Atomic Weight.~--A large number of determinations of this constant have
been made, but the results even of recent investigations do not agree so
closely as might be desired. The value adopted by the International
Committee, 139·0, is based on the work of Brauner and Pavliček,[249]
carried out in 1902. These authors give an account of all the
determinations made up to that date, with critical discussion of the
methods employed and the possible sources of error. The more important
investigations have been based on the ratio La₂O₃ : La₂(SO₄)₃, for the
determination of which the most stringent precautions must be taken. The
synthetic method has generally been employed, on account of the tenacity
with which the oxide clings to traces of sulphuric anhydride. In this
method, the total decomposition of the acid sulphate, and the protection
of the very hygroscopic sulphate, La₂(SO₄)₃, from atmospheric moisture,
constitute the chief difficulties. By this method, H. C. Jones[250] in
1902 obtained a result (138·76) considerably lower than the value found
by Brauner and Pavliček (_loc. cit._) A later research by Brill,[251]
who carried out a synthetic sulphate determination on a minute scale,
using a Nernst microbalance, gave the value 139·5, which, whilst
considerably higher than either of the other figures, shows that Brauner
and Pavliček’s number can hardly be too high.

  [249] _Trans. Chem. Soc._ 1902, ~81~, 1243.

  [250] _Amer. Chem. J._ 1902, ~28~, 23.

  [251] _Zeitsch. anorg. Chem._ 1906, ~47~, 464.


  ~Detection.~--Pure lanthanum compounds show no absorption in the
  visible region, and the pure oxide gives no cathode luminescence. The
  emission spectra show very characteristic lines in the violet and
  ultraviolet. The chief lines are:

  3949·27
  3988·69
  4238·55
  4333·98
  6250·14
  6262·52
  6394·46

  For arc spectra see Exner and Haschek; Eder and Valenta.[252]

  [252] _Sitzungsber. kaiserl. Akad. Wiss. Wien_, 1910, ~119~, IIa, 39.


~Praseodymium~, Pr = 140·6

This element occurs only in small quantities in the commoner rare earth
minerals, and its separation in the pure state is in consequence a
matter of very great difficulty. The salts and their solutions have a
characteristic green colour. The salts are derived from the sesquioxide,
Pr₂O₃, but a dioxide, PrO₂, and an intermediate oxide of uncertain
composition are known. The absorption spectrum has five absorption
bands, one of which coincides with a band in the absorption spectrum of
neodymium; this fact has been interpreted as an indication of the
non-elementary nature of both metals.[253] Difference in the absorption
spectra have been put forward by several workers as indicating the
complex nature of praseodymium, but an exhaustive examination by
Stahl[254] in 1909 showed that there is no reason to doubt that the
metal is really an element.

  [253] Auer von Welsbach, _Sitzungsber. kaiserl. Akad. Wiss. Wien_,
  1903, ~112~, II_a_, July; also Urbain, _Ann. Chim. Phys._ 1900, [vii],
  ~19~, 184.

  [254] _Le Radium_, 1909, ~6~, 215.

The _metal_ is prepared by electrolysis of the fused chloride; in order
to attain the temperature required to fuse the element, a very thin
cathode is employed; if too powerful a current be used, the dioxide is
formed. The metal is purified by remelting it in crucibles of magnesia,
under a layer of anhydrous barium chloride. It has a yellowish shade,
and is more stable in the air than lanthanum and cerium. For physical
properties, see p. 115. No alloys have been prepared.

The _hydroxide_ is thrown down by alkalies as a gelatinous green
precipitate; in the presence of hydrogen peroxide, an hydrated peroxide,
which closely resembles the corresponding lanthanum compound, is thrown
down.

The _Oxides_.--By ignition of salts of volatile acids, Auer von
Welsbach[255] obtained an oxide to which he assigned the formula Pr₄O₇.
More recent work[256] has shown that the composition of the oxide
obtained depends upon the conditions under which the various salts are
decomposed. By fusing the nitrate in presence of potassium nitrate at
400-450°C., Meyer obtained the dioxide, PrO₂; at higher temperatures
this decomposes, giving the intermediate oxides. The formation
of the dioxide is greatly influenced by the presence of other
oxides,[257]--ceric oxide, acting as an oxygen carrier, favouring
whilst the other oxides hinder. The pure dioxide is a brownish-black
powder, which resembles manganese dioxide, but is less stable. It
liberates halogens from the halogen acids, and oxidises manganese salts
to permanganates, but does not completely oxidise ferrous or stannous
salts, losing instead a part of its oxygen in the gaseous form. The
dioxide cannot be obtained in the wet way.

  [255] _Monats._ 1885, ~6~, 477.

  [256] See, _e.g._ Meyer, _Zeitsch. anorg. Chem._ 1904, ~41~, 94.

  [257] Brauner, _Monats._ 1882, ~3~, 1; Marc, _Ber._ 1902, ~35~, 2370;
  Meyer and Koss, _ibid._ 3470.

When heated in a stream of hydrogen, the dioxide yields the
_sesquioxide_, Pr₂O₃, as a greenish-yellow powder,which readily absorbs
oxygen from the air, becoming brown, with formation of the intermediate
oxide.

The _chloride_, PrCl₃,7H₂O, forms large green prisms, very readily
soluble in water; 100 parts of the solvent at 13° take up 334·2 parts of
the hydrated salt, the solution having the specific gravity 1·687. The
anhydrous chloride is a pale green deliquescent powder, which melts at a
red heat to a clear green liquid; ebullioscopic measurements show that
in alcoholic solution it has the simple molecular formula PrCl₃.

The _Bromate_, Pr(BrO₃)₃,9H₂O, has been obtained by James and
Langelier[258] by dissolving the oxide in aqueous bromic acid, and also
by double decomposition. It forms greenish hexagonal prisms, melting at
56·5°, and is readily soluble; 100 parts of water dissolve 190 parts of
this salt at 25°. At 100° it loses five molecules of water, forming the
tetrahydrate Pr(BrO₃)₃,4H₂O, which loses all its water at 130°. The
anhydrous salt begins to decompose at 150°.

  [258] _J. Amer. Chem. Soc._ 1909, ~31~, 913.

The _sulphate_ crystallises with 8 molecules of water of crystallisation
at ordinary temperatures, but hydrates with 15¹⁄₂, 12, and 5 molecules
of water respectively have been described. The octohydrate is
considerably more soluble than lanthanum sulphate enneahydrate. The
anhydrous salt is a bright green powder.

_Praseodymium acetylacetone_ melts at 146°.


~Atomic Weight.~--The value 140·6, adopted by the International
Committee, is based on the work of Jones, v. Scheele, Auer von
Welsbach, and Feit and Przibylla; the work of Brauner, however, points
consistently to a higher atomic weight. Most of these investigators have
used the sulphate method. The first determinations of von Welsbach for
the newly discovered element[259] gave the value 140·8 (see p. 179);
another series of determinations published in 1903[260] gave the mean
value 140·57. Jones[261] obtained the sesquioxide for the synthetic
sulphate operation by reduction of the peroxide in a current of
hydrogen; according to Brauner, this method gives an oxide which is not
perfectly pure, probably by absorption of water vapour and carbon
dioxide from the air. Jones’ mean value was 140·466. v. Scheele[262]
used the same method, as well as a combined oxalate-sulphate method; his
figures vary considerably, the mean value being 140·55. Feit and
Przibylla,[263] using their volumetric method, obtained the value
140·54.

  [259] _Monats._ 1885, ~6~, 477.

  [260] _Sitzungsber. kaiserl. Akad. Wiss. Wien_, 1903, ~112~, 1037.

  [261] _Amer. Chem. J._ 1898, ~20~, 345.

  [262] _Zeitsch. anorg. Chem._ 1898, ~17~, 310.

  [263] _Zeitsch. anorg. Chem._ 1906, ~50~, 249.

Brauner’s earlier work,[264] carried out in 1898, gave the value 140·95.
In 1901 this author[265] carried out an extensive research on the atomic
weight of praseodymium, employing four different methods with
spectroscopically pure material; the mean value of his very concordant
results was 140·97, almost the value he obtained in 1901. A further
investigation into the value of this constant appears desirable.

  [264] _Proc. Chem. Soc._ 1898, ~14~, 70.

  [265] _Ibid._ 1901, ~17~, 65; see also Abegg, III, i. 263.


  ~Detection.~--The maxima of the absorption bands are given by
  Rech[266] as follows:

  [266] _Zeitsch. wiss. Photochem._ 1906, ~3~, 411.

  Yellow  596·4 and 588·2, weak.
  Blue    481·3 very intense.
          468·3 coincident with a neodymium band.
  Violet  444·2

  The arc spectrum is very rich in lines.[267] The most intense, which
  may be used also for identification, are the following:

  [267] Exner and Haschek; Bertram, _Zeitsch. wiss. Photochem._ 1906,
  ~3~, 16; Eder and Valenta, _Sitzungsber. kaiserl. Akad. Wiss. Wien_,
  1910, ~119~, II_a_, 65.

  4008·90
  4100·91
  4118·70
  4143·33
  4179·60
  4189·70
  4206·88
  4223·18
  4225·50
  4241·20
  4305·99
  4429·38
  4496·60
  4510·32


~Neodymium~, Nd = 144·3.

Neodymium is, after cerium, the commonest constituent of the cerium
group in the more important rare earth minerals, and its separation is
therefore by no means so difficult as that of praseodymium. The
compounds of the element obtained by von Welsbach in 1885 were not pure,
being admixed with samarium compounds which had not been completely
separated. Neodymium salts were first prepared free from samarium by
Demarçay[268] in 1898; they are of a violet-rose colour, and show in
solution a well-marked and characteristic absorption spectrum, the bands
being very numerous and sharply defined, and extending over the whole
optical region. In chemical as well as in physical and crystallographic
properties, they show an extremely close resemblance to the compounds of
praseodymium.

  [268] _Compt. rend._ 1898, ~126~, 1039.

On account of the high melting-point, the preparation of the _metal_
presents the same difficulties as that of praseodymium. A current of
90-100 ampères is employed at a potential difference of 15-22 volts;
this suffices to raise the thin carbon cathode to a bright white heat,
and to fuse the liberated metal. For the properties of the element, see
p. 115.

The _sesquioxide_, Nd₂O₃, when perfectly pure, has a light blue or lilac
colour, with a faint reddish fluorescence; the shade varies somewhat
according to the method of and temperature employed for the preparation.
A bluish or violet-red fluorescence is highly characteristic of the
salts, and is particularly noticeable if the powdered recrystallised
oxalate be viewed in a good light. The greyish or brownish colour of the
oxide observed by some authors is probably due to traces of
impurity.[269] The existence of higher oxides of the formulæ Nd₂O₄ and
Nd₂O₅ respectively, which Brauner[270] put forward, has been disputed
by other writers, though it is found[271] that in the presence of ceria
and praseodymia, the sesquioxide can take up more oxygen. Waegner[272]
claimed to have obtained the compound Nd₄O₇ by heating the oxalate in a
stream of oxygen, though his material, as well as that of Brauner,
contained praseodymia. More recently, Joye and Garnier[273] have shown
that the spectrum attributed by Waegner to the hypothetical Nd₄O₇ was in
reality that of an hydrated oxide, 2Nd₂O₃,2H₂O; these authors have also
prepared a second hydrated oxide of the formula 2Nd₂O₃,3H₂O.

  [269] See Waegner, _Zeitsch. anorg. Chem._ 1904, ~42~, 118; also
  Baxter and Chapin, _J. Amer. Chem._ Soc. 1911, ~33~, 1.

  [270] _Chem. News_, 1898, ~77~, 161; _ibid._ 1901, ~83~, 197.

  [271] See Meyer and Koss, _Ber._ 1902, ~35~, 3740; and Marc, _ibid._
  2370.

  [272] _Loc. cit._

  [273] _Compt. rend._ 1912, ~154~, 510.

The _chloride_, NdCl₃,6H₂O, is obtained by crystallisation from aqueous
solutions; it is also precipitated by addition of water to an alcoholic
solution. It forms large deliquescent rose-coloured crystals; 100 parts
of water at 13° dissolve 246·2 parts of the salt, the saturated solution
having the density 1·741; at 100°, 511·6 parts are dissolved. The
solution resembles those of the other chlorides of the group in that it
readily dissolves the rare earth oxalates. When heated in a current of
hydrogen chloride of 130°, the hexahydrate yields a monohydrate,
NdCl₃,H₂O; at 160° the anhydrous chloride is obtained as a very
deliquescent rose-coloured powder, which melts at a red heat to a clear
red liquid. The anhydrous chloride forms an additive compound
NdCl₃,12NH₃, when exposed to the action of ammonia at low
temperatures;[274] by gradually heating this, a large number of other
additive compounds are formed, containing smaller quantities of ammonia.

  [274] Matignon and Trannoy, _Compt. rend._ 1906, ~142~, 1042.

The anhydrous _iodide_, NdI₃, has been obtained[275] by passing hydrogen
iodide over the heated anhydrous chloride, and also by heating the
carbide in iodine vapour. It fuses to a black liquid, which at a higher
temperature suddenly becomes transparent.

  [275] Matignon, _ibid._ 1905, ~140~, 1637.

The _bromate_, Nd(BrO₃)₃,9H₂O, which is exactly similar to the analogous
compound of praseodymium, forms rose-coloured hexagonal prisms, melting
at 66·7°.

The _sulphate_, Nd₂(SO₄)₃,8H₂O, is isomorphous with the corresponding
salt of praseodymium, but is considerably less soluble. Only the one
hydrate is known.

The _nitrates_ show an interesting case of isomorphism with the
corresponding bismuth nitrate hydrates.[276] The stable form of the
neodymium salt is the hexahydrate, Nd(NO₃)₃,6H₂O, whilst the
pentahydrate, Nd(NO₃)₃,5H₂O, is labile. Of the bismuth salts, on the
other hand, the pentahydrate is stable whilst the hexahydrate is labile;
but mixed crystals of both pairs may be obtained, the stable neodymium
hexahydrate with the unstable bismuth compound, and the stable bismuth
pentahydrate with the labile neodymium salt.

  [276] Bodman, _Ber._ 1898, ~31~, 1237.

Many _double carbonates_ are obtained by dissolving the normal carbonate
in excess of the precipitant. The absorption spectra of these solutions,
which have a blue colour, are abnormal and very intense, and have been
suggested as a basis of quantitative estimation.[277]

  [277] Muthmann and Stutzel, _Ber._ 1899, ~32~, 2653.

The _acetylacetone derivative_ forms violet crystals, melting at
144°-145°.

A large number of _organic salts_ of neodymium have been prepared by
James, Hoben, and Robinson.[278]

  [278] _J. Amer. Chem. Soc._ 1912, ~34~, 276.


~Atomic Weight.~--The earlier determinations of this constant were
carried out by the sulphate method, the synthetic process being usually
employed. Auer von Welsbach, at the time of the discovery of
praseodymium and neodymium,[279] gave the values 143·6 and 140·8
respectively for their atomic weights. Brauner, who carried out a
determination in 1898,[280] showed that these numbers should be
interchanged, and gave the value 143·63 for neodymium. Boudouard,[281]
employing the analytical sulphate method, obtained the value 143·05,
whilst in the same year Jones[282] gave the value 143·6. A second
determination by Brauner[283] gave the value 143·89. All these values
are undoubtedly too low, the material being probably contaminated with
other earths.

  [279] _Loc. cit._

  [280] _Proc. Chem. Soc._ 1898, ~14~, 70.

  [281] _Compt. rend._ 1898, ~126~, 900.

  [282] _Amer. Chem. J._ 1898, ~20~, 345.

  [283] _Proc. Chem. Soc._ 1901, ~17~, 66.

In his second determination in 1908, Auer von Welsbach[284] gave the
value 144·54 as the mean of three determinations. Feit and
Przibylla,[285] using their volumetric method, gave the value 144·52,
whilst Holmberg,[286] using material which he considered to have been
the purest obtained up to that time, obtained the figure 144·11. More
recently, Baxter and Chapin[287] have made determinations by treating
the chloride with pure silver nitrate, and weighing the precipitated
silver chloride, as well as by titration. The mean value obtained by the
first method--ratio NdCl₃  : 3AgCl--was 144·272 (extremes 144·250 and
144·298), and by the second method--ratio NdCl₃  : 3Ag--was 144·268
(extremes 144·249 and 144·283), giving the mean value for the whole
series of 144·270.

  [284] _Loc. cit._

  [285] _Zeitsch. anorg. Chem._ 1905, ~43~, 202; _ibid._ 1906, ~50~,
  249.

  [286] _Ibid._ 1907, ~53~, 124.

  [287] _Proc. Amer. Acad._ 1911, ~46~, 215.

The value adopted by the International Committee is 144·3.


  ~Detection.~--The absorption spectra of neodymium compounds have been
  examined by Demarçay, Forsling, von Welsbach, Rech, Schäfers, and
  Baxter and Chapin, with concordant results. The positions of the
  absorption maxima as given by Holmberg[288] from the measurements of
  Forsling are as follows, the weaker bands being omitted:

  [288] _Zeitsch. anorg. Chem._ 1907, ~53~, 83.

  677·5
  621·7
  578·5 } In concentrated solution
  575·4 } these give the
  573·5 } intense absorption
  571·6 } region in the yellow.
  532·3}
  521·6} In concentrated
  520·4} solution these
  512·4} give one intense
  508·7} band.
  474·5
  468·7
  461·0
  427·1

  The arc spectrum is given by Exner and Haschek, Bertram,[289] and
  Eder and Valenta.[290] The most intense lines are as follows:

  [289] _Zeitsch. wiss. Photochem._ 1906, ~3~, 16.

  [290] _Sitzungsber. kaiserl. Akad. Wiss. Wien_, 1910, ~119~, II_a_,
  554.

  3863·52
  3951·32
  4061·27
  4156·30
  4247·54
  4282·67
  4303·78
  4325·87
  4375·11
  4385·81
  4400·96
  4446·51
  4451·71
  4463·09
  4920·84
  5923·35
  5319·98
  5594·58
  5620·75
  6310·69
  6314·69
  6385·32


~Samarium~, Sa = 150·4

The samarium of the earlier chemists (see p. 168) contained a large
proportion of the terbium elements, from which a fairly complete
separation was first effected by Demarçay in 1900.[291] By the
fractional crystallisation of the double magnesium nitrate in presence
of bismuth magnesium nitrate, Urbain and Lacombe[292] succeeded in
preparing samarium compounds, which were shown by spectroscopic
examination[293] to be free from other earths. The element is
intermediate in electropositive character and in the solubility
relations of its salts between neodymium and the terbium earths; its
salts are topaz-yellow in colour, and in concentrated solutions show
absorption in the blue and violet regions. The oxide is almost white in
colour, with only a faint yellow tinge. A systematic investigation of
samarium compounds was carried out by Cleve,[294] but his work was
vitiated by the fact that his material was very impure. More recently,
the pure salts have been examined by Matignon and his pupils.

  [291] _Compt. rend._ 1900, ~130~, 1185.

  [292] _Ibid._ 1904, ~138~, 84 _and_ 1166.

  [293] Eberhard, _Zeitsch. anorg. Chem._ 1905, ~45~, 374.

  [294] _Trans. Chem. Soc._ 1883, ~43~, 362; _Bull. Soc. Chim._ 1885,
  [ii.], ~43~, 53; _Chem. News_, 1886, ~53~, 30, 45, 67, 80, 91, 100.

The melting-point of the _metal_ lies between 1300° and 1400°C., so that
its preparation by the electrolytic method is a matter of great
difficulty. A mixture of the chloride with one-third of its weight of
barium chloride is electrolysed by means of a current of 100 ampères,
using a cathode of only 2·5 mm. thickness; the metal so obtained is
greyish white in colour, and is the hardest of the cerium elements.

The _chloride_ separates from aqueous solution as the hexahydrate,
SaCl₃,H₂O, in large tabular yellow crystals. The anhydrous chloride is
white, but fuses to a chocolate-brown liquid; it forms a large number of
additive compounds with ammonia. When heated in an atmosphere of dry
hydrogen or ammonia, air and moisture being carefully excluded, it
yields the _subchloride_,[295] SaCl₂, as a dark brown crystalline solid,
insoluble in alcohol and all organic solvents. Samarous chloride
dissolves in water, forming a deep brownish-red solution, which rapidly
becomes colourless, with evolution of hydrogen, and precipitation of the
oxide and oxychloride. _Samarous iodide_, SaI₂, may be obtained by a
similar process, and closely resembles the chloride.

  [295] Matignon and Cazes, _Compt. rend._ 1906, ~142~, 83.

The _bromate_, Sa(BrO₃)₃,9H₂O, melts at 75°, and closely resembles the
corresponding compounds of the didymium metals. The _sulphate_
crystallises with 8, and the _nitrate_ with 6 molecules of water. The
_carbonate_, Sa₂(CO₃)₃,3H₂O, can be obtained only by passing carbon
dioxide through an aqueous suspension of the hydroxide; addition of
alkali carbonate to a solution of a samarium salt precipitates hydrated
double carbonates.

The _acetylacetone compound_ melts at 146°-147°C.

Many organic salts have been prepared by James, Hoben, and Robinson
(_loc. cit._).


~Atomic Weight.~--The earlier determinations of this constant were
carried out with material not entirely free from europium. Demarçay[296]
carried out a synthetic sulphate operation with the material which he
obtained free from europium in 1900, and found values between the limits
147·2 and 148·0. The International Committee has adopted the value
150·4, which is based on the work of Urbain and Lacombe[297] in 1904.
These authors made determinations of three series of ratios, obtained by
(_a_) conversion of sulphate octohydrate to anhydrous sulphate, (_b_)
conversion of anhydrous sulphate to oxide, and (_c_) conversion of
sulphate octohydrate to oxide; these gave the values 150·314, 150·533,
and 150·484 respectively, from which the mean atomic weight is
150·44.[298]

  [296] _Loc. cit._

  [297] _Compt. rend._ 1904, ~138~, 1166.

  [298] These numbers are calculated by Brauner (Abegg’s _Handbuch_,
  III. i. p. 285) on the basis O = 16, S = 32·06, H = 1·0076, and are
  somewhat higher than those given by Urbain and Lacombe, who used the
  round numbers O = 16, S = 32, and H = 1.


  ~Detection.~--The absorption spectrum of samarium compounds is only
  visible in fairly concentrated solutions, so that the element cannot
  usually be detected in a mixture by this means. The position of the
  maxima of the strongest bands (Demarçay, _loc. cit._) are:

  476
  463
  417
  402

  These are all in the blue and violet regions; the first and second are
  in the neighbourhood of neodymium and europium bands (_q.v._), and in
  concentrated solutions the bands would partially coincide. Since these
  are the two elements from which the separation is most difficult, and
  are moreover the most constant in their occurrence with samarium, the
  absorption spectrum is of very little use as a test.

  The arc spectrum is very rich in lines,[299] of which the most intense
  are:

  3739·30
  4152·38
  4203·18
  4225·48
  4229·83
  4236·88
  4256·54
  4319·12
  4329·21
  4334·32
  4347·95
  4391·03
  4420·72
  4421·32
  4424·55
  4434·07
  4434·52
  4452·92
  4454·84
  4458·70
  4467·50
  4519·80
  4524·08
  4544·12
  4566·38
  4577·88
  4642·41
  4674·79

  [299] Exner and Haschek; Eder and Valenta; Rütten and Mersch,
  _Zeitsch. wiss. Photochem._ 1905, ~3~, 181.


CHAPTER XIII

THE TERBIUM GROUP

In his examination of the yttria earths in 1842, Mosander described two
new oxides isolated from the old yttria. To one of these, an
orange-yellow earth which yielded colourless salts, he gave the name
Erbia; the second earth, which was colourless and gave rose-coloured
salts, he called Terbia. Bahr and Bunsen examined the yttria oxides in
1866, and obtained only the latter earth, which gave rose-coloured
salts; to this they applied Mosander’s name Erbia, and stated that the
earth to which Mosander had given that name had no existence.
Delafontaine, however, confirmed Mosander’s work, showing that the
orange-yellow earth which yielded colourless salts (Mosander’s Erbia)
had been fractionated out of their material by Bahr and Bunsen in the
double sulphate separation of the cerium group; to avoid further
confusion, however, he proposed to give to this oxide (Mosander’s Erbia)
the name Terbia, leaving for the colourless oxide, which forms
rose-coloured salts (which Mosander had called Terbia) the name Erbia
applied to it by Bahr and Bunsen. This reversed nomenclature has been
generally accepted.

Delafontaine,[300] continuing his work on the earths from samarskite
(see p. 168) announced in 1878 the discovery of a new oxide, Philippia,
intermediate between terbia and yttria; but this was subsequently shown
to be a mixture of yttria and terbia (see p. 133). In the same year,
Lawrence Smith[301] announced the discovery of another oxide, Mosandria,
from the samarskite earths; this was afterwards shown by Lecoq de
Boisbaudran to be a mixture of terbia with gadolinia.[302] In 1880
Marignac[303] announced the discovery of two more new oxides, Y_{α} and
Y_{β} from the same mineral; Y_{β} was afterwards found to be identical
with samaria, whilst Y_{α} was subsequently separated from the old
terbia earths by Lecoq de Boisbaudran, who proposed, with the assent of
Marignac, the name Gadolinium.[304] The terbia left after removal of the
erbia earths and gadolinia was believed by that author to be still a
mixture, a conclusion supported by the work of Hofmann and Kruss in
1893.[305]

  [300] _Compt. rend._ 1878, ~87~, 559.

  [301] _Ibid._ 1878, ~87~, 146.

  [302] _Ibid._ 1886, ~102~, 647.

  [303] _Compt. rend._ 1880, ~90~, 899.

  [304] _Loc. cit._

  [305] _Zeitsch. anorg. Chem._ 1893, ~4~, 27.

In 1886 Demarçay[306] isolated from samaria a new oxide, which he
designated S₁. From his work on this oxide in 1892-1893, de
Boisbaudran[307] concluded that samaria consisted of at least three
oxides, samaria proper, and two new oxides Z_{ξ} and Z_{ε}. In 1896,
Demarçay[308] separated an earth Σ, which showed the spark-spectrum of
Z_{ε} and the reversal spectrum of Z_{ξ}, and finally in 1901[309] he
obtained the new oxide in a fairly pure condition, and gave it the name
Europia.

  [306] _Compt. rend._ 1886, ~102~, 1551.

  [307] _Ibid._ 1892, ~114~, 575; _ibid._ 1893, ~116~, 611 and 674.

  [308] _Ibid._ 1896, ~122~, 728.

  [309] _Ibid._ 1901, ~132~, 1484.

The complicated history of the terbium group has been entirely cleared
up by the work of Urbain and his co-workers during the early years of
the present century, and processes have been devised by which the
separation of the three members of the group from one another, and from
the related elements of the erbium group on the one side, and samarium
on the other, can be satisfactorily accomplished. The chemistry of this
group, therefore, may be regarded as satisfactorily settled, though
relatively little is known of the properties of the elements and their
compounds.

In their general chemical relations, elements of the terbium group
occupy an intermediate position between the cerium group and the
elements of the yttrium group in the narrower sense. In the solubility
relations of the double salts, they are bounded on the one side by
samarium and the less soluble cerium group, on the other by dysprosium
and holmium and the more soluble yttrium group. They show only very
slight differences in electropositive character, and methods based on
differences in basic strength of the oxides, therefore, are of very
little use for separating them from one another. Fractional
precipitation with ammonia separates them in the order terbium,
samarium, gadolinium, and europium--samaria being less strongly basic
than the oxides of gadolinium and europium; this constitutes an
exception to the general rule regarding the solubilities of the double
nitrates and sulphates with increasing electropositive character.[310]
The difficulties of separation are greatly increased by the very small
proportions in which the elements are usually found in rare earth
minerals. Gadolinium usually occurs in the largest quantities; in
consequence of this, there is little doubt that most of the material
described by the earlier workers as terbia consisted very largely of
gadolinia.

  [310] See Lecoq de Boisbaudran, _Compt. rend._ 1890, ~111~, 394.

The group is not characterised by well-marked absorption spectra;
europium and terbium show weak absorption in the blue region. Terbium,
of which the salts are colourless, forms a very strongly coloured
peroxide, analogous to that of praseodymium; small quantities of this
give to the mixed oxides obtained by ignition the characteristic yellow
colour, whilst mixtures richer in the peroxide become correspondingly
darker and darker.


SEPARATION

In the double sulphate separation of the yttrium and cerium groups, the
terbium elements divide themselves between the soluble and the insoluble
portions; if the separation is made as complete as possible by addition
of a large excess of alkali sulphate under suitable conditions, the
larger part of the compounds of the group will be precipitated with the
cerium elements. In the separation of the cerium elements the terbium
elements collect in the most soluble fractions, and the mother-liquors
of the double nitrate crystallisations therefore form a very convenient
source of these elements. A considerable proportion, however, will
usually remain in solution with the double sulphates of the yttrium
group; in the bromate separation of these (see p. 198), the terbium
elements collect in the least soluble fractions. By careful
fractionation under suitable conditions, the double sulphate method may
be used to separate the terbium group completely from the cerium and
yttrium elements. A very convenient method of separating the terbium
group from a rare earth mixture is the ethylsulphate process of Urbain.
By fractional crystallisation of these salts from alcohol or water, the
separation into three groups can be satisfactorily accomplished.

For the separation of the terbium elements from one another, the nitrate
and double nitrate methods are most suitable. Samarium can readily be
separated by crystallisation of the double magnesium nitrates in
presence of bismuth magnesium nitrate; by continuing the fractionation,
europium magnesium nitrate can be separated in a pure state, as there is
a considerable difference between the solubility of this salt and the
corresponding compound of gadolinium;[311] the process, however, is
somewhat long and tedious. For the separation of gadolinium and terbium,
the double nitrates are converted into the simple nitrates, and these
fractionated from nitric acid in presence of bismuth nitrate. The
gadolinium nitrate separates before the bismuth nitrate, and may be
obtained fairly pure in this way, though the process is extremely
tedious, and several thousand recrystallisations are required.[312]
Terbium nitrate has almost the same solubility as bismuth nitrate, and
the two separate together in the middle fractions. The more soluble
nitrates of the erbia earths collect in the mother-liquors.

  [311] James (_J. Amer. Chem. Soc._ 1912, ~34~, 757) employs at this
  stage the fractional crystallisation of the double nickel nitrates.

  [312] See Urbain, _Compt. rend._ 1904, ~139~, 736.


~Europium~, Eu = 152·0

This element is one of the rarest of the whole group, and occurs only in
extremely small quantities. Monazite sand is said to contain about 0·002
per cent. of the oxide, though on account of the remarkable intensity of
some of the stronger lines in the arc spectrum, Eberhard[313] was able
to detect europium with ease in a mixture of rare earth oxides from
that mineral, after the separation of cerium. The _oxide_ has a pale
rose colour; the salts are also faintly coloured, and in solution show
weak absorption bands.

  [313] _Zeitsch. anorg. Chem._ 1905, ~45~, 378.

_Europium sulphate_, Eu₂(SO₄)₃,8H₂O, separates in pink crystals, which
are completely dehydrated at 375°; _europic chloride_, EuCl₃, in the
anhydrous state forms fine yellow needles; _europium oxychloride_,
EuOCl, prepared by heating europic chloride in dry air to 600°, is a
white solid, insoluble in water, but soluble in strong acids; _europous
chloride_, EuCl₂, prepared by reduction of the higher chloride in
hydrogen, is a white amorphous solid, soluble in water to a neutral
solution, which on boiling throws down the oxide, Eu₂O₃.[314] Several
organic salts have been prepared by James and Robinson.[315]

  [314] Urbain and Bourion, _Compt. rend._ 1911, ~153~, 1155.

  [315] _J. Amer. Chem. Soc._ 1913, ~35~, 754.


~Atomic Weight.~--Using the material isolated from samaria,
Demarçay[316] in 1900, by the synthetic sulphate method, found the
atomic weight of europium to be about 151. Urbain and Lacombe[317]
determined the value in 1904, with material free from gadolinium and
samarium, using the three ratios which they employed in the case of the
latter element (see p. 182); their values, corrected by Brauner, were
152·00, 151·93 and 151·94 respectively. Another series of determinations
was carried out by Jantsch[318] in 1908, the same method being employed;
he obtained the mean value 152·03, with an error of ±·02. The
International Committee have adopted the value 152·0.

  [316] _Compt. rend._ 1900, ~130~, 1469.

  [317] _Ibid._ 1904, ~138~, 627.

  [318] _Ibid._ 1908, ~146~, 473.


  ~Detection.~--The absorption spectrum was determined by Demarçay,[319]
  but is not sufficiently intense or characteristic for ordinary
  purposes of detection. The spark spectrum has been investigated by the
  same author (_loc. cit._); it is very bright, and shows the three blue
  rays which characterised Lecoq de Boisbaudran’s Z_{ε}. The reversal
  spectrum shows the characteristic band of Z_{ξ}.

  The pure oxide, according to Urbain,[320] shows no luminescence under
  the influence of cathode rays, but when impure, or very largely
  diluted with lime or gypsum, it gives very bright and characteristic
  spectra.

  [319] _Ibid._ 1900, ~130~, 469.

  [320] _Ibid._, 1906, ~142~, 205, 1518.

  The arc spectrum[321] is very characteristic, and contains some
  exceedingly intense lines, by means of which Lunt[322] has detected
  europium in the sun and in many stars. The lines most suited for
  identification of the element are the following:

  [321] Exner and Haschek; Eder and Valenta, _Sitzungsber. kaiserl.
  Akad. Wiss. Wien_, 1910, ~119~, II_a_, 31.

  [322] _Proc. Roy. Soc._ 1907, ~79~; A, 118.

  3688·57
  3725·10
  3819·80
  3907·28
  3930·66
  3972·16
  4129·90
  4205·20
  4435·75
  4522·76
  4594·27
  4627·47
  4662·10
  6645·44


~Gadolinium~, Gd = 157·3.

Gadolinia is the commonest of the terbia oxides, and occurs in
considerable quantities in some of the rare earth minerals, notably in
samarskite and gadolinite; its separation from the neighbouring oxides,
europia and terbia, is, however, exceedingly difficult, and has only
been satisfactorily accomplished in recent times. The gadolinium
compounds prepared and examined by the earlier workers, as appears from
the atomic weight determinations, must have been associated with earths
of lower atomic weight, and undoubtedly also with small quantities of
terbium. After the isolation of Marignac’s Y_{α}, and the examination of
the element by Lecoq de Boisbaudran, to whom the name gadolinium is due,
further investigations were carried out by Bettendorff[323] and by
Benedicts.[324] Pure gadolinia was probably first obtained by
Demarçay,[325] by fractional crystallisation of the magnesium double
nitrate; the oxide obtained by Urbain and Lacombe[326] by
crystallisation of the nitrates in presence of bismuth nitrate, was
proved to be spectroscopically pure by Eberhard.[327]

  [323] _Annalen_, 1892, ~270~, 376.

  [324] _Zeitsch. anorg. Chem._ 1900, ~22~, 393.

  [325] _Compt. rend._ 1900, ~131~, 343; _ibid._ 1901, ~132~, 1484.

  [326] _Ibid._ 1905, ~140~, 583, etc.

  [327] _Zeitsch. anorg. Chem._ 1905, ~54~, 374.

The gadolinia obtained by ignition of the salts of volatile acids should
be perfectly white; presence of terbia causes it to assume a yellow
colour.[328] The salts are colourless, and their solutions show no
absorption in the visible region, though Urbain[329] has shown that
there are four strong bands in the ultraviolet.

  [328] Eberhard (_loc. cit._) has shown that even in the perfectly
  white oxide, traces of terbia can be distinguished by spectroscopic
  examination.

  [329] _Compt. rend._ 1905, ~140~, 1233.

The _hydroxide_, Gd(OH)₃, is a gelatinous precipitate with strongly
basic properties, rapidly absorbing carbon dioxide from the air. The
_oxide_, Gd₂O₃, also absorbs carbonic anhydride from the air, and is
easily soluble in acids, even after strong ignition. The element is
therefore strongly electropositive. Its position among the yttrium
elements, however, is justified by the properties of the
_platinocyanide_, 2Gd(CN)₃,3Pt(CN)₂,18H₂O, which forms long, pointed red
crystals, with a green metallic lustre, belonging to the rhombic system,
and isomorphous with the corresponding yttrium and erbium salts; the
cerium elements, on the other hand, give yellow platinocyanides, with a
blue metallic lustre, which crystallise in the monoclinic system.

The _nitrate_, Gd(NO₃)₃,6H₂O, separates from aqueous solutions at the
ordinary temperatures in large crystals belonging to the anorthic
system, and is isomorphous with the corresponding compounds of
praseodymium and neodymium.[330] From solutions in strong nitric acid, a
pentahydrate is obtained, which melts at 92°; the hexahydrate melts at
91°. The _sulphate_ separates from aqueous solution as the octohydrate,
Gd₂(SO₄)₃,8H₂O, isomorphous with the corresponding salts of both groups.
The anhydrous sulphate is much less soluble in water at 0° than the
corresponding compounds of the cerium elements. The _selenate_ forms
hydrates with 10 and 8 molecules of water of crystallisation
respectively; these are isomorphous with the corresponding selenates of
yttrium and the erbium metals.

  [330] Lang and Haitinger, _Annalen_, 1907, ~351~, 450.


~Atomic Weight.~--The determinations of this constant made by the
earlier workers were all carried out with impure material and gave
results which were considerably too low. The International Committee
have adopted the value 157·3, which is based on the work of Urbain.[331]
In employing the analytical sulphate method, that author observed that
the anhydrous sulphate did not remain constant in weight when allowed to
remain in a desiccator, and that it could not be accurately weighed. He
therefore determined the ratio Gd₂(SO₄)₃,8H₂O : Gd₂O₃, by converting the
octohydrate directly to oxide, and obtained the mean value 157·24.

  [331] _Compt. rend._ 1905, ~140~, 583.


  ~Detection.~--Pure gadolinium compounds show no absorption in the
  visible spectrum, but there are four strong bands[332] in the
  ultraviolet, viz. 311·6-310·5; 306·0-305·7; 305·6-305·5; and
  305·4-305·0. The arc spectrum[333] is very rich in lines, of which the
  most intense are the following:

  [332] Urbain, _ibid._ 1905, ~140~, 1233.

  [333] Exner and Haschek; Eder and Valenta, _Sitzungsber. kaiserl.
  Akad. Wiss. Wien_, 1910, ~119~, II_a_, 21.

  3082·15
  3100·66
  3422·62
  3545·94
  3549·52
  3585·12
  3646·36
  3671·39
  3719·63
  3743·68
  3768·60
  3796·62
  3814·18
  3852·65
  3916·70
  4037·49
  4050·05
  4063·62
  4070·51
  4073·99
  4085·73
  4098·80
  4130·59
  4184·48
  4251·90
  4262·24
  4325·83
  4327·29
  4342·35
  6114·26

  The spark spectra have been examined by Demarçay,[334] Baur and
  Marc,[335] Urbain[336] and Crookes.[337]

  [334] _Compt. rend._ 1900, ~131~, 343.

  [335] _Ber._ 1901, ~34~, 2460.

  [336] _Loc. cit._

  [337] _Proc. Roy. Soc._ 1905, ~74~, 420.


~Terbium~, Tb = 159·2

Terbia occurs among the rare earth oxides in exceedingly small
quantities, and its separation has in consequence presented such great
difficulties that only within the last few years have terbium compounds
been completely freed from gadolinium and neighbouring elements. In 1886
Lecoq de Boisbaudran,[338] by fractional precipitation of the hydroxides
with ammonia, and subsequent fractional crystallisation of the double
sulphates, obtained an oxide much richer in terbia than any specimen
previously prepared; it was dark yellow in colour. In 1902 Marc[339]
obtained from monazite a very dark oxide containing about 15 per cent.
of terbia, whilst Feit[340] in 1905 obtained a dark brown oxide
consisting of gadolinia with about 13 per cent. of terbia. Pure terbium
compounds were obtained by Urbain in 1904,[341] by fractional
crystallisation of the nitrate from nitric acid, in presence of bismuth
nitrate, and by crystallisation of the double nickel nitrates, and
precipitation with ammonia; he showed that the element was identical
with the Z_{δ} and Z_{β} of de Boisbaudran,[342] with the Γ of
Demarçay,[343] and with the G_{β} and possibly the G_{ζ} of Crookes[344]
(see p. 193).

  [338] _Compt. rend._ 1886, ~102~, 395, 483.

  [339] _Ber._ 1902, ~35~, 2382.

  [340] _Zeitsch. anorg. Chem._ 1905, ~43~, 267.

  [341] _Compt. rend._ 1904, ~139~, 736; 1905, ~141~, 521; 1909, ~149~,
  37.

  [342] _Ibid._ 1895, ~121~, 709; 1904, ~139~, 1015.

  [343] _Ibid._ 1900, ~131~, 343.

  [344] _Trans. Chem. Soc._ 1889, ~55~, 258.

The element gives the white _sesquioxide_, Tb₂O₃, and colourless
salts.[345] The _peroxide_, of which the composition corresponds
approximately to the formula Tb₄O₇, is obtained as a brownish-black
powder by ignition of suitable salts. Its presence, even in small
quantities, gives so deep a colouration to the other earths that some
kind of salt formation seems probable. It is insoluble in cold acids; it
dissolves in hot nitric acid with evolution of oxygen, forming a
solution from which the _nitrate_, Tb(NO₃)₃,6H₂O, melting at 89·3°,
separates on cooling. In hot hydrochloric acid, the peroxide dissolves
with evolution of chlorine, forming solutions from which the _chloride_,
TbCl₃,6H₂O, can be isolated with difficulty; this salt is extremely
deliquescent, and easily forms supersaturated solutions. The _sulphate_,
Tb₂(SO₄)₃,8H₂O, can be precipitated from a sulphuric acid solution of
the oxide by addition of considerable quantities of alcohol; it is
isomorphous with the other sulphate octohydrates, and is completely
dehydrated at 360°.

  [345] The terbium compounds here described have been prepared by
  Urbain (_loc. cit._) from carefully purified material; other compounds
  have been described by Potratz (_Chem. News_, 1905, ~92~, 3), but her
  material contained a large proportion of gadolinium.


~Atomic Weight.~--The value adopted by the International Committee is
159·2, which was obtained by Urbain in 1905 (_loc. cit._) from the
ratio Tb₂(SO₄)₃,8H₂O : Tb₂(SO₄)₃. This is the only determination on
which reliance can be placed, as the material of the earlier workers was
seldom even approximately pure.


  ~Detection.~--Solutions of terbium salts show only one band in the
  visible spectrum, at 487·7 in the blue. This band was observed by
  Lecoq de Boisbaudran in a specimen of terbia containing dysprosia, and
  assumed by him to belong to a new element, Z_{δ} (_loc. cit._) In the
  ultraviolet nine absorption bands have been observed (Urbain, _loc.
  cit._)

  The spark spectrum shows the lines observed by Demarçay in 1900, and
  attributed by him to the new element Γ. Lecoq de Boisbaudran’s element
  Z_{β} showed a green fluorescence with the reversed spark, a
  phenomenon which Urbain has found to be exhibited by pure terbium
  compounds.

  The arc spectrum of Urbain’s pure terbia was examined by
  Eberhard[346]--see also Exner and Haschek, and Eder and Valenta.[347]
  The element may be detected in minerals and earth mixtures by the
  following lines:

  3523·82
  3676·52
  3703·05
  3704·01
  4005·62
  4278·71

  The chief lines in the arc spectrum (Exner and Haschek) are the
  following:

  3324·53
  3509·34
  3531·86
  3561·90
  3568·69
  3600·60
  3628·53
  3650·60
  3659·02
  3704·10
  3711·91
  3848·90
  3874·33
  3899·34
  3925·60
  3939·75
  3977·01
  3982·07
  4005·70
  4012·99
  4278·70
  4752·69

  Pure terbia does not exhibit the phenomenon of cathode luminescence,
  but gadolinia containing a trace of terbia shows a marked green
  fluorescence, which was attributed by Crookes to a new Meta-element,
  G_{β}. A trace of terbia in aluminium oxide causes the latter to
  exhibit a highly characteristic intense white luminescence.

  [346] _Sitzungsber. königl. Akad. Wiss. Berlin_, 1906, ~18~, 385.

  [347] _Sitzungsber. kaiserl. Akad. Wiss. Wien_, 1910, ~119~, II_a_,
  14.


CHAPTER XIV

THE ERBIUM AND YTTERBIUM GROUPS--YTTRIUM AND SCANDIUM

In his examination of the ‘Yttria’ of Gadolin and Ekeberg, during the
years 1839 to 1843, Mosander, by methods based on differences in
strength of the oxides as bases, separated the earth into three new
oxides, yttria proper, the most strongly basic, terbia, intermediate in
strength, and erbia,[348] the least basic. No further separation was
effected until 1878, when Marignac, by fractional decomposition of the
nitrates, separated from erbia a new oxide, for which he proposed the
name Ytterbia; the new oxide was the least basic of the erbia earths. In
the following year, Nilson[349] isolated from ytterbia a still less
basic oxide, by the same method; he proposed the name Scandia, to recall
the fact that it occurred in gadolinite and euxenite, which up to that
time had been found only in Scandinavia. In 1879 also, Soret[350]
announced the discovery of a new element X, evidence for the existence
of which he had obtained during a spectroscopic examination of a mixture
of erbia and terbia earths; the oxide of X was isolated in the same year
by Cleve[351] from the old erbia, by fractional decomposition of the
nitrates, and the name Holmium, from the town of Stockholm, was proposed
for the new element. The same investigation led to the discovery of
Thulium, which derives its name from Thule, an old name for Scandinavia.

  [348] The reversed nomenclature of Delafontaine is here employed (see
  p. 184).

  [349] _Compt. rend._ 1879, ~88~, 642, 645.

  [350] _Ibid._ 1879, ~89~, 521.

  [351] _Ibid._ 1879, ~89~, 478, 708.

Lecoq de Boisbaudran[352] in 1886 showed Cleve’s Holmia to be a mixture
of at least two oxides; he retained the name Holmium for the element
which gave the most characteristic absorption bands of the old holmium,
and proposed the name Dysprosium (from δυσπροσιτος, difficult of access)
for the second element. The name Erbia was retained for the oxide
remaining after the removal of holmia, thulia, and dysprosia from the
old erbia; the homogeneity of this erbia has been called in question,
but is now fairly firmly established. The individuality of
dysprosium[353] and holmium[354] may also be regarded as definitely
established; that of thulium remains doubtful (see p. 204).

  [352] _Ibid._ 1886, ~102~, 1003, 1005.

  [353] Urbain, _Compt. rend._ 1906, ~142~, 785.

  [354] Holmberg, _Zeitsch. anorg. Chem._ 1911, ~71~, 226.

The homogeneity of ytterbia was questioned by Auer von Welsbach[355] in
1906; by fractionation of the ammonium double oxalates, that author
isolated the oxides of two new elements, for which he proposed the names
Aldebaranium and Cassiopeium. By fractionation of the nitrates from
nitric acid solution, Urbain[356] arrived at the same result, and
proposed the names Ytterbium (Neoytterbium) and Lutecium, which have
been adopted by the International Committee. The latter author,
employing the same method in the fractionation of the gadolinite earths,
has recently obtained very strong evidence of the existence in this
group of another element, for which he proposes the name Celtium;[357]
the discovery, however, awaits confirmation.

  [355] _Monats._ 1906, ~27~, 935; 1908, ~29~, 121.

  [356] _Compt. rend._ 1907, ~145~, 759.

  [357] _Ibid._ 1911, ~152~, 141.


SEPARATION

In the separation of the yttrium elements, methods based on differences
in electropositive character are of much greater importance than in the
separation of the cerium and terbium groups, and the method of nitrate
fusion has been very largely employed even in comparatively recent work.
This method, which was introduced by Berlin in 1860, has been of great
value in the separation of yttrium and the ytterbium elements from the
erbium group; it was employed in the isolation of ytterbium by Marignac,
and of scandium by Nilson.

If a concentrated solution of the nitrates be evaporated down, and the
syrupy residue subjected to gradually increasing temperature, the
nitrates of the ytterbium elements and scandium are converted first into
the basic nitrates; at somewhat higher temperatures the erbium salts are
decomposed, whilst yttrium nitrate and the nitrates of any cerium
elements present are the last to break up. If the mixture of basic and
neutral nitrates be dissolved in boiling water, the former, being less
soluble, crystallise out on cooling, and may be separated by this means,
the process being repeated with the filtrate containing the unchanged
nitrates. In this way, the weakly basic scandia and ytterbia quickly
collect in the first fractions, whilst the oxides of the erbia group are
easily separated from the more strongly basic yttria. The presence of
the intermediate terbium group renders the process much less easily
workable.

The process may be modified by raising the temperature to such an extent
that the soluble basic nitrates are converted into insoluble superbasic
nitrates, the temperatures at which this change occurs increasing from
element to element as the positive character becomes more marked; the
mixture of basic and superbasic salts is then extracted with dilute
nitric acid which leaves that latter undissolved and removes the more
positive elements in solution.

Fractional precipitation of the hydroxides by means of ammonia,
alkalies, or alkaline earths has also been frequently employed. A
modification of this process is the precipitation with aniline, carried
out by Kruss;[358] in this method, the solution of the chloride in warm
dilute alcohol is treated with an alcoholic solution of the organic
base. Another modification is the ‘Oxide process’ employed by Auer von
Welsbach[359] for the separation of the cerium elements, and by
Drossbach[360] in the yttrium group. The concentrated solution of the
mixed salts is thoroughly digested with the oxides obtained by
precipitating a fraction of the earths; the more strongly basic oxides
tend to displace the less basic, so that these accumulate in the
insoluble part. The solution is filtered from the undissolved oxides,
another fraction precipitated, and the oxides obtained from the
precipitate digested with the concentrated solution as before.

  [358] _Zeitsch. anorg. Chem._ 1893, ~3~, 108, 353.

  [359] _Monats._ 1883, ~4~, 630.

  [360] _Ber._ 1902, ~35~, 2826.

[Illustration: ~GROUP B~

YTTRIUM DOUBLE SUPLHATES.

                         Fractionate as Bromates.
                                  |||||
       +--------------------------+|||+-------------------------+
       |              +------------+|+------------+             |
       |              |             |             |             |
      ~1~            ~2~           ~3~           ~4~           ~5~
  Gd, Tb, Dy.    Tb, Dy, Ho,   Dy, Ho, Er,   Yt, Er, and   Tm, Yb, Lu,
                     Yt.           Yt.           Sc?           etc.
   For separa-  Transform to   Fractionate   Fractionate    Continue.
    tion of        Ethyl-       by Nitrate    by Nitrate     |||||
    Terbium      sulphates.      Fusion.       Fusion.       |||||
     group.--+      |||           ||            ||           |||||
  |  |  |    |      |||       +---+|            ||           |||||
  Terbium    |      |||       |    |            ||           |||||
  Group.     |      |||       |    |            ||+----------+||||
        +----+      |||       |    |+-----------+||       +---+|||
        |+----------+||       |    ||      +-----+|       |    ||+-----+
        ||     +-----+|       |    ||      |      |       |    |+-----+|
       ~6~     |     ~8~      |   ~10~     |    ~12~      |    ~14~   ||
     Tb, Dy.   |    Ho, Yt.   |  Yt, Er.   |    ~Tm.~     |    ~Lu.~  ||
    Continue   | Fractionate  | Continue.  |   Bromate.   |   Bromate.||
  Ethylsulphate| by Nitrate   |            |              |           ||
   Fractiona-  |   Fusion.    |            |              |      +----+|
      tion.    |       |      |            |              |      | +---+
               |       |      |            |              |      | |
              ~7~      |     ~9~          ~11~          ~13~     | |
             ~Dy.~     |    ~Yt.~         ~Er.~         ~Yb.~    | |
         Ethylsulphate.|  Nitrate.       Nitrate.      Bromate.  | |
                       |                               +---------+ +
                       +----------+                    |           |
                       |          |                   Yb, Lu.     Ct?
                       |          |                  Bromates.  Bromate.
                     ~Ho.~      Ho, Yt.
                 Basic Nitrate.

FIG. 9.--SEPARATION OF THE YTTRIUM ELEMENTS]

The more modern methods of separation combine the above processes with
the methods of fractional crystallisation, for which the bromates and
alkylsulphates of these elements are well adapted. The procedure[361]
which experience shows will lead to a fairly rapid separation is roughly
represented in Fig 9. The double sulphates (B), left in solution after
removal of the cerium and part of the terbium group, are transformed
into the bromates, which are separated by fractional crystallisation
into five main fractions. The least soluble portion, fraction 1,
contains the terbium elements with some dysprosium; in the fractionation
of the terbium group by means of the nitrates and double nitrates, the
dysprosium, with some terbium, collects in the final fractions (fraction
6). Fraction 2 contains terbium, dysprosium, holmium, and yttrium as the
bromates; these are converted into the anhydrous chlorides, from which,
by treatment with sodium ethylsulphate in alcoholic solution, the
ethylsulphates are obtained. By fractional crystallisation, dysprosium
may be obtained in a fairly pure condition (fraction 7), the least
soluble part (fraction 6) containing the terbium with some dysprosium.
Holmium and yttrium collect in the most soluble part (fraction 8), from
which pure holmium can be obtained by the method of nitrate fusion.
Fraction 3 contains yttrium and erbium, with small quantities of
dysprosium and holmium; the latter are readily separated by the nitrate
fusion, which will also allow of a fairly complete separation of yttrium
(fraction 9). Fraction 4 contains yttrium and erbium; scandium if
present will also collect here. Erbium can be obtained pure by the
nitrate fusion; the second fraction from this process contains both
yttrium and erbium, and may be further worked up with the fraction of
similar composition (fraction 10) from fraction 3.

  [361] James, _J. Amer. Chem. Soc._ 1912, ~34~, 757.

The mother-liquors from the bromate separation (fraction 5) contain
thulium and the ytterbium elements; the crystallisation is continued,
and allows of complete separation of thulium and ytterbium, and probably
of lutecium, though the most soluble fractions do not seem to have been
fully separated.


THE ERBIUM GROUP

The oxides of this group, as contrasted with the ytterbia oxides, give
rise to coloured salts, which in solution show definite absorption
spectra in the optical region; the spectrum of erbium salts is
particularly definite and characteristic. Erbium has among the yttrium
elements the place of neodymium among the cerium elements; after yttria,
erbia is the commonest oxide of the yttria group, though on account of
the difficulties of separation the chemistry of erbium is by no means so
complete and definite as that of neodymium. The oxides in order of
decreasing basicity, as shown by the order in which they are thrown down
by ammonia, are: dysprosia, holmia, erbia, thulia; the electropositive
character becomes weaker, therefore--as generally in the rare earth
series--as the atomic weight of the elements increases.


~Dysprosium~, Dy = 162·5

Compounds of this element were probably prepared in the pure state for
the first time by Urbain[362] in 1906, by the fractional crystallisation
of the ethylsulphate. He showed that after fourteen recrystallisations,
the absorption spectrum of the salts and the mean atomic weight of the
element remain unaltered, and that after removal of terbium by the very
efficient ethylsulphate method, all remaining traces of yttrium could be
rapidly removed by crystallisation of the nitrate. The salts have
generally a more or less pronounced yellow colour.

  [362] _Compt. rend._ 1906, ~142~, 785.

The _oxide_, Dy₂O₃, is a white powder which does not alter in
composition when strongly heated in reducing or oxidising atmospheres.
It is remarkable in that it is the most strongly paramagnetic oxide
known, having a coefficient of susceptibility much greater than that of
ferric oxide.[363] The _chloride_ crystallises with 6, the _sulphate_
with 8, and the _nitrate_ with 5 molecules of water of crystallisation.
The _bromate_, Dy(BrO₃)₃,9H₂O,[364] obtained by double decomposition,
melts at 78°. The _platinocyanide_, Dy₂[Pt(CN)₄]₃,21H₂O, forms bright
red cubic crystals, with greenish fluorescence.

  [363] _Compt. rend._ 1908, ~146~, 922.

  [364] Jantsch and Ohl, _Ber._ 1911, ~44~, 1274.

Several other salts are described by Urbain, and by Jantsch and Ohl
(_loc. cit._).


~Atomic Weight.~--Urbain and Demenitroux[365] determined this constant
from the ratio Dy₂(SO₄)₃,8H₂O : Dy₂O₃. The mean value of six
determinations carried out with material obtained by fractional
crystallisation of the nitrate was 162·52; with material purified by the
ethylsulphate crystallisation, the mean of six determinations gave the
value 162·54. The International Atomic Weight is 162·5.

  [365] _Compt. rend._ 1906, ~143~, 598.


  ~Detection.~--Lecoq de Boisbaudran[366] and Urbain[367] give the
  position of the following absorption maxima in the visible and
  ultraviolet regions respectively:

         ┌──────┴──────┐
  753     368·5   338
  475     379·5   332·5
  451·5   365
  427·5   351

  [366] _Ibid._ 1886, ~102~, 1003.

  [367] _Ibid._ 1906, ~142~, 785.

  The arc spectrum of Urbain’s material was examined by Eberhard,[368]
  who gives as most suitable for detection of the element in a mineral
  or oxide mixture the following lines:

  3385·16
  3531·86
  3536·17
  3645·54
  3898·69
  3944·83
  4000·59
  4078·11
  4187·00
  4211·82

  [368] _Publ. astrophys. Observ. Potsdam_, 1909, ~20~, No. 60.

  See also Exner and Haschek, and Eder and Valenta.[369]

  [369] _Sitzungsber. kaiserl. Akad. Wiss. Wien_, 1910, ~119~, II_a_, 9.

  The ultraviolet arc spectrum and the cathode phosphorescence have also
  been examined by Urbain.[370]

  [370] _Loc. cit._


~Holmium~, Ho = 163·5

The individuality of this element can hardly be regarded as perfectly
established, though Holmberg[371] has prepared salts which in solution
show only faint indications of erbium and dysprosium, when tested
spectroscopically. That author fractionated the yttrium elements
obtained from euxenite by a long process of separation, which involved
crystallisation of the _m_-nitrobenzenesulphonates, of the simple
nitrates (two series), of the double ammonium oxalates, and finally
fractional precipitation of the hydroxides by ammonia.

  [371] _Zeitsch. anorg. Chem._ 1911, ~71~, 226; see also Langlet,
  _Abstr. Chem. Soc._ 1907, ~92~, ii. 955.

He determined the _Atomic Weight_ as 163·5, which is the value accepted
by the International Committee, and mapped the absorption spectrum. The
_oxide_, Ho₂O₃, is a pale yellow powder; the _salts_ are yellow, with a
faint orange tinge.


~Erbium~, Er = 167·7

Although erbia was separated by Mosander seventy years ago, it is
doubtful if the perfectly pure oxide has ever been prepared. Whilst the
individuality of the element is well established, its homogeneity has
frequently been called in question. The name ‘Neo-Erbia’ was given by
Cleve[372] to the residue left after the separation from the old erbia
of ytterbia, scandia, thulia, and holmia (with which dysprosia (_q.v._)
was also separated), but the spectrum examination of Kruss and
Nilson[373] led them to regard Cleve’s oxide as still complex. Their
results, however, were explained by the work of Hofmann and his
pupils,[374] who consider erbia to be a homogeneous product; the
homogeneity of the element, therefore, may be considered as established,
though it would be strengthened by a more complete knowledge of the
neighbouring elements, holmium and thulium.

  [372] _Loc. cit._

  [373] _Ber._ 1887, ~20~, 2134.

  [374] _Ber._ 1908, ~41~, 308; also Hofmann, _ibid._ 1910, ~43~, 2631.

The element forms a rose-coloured oxide, and rose-coloured salts, which
give to the compounds of the mixed erbia earths their characteristic
colour. The oxide gives a very definite and characteristic reflection
spectrum, but the salts do not possess this property;[375] the
reflection spectrum remains unchanged in the presence of foreign oxides,
provided no combination occurs. From the atomic weight determinations,
it seems clear that the salts described by Cleve and his pupils[376]
were not pure erbium compounds; a few salts only appear to have been
recently obtained in the pure state for the atomic weight determination
(_q.v._).

  [375] See Kruss and Bugge, _Ber._ 1908, _41_, 3783.

  [376] See _Compt. rend._ 1880, ~91~, 381.

The _sulphate_ separates from aqueous solutions at ordinary temperatures
as the octohydrate, Er₂(SO₄)₃,8H₂O, which forms rose-coloured monoclinic
crystals isomorphous with the corresponding sulphates of the whole
group. The anhydrous sulphate is formed by long heating at 400°, more
quickly at 475°, and can be heated to 630° without decomposition. At
845° a basic salt, Er₂O₃,SO₃, is formed, which begins to decompose at
950°; at 1055° the transformation to the oxide is complete. The ammonium
and potassium double sulphates are easily soluble in cold water.

The _oxalate_ is thrown down in rosettes of bright rosy plates, which
according to Hofmann[377] have the formula Er₂(C₂O₄)₃,10H₂O, even when
dried in the air. Cleve believed the salt to be thrown down as the
enneahydrate. When kept _in vacuo_ over phosphoric anhydride, the
decahydrate passes into the trihydrate, which when heated decomposes,
passing into the oxide at a temperature of 575°. The _nitrate_,
Er(NO₃)₃,5H₂O, separates from aqueous solution as the pentahydrate, in
large stable red crystals. The _platinocyanide_, Er₂[Pt(CN)₄]₃,21H₂O,
has the characteristic red colour with green fluorescence. The
_formate_, Er(HCOO)₃--Cleve, _loc. cit._--is a red powder, obtained by
dissolving the oxide in formic acid; it crystallises from water as the
dihydrate.

  [377] _Loc. cit._


~Atomic Weight.~--The determinations of the earlier workers, being
carried out with impure material, gave results which differ very widely,
and are quite unreliable. Cleve’s value of 1880, for material free from
ytterbia, but not apparently free from earths of lower equivalent, was
166·25; Brauner,[378] using the same material in 1905, obtained the much
higher value 167·14. The determinations of Hofmann and Burger[379] in
1908 gave the mean value 167·38; with purer material, Hofmann in
1910[380] obtained the mean value 167·68, on which is based the value
accepted by the International Committee, 167·7.

  [378] Abegg, III. i. 318.

  [379] _Loc. cit._

  [380] _Loc. cit._


  ~Detection.~--Salts of erbium give in solution absorption spectra
  which are well defined and highly characteristic, though not so
  intense as those of praseodymium and neodymium. Hofmann and Bugge[381]
  give the following absorption maxima for a 10 per cent. solution of
  their pure nitrate in a layer of 15 mm. thickness:

  667 weak
  654 strong
  541 very weak
  523 very strong
  519 shadowy
  492
  487 strong
  450
  442 weak

  [381] _Ber._ 1908, ~41~, 3783.

  The arc spectrum has been mapped by Eder and Valenta[382] and Exner
  and Haschek. The following lines are used by Eberhard[383] for
  purposes of detection:

  3230·73
  3264·91
  3312·56
  3372·92
  3499·28
  3692·85
  3896·40
  3906·47
  3938·79

  [382] _Sitzungsber. kaiserl. Akad. Wiss. Wien_, 1910, ~119~, II_a_,
  18.

  [383] _Publ. astrophys. Observ. Potsdam_, 1909, ~20~, No. 60.


~Thulium~, Tm = 168·5

The thulia isolated in 1879 was described by Cleve[384] as pale rose in
colour; in the following year, having obtained it in larger quantity, he
found that it was white, and dissolved in acids to form colourless
solutions which showed absorption bands in the red and blue. The spectra
of the thulium compounds prepared by Cleve were examined by Thalèn,[385]
who concluded that a new element was certainly present, though it had
not been freed from ytterbium and erbium. Incidental observations on
the new oxide were made by various investigators, but no extensive
researches were carried out upon it until 1911, when James[386]
published an account of the separation and purification by the bromate
method, stating that after some 15,000 operations, his products remained
unaltered; he gives, however, no spectroscopic determinations, though
part of his material, spectroscopically examined by Sir William Crookes,
was described as ‘Very good thulium, with a trace of ytterbium.’ In the
same year Auer von Welsbach[387] published an account of a spectroscopic
investigation, as a result of which he concludes that thulium is a
mixture of at least three elements, of which the second, Tm II, agrees
fairly well in properties, so far as the two accounts allow of
comparison, with the thulium of James.

  [384] _Loc. cit._

  [385] _Compt. rend._ 1880, ~91~, 376.

  [386] J. _Amer. Chem. Soc._ 1911, ~33~, 1333.

  [387] _Zeitsch. anorg. Chem._ 1911, ~71~, 439.

Thulia is described by James as a dense white powder, with a greenish
tinge, which ‘emits a carmine coloured glow, when carefully made to
incandesce.’ The salts have a greenish tint, very susceptible to traces
of erbium; addition of erbium compounds turn the solution first
yellowish-green, then colourless, and finally pink. von Welsbach
describes Thulium II as forming an almost white sesquioxide, which, when
heated in the flame, gives a purplish light quickly succeeded by a
splendid characteristic glow; the salts are pale yellowish-green by
daylight, emerald-green by artificial light, the colour being almost
complementary to that of erbium salts. In solution, salts of Tm II give
the bands at 685 and 464 ascribed by James and other workers to thulium.

Until further researches on these interesting results are published, the
elementary nature of thulium cannot be considered definitely settled; it
appears probable, however, that homogeneous salts of a definite element
were obtained by James. The following salts are described by James
(_loc. cit._).

The _chloride_, TmCl₃,7H₂O, separates at ordinary temperatures from the
concentrated solution of the oxide in hydrochloric acid as greenish
crystals, very soluble in alcohol and water. The _bromate_,
Tm(BrO₃)₃,9H₂O, forms pale bluish-green hexagonal prisms, isomorphous
with the analogous salts of the group. The _sulphate_ and _nitrate_
separate as the octohydrates. The precipitated _oxalate_ has the formula
Tm₂(C₂O₄)₃,6H₂O, and is soluble in excess of alkali oxalate. The
_acetylacetone derivative_ was prepared by dissolving the precipitated
and well-washed hydroxide in alcoholic acetylacetone; it recrystallises
from absolute (?) alcohol as the dihydrate, Tm₂(C₅H₇O₂)₆,2H₂O. The
_phenoxyacetate_, Tm₂(C₆H₅·O·CH₂·COO)₆,6H₂O, was obtained in a similar
manner by addition of the hydroxide to a solution of phenoxyacetic acid
in dilute alcohol.


~Atomic Weight.~--Cleve gave the value 170·7 for this constant, but his
material was very impure. In a footnote to a paper published in 1907,
Urbain[388] pointed out that the value could not be above 168·5.
Analyses of the salts prepared by James agree fairly well with the
theoretical values calculated on this basis, but a systematic
determination with pure material has not yet been made. The
International Committee (1912) have adopted the value 168·5.

  [388] _Compt. rend._ 1907, ~145~, 760.


  ~Detection.~--The element can be detected in solution by its
  absorption spectrum, the most intense bands being in the neighbourhood
  of λ = 685, and λ = 464. For provisional arc spectra see Exner and
  Haschek, and for spark spectra Auer von Welsbach (loc. cit.) and Eder
  and Valenta.[389]

  [389] _Sitzungsber. kaiserl. Akad. Wiss. Wien_, 1910, ~119~, II_a_,
  103.


  ~Ytterbium~ (Neoytterbium, Aldebaranium), Yb = 172·0.
  ~Lutecium~ (Cassiopeium), Lu = 174·0.

The first indication of the complexity of Marignac’s Ytterbium was
furnished on spectroscopic grounds by Auer von Welsbach in 1905;[390] he
showed that a separation could be effected by the fractional
crystallisation of the ammonium double oxalates from concentrated
ammonium oxalate. Three years later[390] he published a full account of
his method, gave atomic weight determinations, and mapped the spectra of
the two new elements. In 1907, Urbain[391] independently effected a
separation by the fractional crystallisation of the nitrates from nitric
acid, and proposed the names Lutecium (from the old name for Paris) and
Neoytterbium for the elements.

  [390] See _Monats._ 1908, ~29~, 204.

  [391] _Compt. rend._ 1907, ~145~, 759.

The two new elements resemble one another so closely in chemical
properties that the account given by Astrid Cleve in 1902[392] of the
compounds of the old ytterbium applies in practically every detail to
the new elements. The oxides are white, and yield colourless salts,
showing in solution no absorption bands in the visible region.

  [392] _Zeitsch. anorg. Chem._ 1902, ~32~, 129.

The _oxides_, R₂O₃, though perfectly white, are coloured yellow or brown
by the faintest traces of thulium. They are attacked by acids only
slowly in the cold, but dissolve readily on warming; lutecia is slightly
the less strongly basic. The _chlorides_ crystallise with six molecules
of water, and are extremely soluble and deliquescent; when heated in a
stream of hydrogen chloride, they form oxychlorides of the type ROCl.
The _platinocyanides_ crystallise with 18 molecules of water, and have
the characteristic appearance of the analogous compounds of the yttrium
elements. The _sulphates_ crystallise at all temperatures as the normal
octohydrates, and are moderately easily soluble in water; conductivity
measurements show that they are partially hydrolysed in solution. The
_nitrates_ crystallise from concentrated aqueous or nitric acid
solutions as the tetrahydrates; by evaporation of the aqueous solutions
over sulphuric acid, the trihydrates are obtained. These compounds are
anomalous among the rare earth nitrates, by reason of their low water
content. The neutral _carbonates_ are thrown down by ammonium carbonate
as the tetrahydrates; if a stream of carbon dioxide be led into aqueous
suspension of the hydroxides, _basic carbonates_ of the formula
R(OH)CO₃,H₂O, are obtained. The _oxalates_ are precipitated as the
decahydrates; they are readily soluble in excess of alkali oxalate.

Many other salts of the old ytterbium have been prepared.


~Atomic Weights.~--The values determined by Urbain (_loc. cit._) for the
fractions obtained by the nitrate method gave the number 170·1 for the
least soluble fraction free from terbium, and 173·4 for the most soluble
fraction. Auer von Welsbach (_loc. cit._) obtained the values 172·9 and
174·2 for the least soluble and most soluble fractions from the double
oxalate crystallisation respectively. More recently[393] he has
determined these constants with highly purified material, employing a
modified method. The weighed anhydrous sulphates are transformed into
the oxalates, which are then ignited to the oxides. He obtained the
values Yb = 173·00, Lu = 175·00.

  [393] _Monats._ 1913, ~34~, 1713.

The values adopted by the International Committee are Yb = 172·0 and Lu
= 174·0.


~Spectra.~--The spark spectra are of more use in distinguishing the two
elements than the arc spectra. The spark spectrum of the old ytterbium
was mapped by Exner and Haschek,[394] and of the two compounds by both
discoverers (_loc. cit._). See also Eder and Valenta.[395]

  [394] _Sitzungsber. kaiserl. Akad. Wiss. Wien_, 1899, ~108~, II_a_,
  1123.

  [395] _Ibid._ 1910, ~119~, II_a_, 3.

The arc spectra have been mapped by Eder and Valenta (loc. cit.) and by
Exner and Haschek; the latter authors give as the most intense lines the
following:

     Yb                 Lu
           ┌────────────┴──────────────┐
  3031·26   2615·50   3397·21   4124·87
  3107·99   2911·53   3472·65   4184·40
  3289·50   3077·75   3507·57   4518·74
  3464·47   3198·27   3508·55   5476·88
  3988·16   3254·45   3554·58   5983·92
  5556·67   3281·89   3568·00   5984·32
            3312·30   3624·10   6222·10
            3359·74   3636·41   6463·40
            3376·69   3876·80


~Celtium~

The separation of Marignac’s ytterbium into the two elements described
above was accomplished by Urbain with the yttria earths extracted from
xenotime. In carrying out the same process with the ytterbia earths
from gadolinite, that author[396] obtained from the mother-liquor an
earth for which the coefficient of magnetisation was found to be 4·1 ×
10⁻⁶; lutecia has a coefficient three to four times as great. A
spectroscopic examination revealed the presence of lines which did not
correspond with those of any known body, and Urbain considered that a
new element, for which he proposed the name Celtium, with the symbol Ct,
must be present. Lutecia from xenotime shows no trace of the new
element.

  [396] _Compt. rend._ 1911, ~152~, 141.

Spectroscopic evidence for the existence of a third ytterbium element
had previously been brought forward by Auer von Welsbach[397] and also
by Exner and Haschek.[398]

  [397] _Monats._ 1908, ~29~, 204.

  [398] Exner and Haschek, _Sitzungsber. kaiserl. Akad. Wiss. Wien_,
  1910, ~119~, II_a_, 771.

The new element appears to be intermediate between lutecium and
scandium, and therefore may be expected to have a higher atomic weight
than the former element. Its chloride is more volatile than that of
lutecium, less volatile than that of scandium; its hydroxide is more
feebly basic than that of lutecium, but more strongly basic than that of
scandium.

Urbain (_loc. cit._) gives the following as the principal lines in the
spectrum; strong lines are denoted by a single, very strong by a double,
asterisk:

  2459·4
  2469·3
  2481·6 *
  2536·9 *
  2677·7
  2685·2 **
  2729·1 *
  2737·9
  2765·8 **
  2834·3 *
  2837·3 *
  2845·2 *
  2870·2
  2885·1 *
  2903·9 *
  2931·9
  2949·5 *
  3080·7 **
  3118·6 **
  3171·4 *
  3197·9 **
  3326·0 *
  3391·5 *
  3665·6


~Yttrium~, Yt = 89·0

Since the separation of yttria proper from the old yttria earths by
Mosander, in 1842, the individuality of yttrium has been well
established. The yttria of the workers of the sixties and seventies, to
judge from the atomic weight determinations, must have been very impure,
but no doubts were raised as to its homogeneity. By examination of the
cathode luminescence spectra, Crookes[399] concluded that the oxide was
of a complex nature; Lecoq de Boisbaudran, however, showed that the
phenomena observed by Crookes were due to traces of impurity in his
material, a conclusion confirmed by the work of Baur and Marc.[400]

  [399] _Trans. Chem. Soc._ 1889, ~55~, 255.

  [400] _Ber._ 1901, ~34~, 2460.

The oxide is the most strongly basic of all the yttria earths; in the
basicity methods of separation, therefore, it collects in the end
fractions, and is easily separated from the erbia and ytterbia earths by
the nitrate fusion and similar processes. The terbia earths, however,
which are comparable to it in basic strength, cannot be easily separated
by such methods; processes of fractional crystallisation are very
convenient in this case, since yttrium falls, with regard to the
solubility of its simple salts, among the erbium group--between holmium
and erbium generally--which is easily separated from the less soluble
terbium elements. The separation of yttrium, therefore, affords an
example of the combination of methods of both kinds.

The methods for the separation and purification of yttrium have recently
been exhaustively examined by Meyer and Wuorinen.[401] They consider the
chromate method suitable only if the terbium elements have already been
removed. The ethylsulphate method is said to be tedious, whilst the
ferrocyanide method indeed effects very rapid concentration, but with
great loss. For purposes of concentration they find the most suitable
method in the fractional hydrolysis of the phthalates; these salts are
soluble in cold water, but hydrolyse when the solution is warmed, the
most positive elements remaining of course longest in solution. For the
final purification, they recommend fractional precipitation of the
iodate from nitric acid solution; yttrium iodate being more soluble than
the iodates of the erbium and ytterbium group, the latter collect in the
first precipitates.

  [401] _Zeitsch. anorg. Chem._ 1913, ~80~, 7; Meyer and Weinheber,
  _Ber._ 1913, ~46~, 2672.

Pure yttria is quite white, and gives rise to colourless salts, which in
solution show no absorption spectrum in the visible region. A very large
number of yttrium compounds have been prepared, of which sufficiently
detailed accounts have been given in the general description of rare
earth compounds. For an exhaustive treatment, the reader is referred to
Abegg’s ‘Handbuch.’

The _metal_ has probably not been obtained in the pure state; impure
yttrium has been obtained by Winkler[402] by the action of magnesium on
the oxide, and by Cleve[403] by the action of sodium on a mixture of the
chloride with common salt, and by electrolysis of the mixture of fused
chlorides. It is described as a greyish metal, resembling iron in
appearance; it oxidises in the air and readily decomposes boiling water.
The _hydroxide_ is thrown down as a gelatinous precipitate by alkalies;
ammonia throws down basic salts, but in presence of hydrogen peroxide an
hydrated _peroxide_ is obtained. The _oxide_ absorbs carbon dioxide from
the air, and liberates ammonia from ammonium salts.

  [402] _Ber._ 1890, ~23~, 772.

  [403] _Bull. Soc. Chim._ 1874, [ii.], ~21~, 344; Cleve and Höglund,
  _ibid._ 1873, [ii.], ~18~, 193; see also Popp, _Annalen_, 1864, ~131~,
  359.

The anhydrous _chloride_ has been prepared by many authors; it melts at
a relatively low temperature, 680°, and is the most easily volatilised
of all the rare earth chlorides. After fusion, it forms a mass of
brilliant white lamellæ.[404] It is characterised by the ease with which
it dissolves in pyridine. From aqueous solution it separates as the
hexahydrate, YtCl₃,6H₂O, which melts at 160°. The _bromide_ separates
from solution as the enneahydrate, YtBr₃,9H₂O; the _bromate_[405] also
separates with 9 molecules of water of crystallisation.

  [404] _Compt. rend._ 1902, ~134~, 1308.

  [405] James and Langelier, _J. Amer. Chem. Soc._ 1909, ~31~, 913.

The _nitrate_ cannot be obtained anhydrous; the normal hydrate,
Yt(NO₃)₃,6H₂O, loses 3 molecules of water at 100°, but further heating
converts it into basic salts. A _basic nitrate_, 3Yt₂O₃,4N₂O₅,20H₂O, is
described by James and Pratt[406] as stable at ordinary temperatures,
and in contact with solutions of the normal nitrate. The _sulphate_
octohydrate is isomorphous with analogous compounds of the rare earth
elements, and with the _selenate_, Yt₂(SeO₄)₃,8H₂O; the latter compound
can also form an enneahydrate. The _phosphate_, YtPO₄, occurs in nature
in the mineral xenotime, and has been obtained in the laboratory in the
crystalline form; many other phosphates have been prepared. The
_platinocyanide_, Yt₂[Pt(CN)₄]₃,21H₂O, has the characteristic red colour
with greenish-blue fluorescence.

  [406] _J. Amer. Chem. Soc._ 1910, ~32~, 873.

Many _organic yttrium salts_ have been prepared by James and Pratt[407]
and by Tanatar and Voljanski.[408]

  [407] _J. Amer. Chem. Soc._ 1911, ~33~, 1330.

  [408] _Vide Abstr. Chem. Soc._ 1910, ~98~, i. 809.


~Atomic Weight.~--The numbers obtained by the investigators who have
determined this constant vary to such an extent that considerable
uncertainty attaches to the value, 89·0, at present accepted by the
International Committee. The determinations carried out prior to 1870
gave such diverse results that they are of little use in fixing the
constant; since that date, all the investigations, with the exception of
the most recent, have given values below 90, the sulphate method being
generally employed.

Cleve and Höglund,[409] in 1883, carried out six determinations by the
synthetic method; their results were concordant, and gave the mean value
89·57. Brauner considers this result if anything too low, as traces of
undecomposed acid sulphate may have been present in the anhydrous
sulphate. The same method was employed again by Cleve in 1884;[410] the
mean of twelve very concordant results gave the number 89·11.

  [409] _Loc. cit._

  [410] _Compt. rend._ 1883, ~95~, 1225.

Much stress is laid by Brauner[411] on an unpublished determination of
Marignac, carried out with material entirely free from terbia, which
gave the value 88·88. H. C. Jones in 1895[412] carried out two series of
determinations with material purified by Rowland’s method, _i.e._
precipitation with potassium ferrocyanide;[413] the results in both
series were very concordant, the synthetic method giving the value
88·95, the analytical method the value 88·97. This work has been taken
by the International Committee as the basis for the accepted value.
According to Brauner, the ferrocyanide method does not give perfectly
pure material.[414]

  [411] Abegg’s _Handbuch_, III. i. 328.

  [412] _Amer. Chem. J._ 1895, ~17~, 154.

  [413] Rowland, _Chem. News_, 1894, ~70~, 68; compare also Crookes,
  _ibid._ ~70~, 81-82. Bettendorff (see Böhm, _Die Darstellung der
  seltenen Erden_, I. 480) has also used the method.

  [414] See also Meyer and Wuorinen (_loc. cit._).

Egan and Balke[415] have recently found the ratio Yt₂O₃ : 2YtCl₃ to be
very suitable as a basis for atomic weight determinations; the oxide is
converted into the anhydrous chloride in a quartz flask. In a
preliminary experiment, they obtain as a mean of three consistent
determinations the provisional value 90·12; the yttria employed was
considered to contain not more than one-half per cent. of erbia.

  [415] _J. Amer. Chem. Soc._ 1913, ~35~, 365.

Recent work by Meyer and his co-workers[416] indicates that the accepted
value is too high. Preliminary work with the synthetic sulphate method
gave the values (corrected) 88·71 and 88·73; the mean value of six
analytical sulphate determinations, made on material carefully purified
by the iodate method, was 88·75, the extreme values being 88·71 and
88·76. They consider that the true atomic weight is 88·7, the value of
the second decimal figure being a little uncertain.

  [416] Meyer and Wuorinen; Meyer and Weinheber, _loc. cit._


  ~Detection.~--The spark spectrum of yttrium has been examined by many
  authors, and the ultraviolet as well as the visible regions have been
  mapped; _vide_ Exner and Haschek; Eder and Valenta, also
  Becquerel.[417]

  [417] _Compt. rend._ 1908, ~146~, 683.

  The arc spectrum has been examined by Kayser, Eberhard,[418] and Eder
  and Valenta;[419] Exner and Haschek give the following as the most
  intense lines:

  [418] _Zeitsch. wiss. Photochem._ 1909, ~7~, 245.

  [419] _Sitzungsber. kaiserl. Akad. Wiss. Wien_, 1910, ~119~, IIa, 1.

  3216·83
  3242·42
  3328·02
  3600·92
  3611·20
  3621·10
  3633·28
  3664·78
  3710·47
  3774·52
  3788·88
  3950·52
  3982·79
  4077·54
  4102·57
  4128·50
  4143·03
  4177·74
  4302·45
  4309·79
  4348·93
  4375·12
  4883·89
  6191·91
  6435·27

  Pure yttrium compounds should be colourless, show no absorption in the
  visible region, and yield a perfectly white oxide.


~Scandium~, Sc = 44·1

The scandia obtained by Nilson in 1879 was isolated from the minerals
gadolinite and euxenite; it consisted very largely of ytterbia, as shown
by spectrum examination[420] and by atomic weight determinations, which
gave the value 90. In the same year[421] Cleve prepared the oxide in a
much purer state, using as his source the minerals gadolinite and
keilhauite; he described several salts, carried out atomic weight
determinations by the analytical and synthetic sulphate methods, and
showed that scandium corresponds with the Eka-boron of which the
existence was predicted by Mendelejeff in 1871.[422] Starting from a
large quantity of euxenite, Nilson[423] in the following year prepared
several grams of approximately pure scandia, which contained only traces
of ytterbium.

  [420] Thalén, _Compt. rend._ 1879, ~88~, 642; 1880, ~91~, 45.

  [421] _Compt. rend._ 1879, ~88~, 419.

  [422] See also Mendelejeff, _Ber._ 1881, ~14~, 2821.

  [423] _Ber._ 1880, ~13~, 1439.

The investigation of scandium, which occurs only in extremely small
quantities in the minerals employed by Nilson and Cleve, and was
therefore believed to be exceedingly rare, was not continued until 1908,
when Sir William Crookes[424] made a systematic investigation of a large
number of minerals in order to find a convenient source of the element.
He showed that scandium is present in many rare earth minerals, and
selected as the most suitable for the extraction of the element a
complex mineral named Wiikite, some specimens of which he found to
contain over 1 per cent. of scandia (see p. 70). The mineral was
decomposed by fusion with potassium hydrogen sulphate, and scandia
extracted from the rare earths by the nitrate fusion. The separation
effected on these lines was very thorough, Crookes considering a
specimen of scandia unsatisfactory if it showed any trace of the
dominant ytterbium line, 3694·344, on an over-exposed plate, or if it
gave an atomic weight for the element higher than 44·1.

  [424] _Phil. Trans._ 1908, A, ~209~, 15.

A systematic investigation of the common rocks and minerals for scandium
was carried out by Eberhard in 1908, as a result of which processes for
the extraction of the oxide from wolframite were worked out by R. J.
Meyer (see pp. 3, 131). Wolframite is a tungstate of iron and manganese,
containing, in addition to other oxides, small quantities of the rare
earths, of which considerable proportions are found to be scandia. The
mineral is fused with soda in the usual way, and the rare earths
concentrated by the oxalate method. Scandium is precipitated as the
fluoride by addition of sodium silicofluoride to the boiling acid
solution, and purified by precipitation as the double ammonium
tartrate.[425]

  [425] Meyer and Goldenberg, _Chem. News_, 1912, ~106~, 13.

Whilst the researches of Crookes and Eberhard have shown how widely
distributed the element really is, the minerals which they found richest
in scandium still contained extremely small quantities of the oxide. The
discovery of the mineral Thortveitite (see p. 44), which contains about
37 per cent. of scandia, is therefore of the greatest scientific
interest, and will doubtless allow of a very searching examination of
the properties of this interesting element.

Whilst the low atomic weights of scandium and yttrium place them, to
some extent, apart from the other rare earth elements, the latter
element at least is so closely allied in properties to the other members
of the group that yttria is one of the typical oxides of the family.
Scandium and its compounds, however, present many peculiarities of
behaviour when compared with the typical members, on the grounds of
which Urbain[426] has contended that scandia should not be classed
among the rare earths at all. Whilst this contention is perhaps rather
extreme, especially in view of the fact that in nature scandia always
occurs with other yttria oxides, it must be admitted that in many
respects the element is anomalous. The oxide is the weakest base of the
whole group, yet the oxalate is comparatively readily soluble in mineral
acids (compare p. 132), and the potassium double sulphate is almost
insoluble in potassium sulphate. The sulphate is altogether exceptional
in that it is very easily soluble in water, and crystallises out with 6
molecules of water of crystallisation. The fluoride and the carbonate
both dissolve readily in excess of precipitant, whilst sodium
thiosulphate precipitates a basic salt from neutral solutions.

  [426] _Chem. News_, 1905, ~90~, 319.

Meyer has pointed out the close resemblance between beryllium and
scandium. The oxide and salts are colourless; the latter have a peculiar
sweet astringent taste, and readily yield basic salts.

The _hydroxide_, Sc(OH)₃, is thrown down by alkalies as a bulky white
gelatinous mass; the _oxide_ is a white powder, less readily soluble in
dilute acids than most of the rare earths. The _fluoride_ is important
on account of its insolubility in mineral acids, which exceeds that of
all the other rare earth fluorides, and approaches that of thorium. It
is thrown down from neutral or acid solutions by addition of
hydrofluoric acid or a soluble fluoride; if the solution be boiled, a
soluble silicofluoride will also precipitate scandium fluoride, though
no precipitate is obtained in the cold. This behaviour is due to the
ease with which the silicofluoride is hydrolysed at high temperatures,
according to the equation:

  Sc₂(SiF₆)₃ + 6H₂O = 2ScF₃ + 3SiO₂ + 6H₂F₂

and is of great value in separating scandium from the other earths. The
fluoride is extremely resistant to acids, being completely decomposed
only by fused bisulphate. In the absence of acids, the freshly
precipitated fluoride dissolves in excess of concentrated alkali
fluoride, forming double salts; in this behaviour, scandium resembles
zirconium, but differs from thorium and the cerium and yttrium
elements.

The _chloride_ separates from solution at ordinary temperatures as the
dodecahydrate, Sc₂Cl₆,12H₂O, which loses 9 molecules of water when kept
for six hours at 100°. The trihydrate Sc₂Cl₆,3H₂O, is converted into
scandia at a red heat, with the loss of 6 molecules of hydrogen
chloride. The _iodate_, Sc(IO₃)₃,18H₂O, is obtained as an almost
insoluble white crystalline powder by addition of ammonium iodate to a
salt in solution; hydrates with 15, 13, and 10 molecules of water are
known, and at 250° the anhydrous compound is obtained. It resembles the
iodates of the cerium and yttrium group in being soluble in strong
nitric acid, but the separation of thoria and scandia by this method is
tedious and unsatisfactory.[427]

  [427] Meyer, Winter and Speter, _Zeitsch. anorg. Chem._ 1911, ~71~,
  65.

The _platinocyanide_, Sc₂[Pt(CN)₄]₃,21H₂O, was obtained by Crookes[428]
by double decomposition of the sulphate with barium platinocyanide, in
crimson monoclinic prisms, with a green fluorescence. It dissolves in
water to a colourless solution. Orlov[429] shows that it can occur also
in a second form, stable at higher temperatures; this is yellow, with a
blue fluorescence and crystallises with 18 molecules of water. The two
modifications resemble respectively the platinocyanides of the yttrium
and of the cerium elements; in this respect, therefore, scandium
occupies an intermediate position between the two groups.

  [428] _Phil. Trans._ 1910, A, ~210~, 359.

  [429] _Abstr. Chem. Soc._ 1913, ~104~, i. 27.

The _sulphate_, Sc₂(SO₄)₃, is obtained anhydrous by evaporating the
excess of acid from a solution of the oxide in the concentrated acid,
care being taken to avoid too high a temperature. The compound dissolves
very easily in water, and slowly hydrates itself with evolution of heat;
no crystals can be obtained from the solution until it has been
concentrated to the consistency of a syrup, when on cooling it slowly
deposits the hexahydrate. This effloresces in a dry atmosphere, forming
the pentahydrate, which appears to be the most stable hydrate at
ordinary temperatures. According to Nilson, the hexahydrate loses 4
molecules of water when maintained at 100°. At 250° it becomes
anhydrous; above that temperature, basic salts are formed. The
_potassium double sulphate_, 3K₂SO₄,Sc₂(SO₄)₃, was shown by Nilson to
resemble the analogous cerium compounds in being insoluble in a
saturated solution of potassium sulphate. The _nitrate_, Sc(NO₃)₃,4H₂O,
separates from concentrated solutions over sulphuric acid as the
tetrahydrate; it is very soluble in water and alcohol, and extremely
deliquescent.

The _carbonate_, Sc₂(CO₃)₃,12H₂O, is thrown down by addition of ammonium
carbonate as a bulky white precipitate, easily soluble in a hot solution
of the precipitant; the solubility in excess may be used in the
separation of scandia from yttria. Addition of water to such solutions
causes separation of a basic carbonate, but crystalline _double
carbonates_ may be obtained by evaporation of concentrated solutions
containing a large excess of alkali carbonate. The sodium compound,
Sc₂(CO₃)₃,4Na₂CO₃,6H₂O, is very sparingly soluble, and has been used in
the separation from thorium. The _oxalate_, Sc₂(C₂O₄)₃,5H₂O, differs
from other oxalates of the group, which generally separate with 10
molecules of water of crystallisation, not only in its water content,
and in its solubility in acids, but also in the ease with which it forms
double oxalates soluble in excess of alkali oxalate; in this latter
property it shows a further resemblance to zirconium and thorium. The
_formate_ and _acetate_ have the formulæ Sc(OH)(HCOO)₂,H₂O and
Sc(OH)(CH₃COO)₂,2H₂O, respectively. A large number of organic salts have
been described by Sir William Crookes.[430]

  [430] _Loc. cit._; see also Meyer, _Zeitsch. anorg. Chem._ 1908, ~60~,
  134; Meyer and Winter, _ibid._ 1910, ~67~, 398.


~Atomic Weight.~--The mean values obtained by Cleve[431] in 1879 were
44·96 and 45·20 by the analytical and synthetic sulphate methods
respectively. In the following year Nilson,[432] using purer material,
obtained the value 44·13 by the synthetic method. Meyer and others
(_loc. cit._) have criticised Nilson’s estimation on the ground of his
empirical method of obtaining the neutral anhydrous sulphate.
Determinations made with material purified from thorium by the iodic
acid method gave the values 44·11, 44·11, 44·20; material purified by
the double ammonium tartrate method gave the atomic weight 43·90. Meyer
has shown that small quantities of thoria in the oxide cannot be
detected spectroscopically; the value of the magnetisation coefficient,
however, showed the oxide obtained by the last method to be free from
thoria, and he considers another determination of the atomic weight to
be necessary.

  [431] _Loc. cit._

  [432] _Loc. cit._

The value accepted by the International Committee is 44·1.


  ~Detection.~--Scandium gives no absorption spectrum in the visible
  region. The spark spectrum has been examined by Thalèn (_loc. cit._)
  and Nilson;[433] see also Exner and Haschek, Lockyer and
  Baxendall,[434] and Crookes (_loc. cit._). The arc spectrum has been
  examined by Fowler,[435] Eder and Valenta,[436] and Exner and Haschek.

  [433] _Compt. rend._ 1880, ~91~, 56, 118.

  [434] _Proc. Roy. Soc._ 1905, ~74~, 538.

  [435] _Phil. Trans._ 1908, A, ~209~, 47.

  [436] _Sitzungsber. kaiserl. Akad. Wiss. Wien_, 1910, ~119~, II_a_,
  576.

  The most intense lines of the arc spectrum are the following:

  3353·90
  3372·33
  3558·69
  3567·89
  3572·73
  3576·53
  3614·00
  3630·93
  3642·99
  3907·69
  3912·03
  4020·60
  4023·88
  4247·02
  4314·31
  4320·98
  4325·22
  4374·69
  4400·63
  4415·78
  6305·94

  Fowler (_loc. cit._) examined the arc spectrum with reference to solar
  spectra. For detection of the element in minerals see Crookes (_loc.
  cit._) and Eberhard (_loc. cit._).

  The purity of scandium preparations may be determined by the following
  tests:

  (1) Precipitation with thiosulphate in boiling solution should remove
  all the rare earth content from solution.

  (2) The iodate test for thorium should give no result.

  (3) The oxide must be perfectly white, and salt solutions show no
  absorption.

  (4) R. J. Meyer has found that whilst 0·5 per cent. of thoria cannot
  be detected spectroscopically in scandia, the magnetisation
  coefficient affords an exceedingly delicate test. The value for pure
  scandia is -0·12 × 10⁻⁶, the oxide being diamagnetic; for scandia with
  0·5 per cent. thoria the coefficient was found to be +0·04 × 10⁻⁶, the
  mixture being paramagnetic.


CHAPTER XV

THE GROUP IVA ELEMENTS--TITANIUM

The oxides zirconia and thoria were generally classed among the rare
earths by the earlier chemists. This view was based partly upon the mode
of occurrence of the oxides, which are very generally associated in
nature with rare earths, and were believed to be equally sparingly
distributed, and partly on fallacious chemical analogies. Thus Berzelius
regarded thoria as a monoxide, ThO, and classed it with the other earth
oxides, magnesia, lime, ceria, lanthana, etc., to all of which the
general formula RO was assigned. Zirconia was regarded as a sesquioxide,
Zr₂O₃, analogous to alumina, Al₂O₃, which in turn showed many points of
resemblance to the rare earths. The introduction of the periodic
classification, and a wider knowledge of the chemical properties of the
oxides, have gradually altered the older conceptions, and zirconia and
thoria are now only classed under the head ‘Rare Earths’ when that term
is used in its widest sense. More generally, the term is restricted to
the oxides of the cerium and yttrium elements, which, whilst they cannot
all be placed in Group III of the table, yet constitute a series with
properties which entitle them to be considered in that relation.

The elements which fall into group IVA of Mendelejeff’s classification
are titanium, zirconium, cerium, and thorium; the elements of lower
atomic weight, carbon and silicon, are placed by some authors in Group
IVB, by others in Group IVA. It is a feature of the periodic system that
the members of the A and B families show great differences in the end
groups, I and VII, II and VI, which disappear as the middle groups are
approached; in group IV the families A and B show only slight
differences in properties, corresponding to their amphoteric character
and electrochemical indifference, so that the elements carbon and
silicon may be placed as well in the one as in the other. Generally they
are placed in family B.

In its tetravalent condition, titanium shows a close relationship to
silicon; the similarity is manifested by the ease with which the dioxide
replaces silica in many minerals, and the isomorphism of many titanates
with corresponding silicates. Yet the strengthening of electropositive
character, which always accompanies the change in atomic weight in
descending a vertical column of the table, is very apparent in the case
of titanium, and its ability to form salts in the tetravalent state is a
very important property. This strengthening of the electropositive
character is still more marked in the case of the succeeding elements.
The salts of zirconium are highly hydrolysed in solution, but they are
considerably more stable than those of tetravalent titanium; the ceric
salts show the same change, whilst thorium salts are comparatively
stable in solution, and can be recrystallised from water without change.
Zirconium hydroxide will not dissolve in alkalies, though zirconates may
be obtained in the dry way; thorium hydroxide shows no acidic properties
whatever.

The change in electrochemical character is accompanied by corresponding
changes in physical properties of the elements and their compounds. With
the exception of cerium, which has a very low melting-point (623°), the
elements fuse only at high temperatures; titanium is the most
refractory, zirconium melts at over 1500°, and thorium at about 1450°.
The boiling-points of the chlorides rise as the series is descended;
titanium tetrachloride boils at 136°, zirconium and thorium chlorides at
400°-450° and 950° respectively; zirconium chloride partly sublimes,
whilst ceric chloride decomposes when heated.

The elements of Group IVA are distinguished from the rare earth elements
by their much less strongly marked electropositive character. This is
apparent not only in the amphoteric nature of the oxides, and in the
ease with which the salts are hydrolysed in solution, but in the more
pronounced tendency to the formation of complex salts. The complex
fluorides of the type K₂RF₆ are peculiarly characteristic, and in the
case of titanium and zirconium have been very important for purposes of
analysis and atomic weight determination. The solubility of zirconium
and thorium salts in excess of alkali oxalate or carbonate is also in
harmony with the less pronounced electropositive character of these
elements. The sulphates of titanium and zirconium appear to be of
complex constitution, whilst their neutral chlorides cannot be obtained
from solution. As is to be expected from its high atomic weight, thorium
approaches most nearly to the rare earths in chemical properties; thus
it forms stable double nitrates of the type R₂Th(NO₃)₆ and its salts,
especially the sulphate, resemble those of the rare earth elements in
their solubility relations.

The elements titanium, zirconium, and thorium are distinguished also by
the fact that they form no definite hydroxides. The precipitates thrown
down from solutions of the salts, on addition of alkali, are hydrated
oxides, which lose water continuously when dried, giving rise to no
definite chemical individuals until constant weight is reached with the
anhydrous oxides. The hydroxides have the further characteristic, common
also to the other members of Group IV, of readily forming colloidal
solutions and gels, a property possessed to some extent also by the
elements themselves, and particularly by zirconium, which, when reduced
from its compounds, shows a great tendency to go into colloidal solution
merely on washing. Highly characteristic also is the property of forming
‘meta’-oxides (acids) and ‘meta’-salts, which is common to all the Group
IV elements which have solid oxides.

In presence of hydrogen peroxide, alkalies throw down characteristic
hydrated peroxides, which have definite acidic properties in the case of
titanium: the zirconium compound is less strongly acidic, the cerium
compound shows no tendency to salt formation, whilst if hydrogen
peroxide be added to a neutral or faintly acid solution of a thorium
salt, the precipitate is a peroxy-salt, containing some acid grouping,
_e.g._ SO₄,NO₃.

With regard to valency, the elements in the typical compounds are
tetravalent. Titanium forms three series of salts, in which the element
is respectively di-, tri-, and tetravalent; salts of the first two
series have powerful reducing properties, and the compounds in which
the metal is tetravalent are most stable. Zirconium is always, with the
doubtful exception of its peroxy-compounds and the lower oxides,
tetravalent. Cerium, as already described, can form two series of
compounds, in which it is respectively tri- and tetravalent; thorium,
like zirconium, is always tetravalent.


~Titanium~, Ti = 48·1

Though generally classed among the rare elements, titanium is probably
at least as widely distributed in nature as most of the common metals.
It occurs as the dioxide in small quantities in all the common silicate
rocks and minerals, and in traces in the animal and vegetable kingdoms;
the element has been identified in the sun and in many stars, and has
been found in meteorites. Probably the commonest mineral in which the
element occurs in quantity is ilmenite, or titaniferous ironstone, which
occurs in enormous quantities in many parts of the world (see p. 57).
The pure dioxide occurs in the three forms Rutile, Brookite, and Anatase
(_q.v._), in which it is said to be isotrimorphous with tin dioxide.
Other important titanium minerals are Perovskite, Titanite or Sphene,
the Euxenite series, and other minerals of the tantalo-columbate group
(see Part I).

The commercial sources of titanium compounds are the minerals rutile and
ilmenite. These may be opened up by fusion with alkali or alkali
carbonate; the residue after extraction with water is dissolved in acid,
and precipitated with ammonia; the mixture of iron and titanium oxides
thrown down may be separated by one of the methods outlined on p. 339.
Fusion with potassium bisulphate has also been employed. A very
satisfactory method is that of Stähler,[437] in which the ore is fused
with carbon in the electric furnace. The carbides so obtained are heated
in a stream of chlorine, when the volatile titanium tetrachloride
distils over, and may be obtained quite pure by redistillation; by
appropriate methods, the required compounds may be obtained from this.
(See also pp. 326-7.)

  [437] _Ber._ 1904, ~37~, 4405; 1906, ~38~, 2619.

_The Metal._--The difficulty of isolating metallic titanium in the pure
state is very great, on account of its great affinity for nitrogen,
oxygen, hydrogen, carbon, etc., the ease with which it forms alloys with
all the common metals, and the extremely high melting-point; in
consequence, it is only within recent times that the element has been
obtained in a state approximately approaching purity, and the accounts
given of its physical properties vary very widely.

Berzelius prepared an impure titanium (Ti = 86 per cent.) by reduction
of potassium titanofluoride with potassium; the method was modified by
Wöhler, who heated a tube containing two boats, of which one was filled
with the fluoride, the other with sodium, reduction being effected by
the sodium vapour. Many authors have attempted the reduction of titanium
tetrachloride by means of hydrogen. By heating the tetrachloride with
sodium in a cast iron bomb, Nilson and Pettersson obtained a product
containing 95 per cent. of the element. Reduction of the dioxide by
means of sodium, magnesium, silicon, or aluminium has not been found to
yield good results, by reason of the ease with which titanium alloys
with these elements. Reduction of the dioxide with carbon yields good
results only when precautions are taken to avoid the formation of the
compound which the element so readily forms with carbon and nitrogen.
Moissan[438] found that if temperatures high enough to decompose this
compound were used, the product contained as the only impurity carbon,
which could be partly removed by fusing with the dioxide; the product
then contained 98 per cent. of titanium.

  [438] _Compt. rend._ 1895, ~120~, 290.

The element has been obtained in the fused condition by Weiss and
Kayser,[439] who pressed the amorphous form into sticks, under a
pressure of 70,000 atmospheres, and employed these as pencils for the
electric arc _in vacuo_; the metal fused, forming globules on the ends
of the electrodes, which were detached after the apparatus had been
allowed to cool.

  [439] _Zeitsch. anorg. Chem._ 1910, ~65~, 388.

The amorphous element is a dark powder, resembling finely divided iron
(Ferrum reductum), of density 3·5-3·6. The specific heat rises rapidly
with the temperature, so that the atomic heat has the values 5·40
between 0° and 100°, 6·18 between 0° and 210°, 7·13 between 0° and 300°,
and 7·77 between 0° and 440°. The amorphous element is said to be
paramagnetic.

The fused carbonaceous product of Moissan formed an extremely brittle
mass, with a shining white lustre on the fractured surface, sufficiently
hard to scratch quartz and steel; its density was determined as 4·87.
The product of Weiss and Kayser was also extremely hard and brittle;
when rubbed against steel, it gave bright sparks. Its density was found
to be 5·174, and the heat of combustion for the gram-atom, 97·79 K.

The amorphous variety is fairly stable in air, but burns vigorously when
heated in air, oxygen, or halogens. Heated in nitrogen or ammonia, it
reacts vigorously, forming the nitride TiN; if carbon is present, a
peculiar substance of uncertain composition, known as _titanium
cyanonitride_, is formed. This substance is also obtained when air is
passed over a heated mixture of the dioxide with coke, and is found in
blast-furnaces in which ores containing small quantities of titanium are
worked; it forms brilliant red cubes, which are extremely hard and
resistant to acids. This substance, as well as the nitride itself,
yields ammonia when heated in steam, and has been proposed as a medium
for ‘fixing’ atmospheric nitrogen (see p. 337).

The amorphous element also absorbs hydrogen, when heated in the gas, but
no definite hydride is known. It combines when heated with almost all
the known non-metals, and forms alloys with all the common metals.
Moissan[440] claims to have prepared a compound as hard as diamond by
heating titanium with boron in the electric furnace. The element attacks
steam at a red heat.

  [440] _Loc. cit._

The element is fairly resistant to acids in the cold, but is readily
attacked, with evolution of hydrogen, on warming. Hot dilute
hydrochloric acid gives the trichloride; but dilute sulphuric acid is
variously reported to give the di- and tri-salt. Hot nitric acid
oxidises it readily, forming the so-called metatitanic acid.
Hydrofluoric acid attacks it very readily, forming the tetrafluoride.


COMPOUNDS OF DIVALENT TITANIUM.

The compounds of divalent titanium show resemblances to those of
divalent iron, chromium and vanadium, but on account of the great
difficulty of preparing them and protecting them from oxidation, little
is known of their properties and behaviour; even the colour of the salts
in solution is not known with certainty. In its divalent state; the
element does not appear to act as a strongly positive metal; the salts
in solution are said to show an acid reaction, whilst the precipitates
thrown down with alkali oxalates and acetates are soluble in excess of
the precipitant, forming deeply coloured solutions. With sodium
phosphate the soluble salts give a bluish-black precipitate, with
potassium ferrocyanide and ferricyanide, dark brown and reddish-brown
precipitates respectively. They are distinguished from salts of the
higher oxides of titanium by the brown colouration produced by potassium
thiocyanate in presence of hydrochloric acid.[441]

  [441] v. d. Pfordten, _Annalen_, 1886, ~234~, 257; 1887, ~237~, 201;
  see also _Ber._ 1889, ~22~, 1485.

The _hydroxide_ is thrown down from solutions by addition of alkali,
alkali carbonate, alkali cyanide, or ammonium sulphide, as a black
precipitate. It cannot be transformed to the corresponding oxide by
drying, since it attacks the water with evolution of hydrogen, forming
the dioxide. The _monoxide_, TiO, has probably never been obtained in
the pure state; it is formed by reduction of the dioxide with zinc or
magnesium. Moissan[442] obtained it in the form of black prismatic
crystals by treating the dioxide with the calculated amount of charcoal
in the electric furnace. The _sulphide_, TiS, is an extremely stable
compound; it can be prepared by heating the higher sulphides in a stream
of hydrogen to a very high temperature, and then forms pseudomorphs
after these.[443] It is a dark red metallic mass, which reacts in the
air only when heated, forming the dioxide; dilute acids and alkalies
have no action on it, concentrated nitric acid oxidises it slowly.

  [442] _Loc. cit._

  [443] See v. d. Pfordten (_loc. cit._); Thorpe, _Chem. News_, 1885,
  ~51~, 260.

The _dichloride_, TiCl₂, is obtained in the impure state as a black
powder by decomposition of the trichloride at a red heat: the
tetrachloride is formed at the same time, and volatilises.[444]
According to v. d. Pfordten,[445] it is obtained by reduction of the
tetrachloride by sulphuretted hydrogen or sodium amalgam in the cold.
The latter author states that it dissolves in alcohol or water in
absence of air to a dark brown solution; Friedel and Guérin, however,
state that it acts energetically on these solvents with evolution of
hydrogen, forming a yellow solution. When heated in the air it burns,
evolving fumes of the tetrachloride and leaving a residue of the
dioxide. The _iodide_, TiI₂, has been obtained by Defacq and Copaux[446]
by reduction of the tetraiodide with silver or mercury, as a black,
lustrous, infusible sublimate. It is insoluble in organic solvents, but
reacts with water and aqueous alkalies, and is readily attacked by
acids. Hydrogen at a bright red heat reduces it to amorphous titanium.

  [444] Friedel and Guérin, _Compt. rend._ 1875, _81_, 889; 1876, ~82~,
  509, 872.

  [445] _Loc. cit._

  [446] _Compt. rend._ 1908, ~147~, 65.


COMPOUNDS OF TRIVALENT TITANIUM.[447]

  [447] Compounds of trivalent titanium are frequently referred to in
  English chemical and technical literature as ‘Titanous Compounds,’ the
  salts of the tetravalent element being tacitly recognised as ‘Titanic
  Compounds.’ In view of the existence of compounds of divalent
  titanium, to which the name ‘Titanous Compounds’ might be more
  logically applied, the former nomenclature cannot be regarded as
  altogether satisfactory, and it is therefore not adopted here.

These salts are obtained when the element is dissolved in hydrochloric
and sulphuric acids, and by reduction of the compounds of tetravalent
titanium in solution by means of zinc and hydrochloric acid, or by
electrolysis. According to Diethelm and Forster[448] the reduction may
also be effected by hydrogen in presence of finely divided platinum. The
salts have strong reducing properties, transforming nitro-bodies to
amines and decolourising azo-derivatives very rapidly; they reduce
unsaturated bodies, and reduce dyes to the leuco-bases; they reduce
sulphurous acid to sulphur, precipitate gold, silver and mercury from
their salts, and reduce cupric and ferric salts to cuprous and ferrous
compounds respectively. The salts are green or violet in solution,
showing the phenomenon of hydrate-isomerism which is exhibited by the
chromic salts; they are to some extent hydrolysed in aqueous solution,
as shown by the acid reaction of the chloride. They resemble the salts
of ferric iron and aluminium in giving precipitates of basic salts when
boiled with sodium acetate or sodium formate, and in giving no
precipitate with alkalies in the presence of organic hydroxy-acids.
Ferrocyanide and ferricyanide give brown precipitates.

  [448] _Zeitsch. physikal. Chem._ 1908, ~62~, 129.

The _hydroxide_, Ti(OH)₃,_x_H₂O, is thrown down as a dark precipitate
with strong reducing properties; it attacks water with evolution of
hydrogen, forming the dioxide; when an aqueous suspension is shaken with
air, autoxidation occurs, hydrogen peroxide being formed. The
_sesquioxide_, Ti₂O₃, has been prepared by Friedel and Guérin[449] by
heating the dioxide to a white heat in a current of hydrogen and
titanium tetrachloride; it forms black lustrous crystals, isomorphous
with hæmatite. The _sulphide_, Ti₂S₃, is best obtained by reduction of
the disulphide, at a moderate temperature, in a stream of hydrogen or
nitrogen, but is also prepared by the action of a mixture of carbon
disulphide and sulphuretted hydrogen on the dioxide at a high
temperature. It is a dark grey metallic powder, stable towards air,
water, alkalies and dilute acids.

  [449] _Loc. cit._

_Titanium Nitride_, TiN, is obtained in all reduction processes in which
titanium compounds are used, if air or nitrogen is admitted; it is
formed when the element is heated in nitrogen, and by the action of
ammonia on the chloride. It forms lustrous, bronze-coloured leaflets,
which appear blue or violet when powdered. It is extremely hard, and
very stable, but is attacked by alkalies with evolution of ammonia. It
reduces the oxides of copper and lead in the fused state. Ruff and
Eisner have shown that it is a true nitride of the trivalent element,
and that only one nitride exists.[450]

  [450] _Ber._ 1905, ~38~, 742; 1908, ~41~, 2250.

The _fluoride_, TiF₃, has been obtained as an insoluble violet powder by
reduction of potassium titanofluoride, K₂TiF₆, with hydrogen. From a
solution it may be obtained by reduction of the same salt with zinc and
hydrochloric acid, or sodium amalgam. It forms complex salts with alkali
or ammonium fluoride, of which the compound (NH₄)₃TiF₆ is an example;
this salt appears to be isomorphous with the analogous compounds,
(NH₄)₃VF₆, (NH₄)₃CrF₆, and (NH₄)₃FeF₆. By autoxidation in the air, the
solutions form fluoroxypertitanates. The complex salts appear to exist
in two forms, a violet insoluble form and a green soluble modification.

The _chloride_, TiCl₃, is obtained anhydrous by reduction of the
tetrachloride--mercury, silver, and hydrogen being the most suitable
agents. Heated in hydrogen, it breaks up, forming the tetrachloride and
the dichloride; heated in air it burns, evolving the tetrachloride and
leaving a residue of dioxide. In solution, in combination with alkali
chlorides, and as the solid hydrate, it exists in the green and violet
forms. Concentrated aqueous solutions deposit the violet hexahydrate,
TiCl₃,6H₂O. If such a solution be covered with ether, and saturated at
0° with hydrogen chloride, the green modification is formed, and may be
extracted by the ether; it is stable only in the presence of
hydrochloric acid. In the violet form, all the chlorine is in the ionic
condition, and can be removed by silver nitrate; similar determinations
have not been made with the green form, but it is most probable, as in
the case of the analogous chromic salts, that only part of the chlorine
content can be removed by silver nitrate. Böck and Moser[451] have
recently described a brown substance, obtained by the action of the
silent electric discharge on a mixture of hydrogen and the vapour of
titanium tetrachloride at the ordinary temperature, which they believe
to be a monotropic modification of the ordinary violet trichloride; the
change of this brown form to the violet form is irreversible.

  [451] _Monats._ 1912, ~33~, 971; 1913, ~34~, 1825.

The _bromide_ and _iodide_ resemble the chloride, but are very unstable.

The _sulphate_, Ti₂(SO₄)₃, is obtained as a green crystalline powder by
heating with sulphuric acid the violet solution obtained by reduction of
a solution of the dioxide in sulphuric acid. It dissolves in dilute
acids, forming violet solutions. With alkali sulphates it forms
_titanium alums_, which can be recrystallised from dilute sulphuric
acid, and have the general formulae, properties, and crystal form of the
other alums. An _acid sulphate_, 3Ti₂(SO₄)₃,H₂SO₄,25H₂O, is obtained by
electrolytic reduction of a strongly acid solution of the dioxide in
sulphuric acid, or by treating the chloride repeatedly with hot dilute
sulphuric acid. It forms a crystalline violet powder, with silky lustre,
insoluble in alcohol, ether, and 60 per cent, sulphuric acid; it
dissolves slowly in water, forming a violet solution. When the aqueous
solution is treated with excess (2¹⁄₂ molecules) of alkali sulphate, it
forms sparingly soluble _double sulphates_, which separate in bright
blue crystals; the compounds Ti₃(NH₄)(SO₄)₅,9H₂O, and Ti₃Rb(SO₄)₅,12H₂O,
have been obtained in this way.

_The Use of Salts of Trivalent Titanium in Volumetric Analysis._--Owing
to their powerful reducing properties, these salts have been proposed as
very convenient reagents in volumetric analysis,[452] the chloride being
most useful in this respect. The estimations must be carried out in
absence of air, to avoid atmospheric oxidation; generally the apparatus
is filled with carbon dioxide. The titanium solutions for use must also
be preserved from the oxidising action of the air.

  [452] See Knecht, _Ber._ 1903, ~36~, 166; Knecht and Hibbert, _ibid._
  1903, ~36~, 1549; 1905, ~38~, 3318; 1907, ~40~, 3819.

For estimation of ferric salts, an aliquot quantity is titrated directly
with the titanium solution, ammonium thiocyanate being used as
indicator. Ferrous salts and ferric salts in the same solution are
easily estimated by titrating the former with permanganate solution, or
better with hydrogen peroxide, and then estimating the total ferric salt
with the titanium solution. Oxidising agents like nitrates and chlorates
can be estimated in acid solution by treatment with an excess of a
ferrous salt, and estimation of the ferric compound formed by means of
titanium. Azo-bodies and organic dyes can be titrated directly, if
soluble in hydrochloric acid, the disappearance of colour marking the
end of the reaction; nitroso-compounds can also be estimated in this
way. If the compound is insoluble, it may be reduced in hydrochloric
acid suspension with excess of the titanium salt, and the excess then
determined by means of ferric iron. Insoluble dyes may also be converted
into soluble sulphonic acids, and estimated directly in solution.
Ammonium persulphate may be estimated by reduction with excess of the
chloride, and back titration of the excess with ferric iron. Hydrogen
peroxide may be estimated directly, the disappearance of the yellow
colour formed at the first addition marking the end of the reaction. Tin
may be estimated by addition of an excess of a ferric salt, and
estimation of the excess by titanium in the usual way. Cupric salts also
may be estimated directly,[453] the end point being reached when the
bluish-green solution becomes colourless.

  [453] Moser, _Chem. Zeitg._ 1912, ~36~, 1126.


COMPOUNDS OF TETRAVALENT TITANIUM

The compounds of tetravalent titanium are much more stable than the
compounds in which the element has a lower valency, and are very readily
formed from them. The dioxide is amphoteric in character, and acts as a
weak acid as well as a weak base; the salts it forms with acids as well
as those it forms with bases are very easily hydrolysed, with separation
of the hydrated oxide. Titanium salts, therefore, can only be held in
solution by a considerable excess of acid. The tendency to the formation
of complex compounds is very pronounced, particularly in the case of the
fluoride, oxalate and tartrate.

The _hydroxide_, or _hydrated oxide_, is capable of existing in two
modifications, according to the conditions under which it is thrown
down, though the two can hardly be said to be very definitely
differentiated. The α or ortho form is obtained as a voluminous white
precipitate by the addition of ammonia or alkali hydroxide in the cold
to a freshly prepared solution of a titanium salt. It is insoluble in
water and alcohol, but dissolves readily in dilute mineral acids, and to
some extent also in dilute alkalies. The water content is very variable,
and no definite hydrate or hydroxide can be prepared; when the substance
is heated, it loses water continuously, and at a definite temperature
glows, doubtless by reason of some polymeric change. If it be maintained
for some time at a temperature somewhat below the normal temperature of
glowing, this phenomenon no longer occurs when the temperature is
further raised.

The β modification, or metatitanic acid, as it is called, is obtained by
hydrolysis of the salts by boiling, or by addition of alkali at 100°, as
a fine white precipitate. It is almost insoluble in dilute acids and
alkalies, but dissolves in water to a colloidal solution; when heated it
does not glow. The β form is also obtained when the metatitanates are
treated with water; these compounds hydrolyse very readily, but the
precipitated dioxide carries down alkali by adsorption.

The _dioxide_, TiO₂, occurs crystalline in nature in the three forms
Rutile, Brookite, and Anatase, all of which can be prepared by
laboratory methods;[454] the amorphous form is obtained by ignition of
the hydrated oxide, and of suitable salts. The oxide melts at 1560°,
forming a mobile (?) liquid of density 4·1; for the physical properties,
see the accounts of the naturally occurring forms in Chapter V. When
heated in a current of hydrogen or carbon monoxide, it gives rise to
intermediate oxides, Ti₃O₄, Ti₇O₁₂, etc., which are not very well known,
and are of doubtful individuality. It reacts when heated in chlorine,
and with many non-metallic chlorides, forming the tetrachloride; with
carbon disulphide at high temperatures it gives the disulphide, ammonia
at a red heat forms the nitride. It is exceedingly resistant to acids,
but is attacked slowly by boiling sulphuric acid, more quickly by fused
bisulphate.

  [454] See p. 79; also Hautefeuille, _Ann. chim. phys._ 1863, [iv.],
  ~4~, 129.

_Titanium disulphide_, TiS₂, is obtained in the pure state when a
mixture of the vapour of the tetrachloride and sulphuretted hydrogen is
led through a strongly heated porcelain tube. It is a fairly stable
substance, forming metallic crystals which yield the dioxide when heated
in air. When heated in a stream of hydrogen or nitrogen it yields one or
other of the lower sulphides according to the temperature employed. It
is not attacked by water, but dissolves in acids, and is decomposed by
boiling potash, forming a titanate; it is insoluble in alkali sulphides.

The _carbide_, TiC, was prepared by Moissan by heating the oxide with
carbon in the electric furnace; any excess of carbon separates on
cooling as graphite. It has the density 4·25, and resembles the fused
element in appearance. It dissolves in nitric but not in hydrochloric
acid.

_Titanium tetrafluoride_, TiF₄, is obtained by the action of fluorine on
the element or the carbide, and by the action of anhydrous hydrofluoric
acid on the element or the tetrachloride. It is a white powder, and
boils at 284°; it is very hygroscopic, and dissolves easily in alcohol
and water, showing little tendency to form basic salts. From the
concentrated aqueous solution it separates as the dihydrate, TiF₄,2H₂O;
basic salts are obtained only by repeated evaporation with water. The
anhydrous compound forms additive products with ammonia and with
pyridine.

With aqueous hydrofluoric acid it forms the complex H₂TiF₆, as shown by
conductivity measurements, and the fact that only a slow and incomplete
precipitation of the hydroxide is effected by addition of ammonia. The
solution dissolves metallic oxides and carbonates, forming
_titanofluorides_, which are for the most part isomorphous with the
corresponding silicofluorides, stannofluorides, and zirconofluorides.
They are very stable crystalline salts, of the general formula R´₂TiF₆;
many salts of the types R´´TiF₆, R´₃TiF₇, etc., have been prepared. The
most important is the potassium salt, K₂TiF₆, which crystallises from
acid solutions in monoclinic tablets; from aqueous solution it separates
as the monohydrate, K₂TiF₆,H₂O, isomorphous with the compounds
K₂CbOF₅,H₂O and K₂WO₂F₄,H₂O. The hydrate loses its water at 100°, and
melts at a white heat without decomposition. It is moderately soluble in
hot, very sparingly soluble in cold water, and hence is readily
recrystallised.

The _tetrachloride_, TiCl₄, is important, on account of its low
boiling-point, for the separation and purification of titanium
compounds. In physical as well as chemical properties, it resembles the
chloride of a non-metallic element rather than a normal salt, and is
distinguished by the ease with which it combines or reacts with the most
widely differing organic compounds. It is prepared by the action of
chlorine upon the element, the carbide, or a mixture of the dioxide with
carbon, and by the action of chloroform or carbon tetrachloride upon the
dioxide at a bright red heat. It is a colourless, transparent liquid, of
density 1·76 at 0°; it freezes at -23°, and boils at 136° under
atmospheric pressure. In moist air it fumes excessively, yielding
hydrogen chloride by hydrolysis: TiCl₄ + H₂O = TiOCl₂ + 2HCl, and is
decomposed by water with separation of the hydrated oxide. If the
compound be added slowly to a large quantity of cold water, and the
clear solution warmed, the oxide formed by hydrolysis remains in
colloidal solution.

The chloride dissolves in fuming hydrochloric acid, forming a deep
yellow solution, which becomes colourless when diluted. The solution
appears to contain the unstable complex acid H₂TiCl₆, or its ions; by
addition of ammonia, or organic bases, salts of the type (NH₄)₂TiCl₆ can
be obtained as yellow crystalline solids. An interesting property of the
chloride is its ability to form stable additive compounds with the
chlorides of negative elements. A long series of these are known, of
which the compounds TiCl₄,PCl₃, TiCl₄,PCl₅, TiCl₄,POCl₃, and
TiCl₄,2POCl₃ may be considered examples; for the most part, they can be
distilled without decomposition. A very long series of compounds, partly
additive and partly condensation products, with all kinds of organic
substances, is also known.

A series of _oxychlorides_, or _basic chlorides_, TiCl₃(OH), TiCl₂(OH)₂,
and TiCl(OH)₃, has been obtained by addition of hydrochloric acid, in
certain quantities and concentrations, to the chloride; they are
amorphous solids, of which little is known.

The _tetrabromide_, TiBr₄, is a yellow crystalline solid, melting at 39°
and boiling at 230°. Its solutions in concentrated hydrobromic acid are
of a blood-red colour, and by treatment with ammonia and organic bases
yield deep red crystalline salts of the type (NH₄)₂TiBr₆. The
_tetraiodide_, TiI₄, is a reddish-brown metallic-looking solid, melting
at 150°, and boiling at 360°; no complex salts are known.

_The sulphates._--Many compounds of doubtful composition and
individuality have been described as titanium sulphates, but relatively
little is known with certainty of this class of derivatives. The most
stable seems to be the _titanyl sulphate_, TiOSO₄, obtained as a white
powder, which is slowly hydrolysed by water, by evaporating a solution
of the dioxide in concentrated sulphuric acid. Under suitable
conditions, _e.g._ when separated from acid or alcoholic solutions, it
is said to form hydrated compounds; the mono-, di- and penta-hydrate
have been described. When solutions of this compound in concentrated
sulphuric acid are treated with concentrated aqueous solutions
of alkali sulphates, salts of the formulæ (NH₄)₂TiO(SO₄)₂,H₂O and
K₄(TiO)₃(SO₄)₅,10H₂O, are obtained. By treating solutions of the dioxide
in a large excess of concentrated acid with solutions of calcium or
strontium sulphate in sulphuric acid, salts of the type R´´Ti(SO₄)₃ are
obtained; the barium salt has the formula 3Ti(SO₄)₂,2BaSO₄. All these
compounds are rapidly hydrolysed by water.

_Phosphoric Acid Derivatives._--Solutions of titanium compounds are
completely precipitated by the addition of phosphoric acid, or soluble
phosphates, even in presence of a large excess of mineral acid, but the
composition of the precipitate obtained is unknown. By heating the oxide
with orthophosphoric acid, a crystalline compound, TiO₂,P₂O₅, is
obtained, and various alkali double phosphates may be prepared by
suitable fusions.

Concentrated aqueous oxalic acid solutions readily dissolve one
equivalent of titanium dioxide, forming greenish-yellow solutions which
contain _titanyl oxalate_, TiO(C₂O₄). From alcoholic solution, this
substance can be precipitated by ether as the alcoholate,
TiO(C₂O₄),C₂H₅OH, a micro-crystalline precipitate soluble in water and
alcohol. _Titanyloxalic acid_, TiO(HC₂O₄)₂,H₂O, and its salts are stable
compounds; the latter are obtained by dissolving the dioxide in alkali
binoxalate, the acid itself being obtained by treatment of the sparingly
soluble barium salt with sulphuric acid.

Complex acids are also formed with tartaric acid, and other organic
hydroxy-acids; from its solutions in these acids, the dioxide cannot be
again precipitated by boiling, or by addition of alkalies.

_Titanates and Pertitanates._--On account of the weakly acid character
of the dioxide, stable titanates can be prepared only in the dry way.
The dioxide resembles silica in the conditions under which it forms
salts, and in the nature, and generally the crystallographic properties,
of the products obtained. The commonest salts are the metatitanates of
the formulae R´₂TiO₃ and R´´TiO₃, which are obtained by fusing the
dioxide with metallic oxides and carbonates, sometimes with addition of
a suitable agent to act as a crystallising medium, _e.g._ sodium
tungstate, calcium chloride, magnesium chloride, etc. Calcium
metatitanate, CaTiO₃, prepared by heating titanium dioxide with calcium
carbonate in presence of calcium chloride, is identical in properties
with the naturally occurring compound, Perovskite (_q.v._).
Orthotitanates of divalent metals only are known; these have the general
formula R´´₂TiO₄, and are prepared by similar methods. The iron compound
FeTiO₃ is also identical in properties with the mineral ilmenite, and
isomorphous with the sesquioxides Fe₂O₃, Ti₂O₃. Magnesium titanates of
both the ortho type (Mg₂TiO₄) and the meta type (MgTiO₃) have been
prepared in the laboratory; the latter is identical with the mineral
Geikielite (_q.v._).

The compounds prepared in this way are all insoluble in water, doubtless
by reason of the slowness with which such compact solids can be
attacked; they dissolve easily in dilute acids. The weakly acid
character of titanium dioxide is shown by the fact that if the fusion
with metallic carbonates be carried out in vessels so adjusted that the
carbon dioxide exerts a pressure of one atmosphere, a condition of
equilibrium is reached, in which a considerable part of the carbonate
remains unattacked. In the presence of hydrogen peroxide, however, the
acidic properties are considerably strengthened, and the per-salts can
be obtained in the wet way.

Addition of hydrogen peroxide to a neutral or acid solution of a
titanium compound gives a yellow colour, due to the formation of a
peroxide, TiO₃,aq. Such solutions have the same oxidising powers as
hydrogen peroxide, but do not give the blue colouration with chromium
salts. By treatment of the solution with dilute alkalies, an hydrated
peroxide is thrown down, which, when dried over phosphoric anhydride,
has the formula TiO₃,3H₂O, and forms a yellow, horny mass. The freshly
precipitated peroxide dissolves in acids and alkalies; from the latter
solutions, by addition of hydrogen peroxide and alcohol, pertitanates of
various composition can be obtained, of which the following are
examples : Na₂O₂,TiO₃,3H₂O; (NH₄)₂O₂,TiO₃,H₂O₂; BaO₂,TiO₃,5H₂O;
K₂O₄,K₂O₂,TiO₃,10H₂O, etc. These salts lose hydrogen peroxide when
treated with dilute acids, and their constitutions are unknown.

An interesting series of fluoroxypertitanates has been prepared by
oxidation of the solution of titanium dioxide in hydrofluoric acid with
hydrogen peroxide, and addition of metallic fluorides. The ammonium
compound, (NH₄)₃TiO₂F₅, crystallises in yellow octahedra, isomorphous
with the salts ZrF₄,3NH₄F and CbOF₃,3NH₄F. The potassium salt, K₂TiO₂F₄,
crystallises well from water, and is easily obtained in the pure state;
various barium salts are known. Similar compounds with oxalic acid have
also been prepared.


~Atomic Weight of Titanium.~--The first reliable determinations of this
constant were carried out by H. Rose in 1829. He determined the ratio
TiCl₄ : 4AgCl, by dissolving the pure tetrachloride, weighed in sealed
glass bulbs, in water in closed flasks, precipitating the dioxide by
ammonia, and weighing the silver chloride obtained by adding silver
nitrate to the filtered and acidified solution. He obtained the values
48·27 and 48·13, which agree very well with the accepted value, 48·1. In
the same year, Mosander, using a method not specified, obtained the
value 47·15. Determinations carried out by Pierre (1847) and Demoly
(1849) led to widely varying results. A series of determinations carried
out by Thorpe during the years 1883-1885 gave consistent results. The
method used was the precipitation of silver halide from the
tetrachloride and tetrabromide, and the mean value of seven series of
determinations gave the number 48·08. The International Committee have
adopted this result as the basis of the accepted value, 48·1.


  ~Detection.~--The specific reactions for the element are frequently
  masked by the presence of other metals, especially of iron, columbium,
  tantalum, and vanadium, which most frequently accompany it in nature,
  and from which a quantitative separation is frequently very difficult
  (see p. 338). The most characteristic reactions are the following:

  (1) Reduction in acid solution by means of tin or zinc gives an
  intense violet colour, due to the formation of trivalent titanium
  salts. Various colours are given by vanadium, columbium, and tungsten,
  so that the test is not decisive if these are present.

  (2) Hydrogen peroxide in acid solution gives a reddish-yellow colour,
  which is very delicate, and is used in quantitative estimation;
  vanadium compounds interfere.

  (3) In sulphuric acid solution, characteristic colours are obtained
  with many phenolic compounds; thymol gives a blood-red colour which is
  exceedingly intense.

  (4) A very characteristic and intense colour is given in acid solution
  on the addition of 1:8-dihydroxynaphthalene-2:4-disulphonic acid
  (chromotropic acid).

The methods for the estimation of titanium are given in Chapter XXII.


CHAPTER XVI

THE GROUP IVA ELEMENTS (_continued_)--ZIRCONIUM AND THORIUM


~Zirconium~, Zr = 90·6

The oxide zirconia was isolated as a new earth from zircon from Ceylon
by Klaproth in 1789; six years later the new earth was obtained also
from hyacinth, the gem-variety of the same mineral. The new oxide was
examined in 1818 by Berzelius, who pointed out its resemblance to
alumina, and gave it the formula Zr₂O₃; during the next two decades he
thoroughly investigated its properties, preparing the element itself,
and determining its equivalent. In 1857 a determination of the vapour
density of the chloride, by Deville and Troost, showed that the element
is really tetravalent, and that the formula ZrO₂ must be assigned to the
oxide; this formula was shown to accord with the isomorphism of rutile
(TiO₂) and zircon (ZrO₂,SiO₂) by Rose in 1859, whilst in the following
year Marignac observed the isomorphism between the fluozirconates of
zinc and nickel and the fluosilicates, fluotitanates, and fluostannates
of these metals. The homogeneity of the oxide has been questioned;
Svanberg in 1845 considered it to be a mixture of at least three earths,
whilst Sorby and Forbes in 1869 claimed to have discovered in it a new
oxide, ‘Jargonia.’ These claims, however, have been shown to have been
founded on inaccurate experimental work, and the individuality of the
element is at the present time considered to be well established.

Zirconium is fairly widely distributed in nature, but generally in very
small quantities, and can be rightly classed as one of the rarer
elements. It occurs in some silicates, and in small quantities in almost
all the rare earth minerals. The most important source of the element
and its compounds was until quite recently the mineral Zircon, with its
gem-varieties Hyacinth and Jargon, and the large number of secondary
altered zircon minerals. Since its discovery in 1892, however, the
naturally occurring oxide, Baddeleyite,[455] has become increasingly
important for the extraction of zirconium compounds, especially for the
preparation of the pure oxide for fire-resistant materials.

  [455] For accounts of the zircon minerals, see pp. 47 and 75, and the
  alphabetical list.

The minerals may be treated by any of the usual methods. Zircon may be
fused with alkali or alkali carbonate; the cooled melt is extracted with
water, and the insoluble alkali zirconate decomposed by dilute acids;
from the solution, zirconia is thrown down by alkalies. Potassium
hydrogen fluoride and potassium hydrogen sulphate may be used for the
treatment either of zircon or of baddeleyite; in the first case, the
potassium fluozirconate formed may be dissolved by boiling with dilute
hydrofluoric acid, and separates out readily on cooling, whilst the
fluosilicate formed is not dissolved; the second treatment yields the
sulphate, which may also be dissolved out by dilute acid. A very
convenient method consists in reducing with carbon, either alone or in
presence of lime, at the temperature of the electric arc; the infusible
zirconium carbide is formed, whilst silica, if present, is reduced to
the carbide, which is volatile at that temperature and is therefore
driven off. The zirconium carbide may be dissolved in warm aqua regia.

In all these methods the compounds obtained are contaminated with iron,
which clings to zirconium very tenaciously. Many methods have been
devised for its removal. A very suitable method is the thiosulphate
precipitation. Zirconia is thrown down quantitatively, mixed with
sulphur, from a not too strongly acid solution by addition of sodium
thiosulphate at the boiling-point, sulphur dioxide being at the same
time evolved, by decomposition of the potential thiosulphuric acid
formed by hydrolysis. Thorium and titanium accompany the zirconium, but
iron, aluminium, and the rare earths remain in solution. Another method
depends on the fact that zirconium is not precipitated from alkaline
solution by ammonium sulphide in the presence of tartaric acid, whereas
this reagent does not inhibit the precipitation of ferrous sulphide.
Iron may also be removed from a solution in concentrated hydrochloric
acid by means of ether, in which medium ferric chloride is easily
soluble. Zirconium compounds may be obtained free from iron by repeated
crystallisations of the oxychloride.

Zirconium forms only one series of compounds, in which the metal is
tetravalent. Its chemical behaviour accords well with its position in
the periodic classification. It is somewhat more electropositive than
titanium, as shown by the fact that the hydroxide will not dissolve in
alkalies, though zirconates may be obtained by the fusion methods; the
oxide, however, is still a weak base, and the salts are to a large
extent hydrolysed in solution. The formation of a stable oxychloride,
which can be recrystallised without change in composition, shows clearly
the strengthening of the electropositive character. It has still,
however, in a high degree, the property of forming complex salts, which
is characteristic of the less electropositive metals.

The group relations are borne out by the isomorphism of many related
salts. The hydroxide and oxide show polymeric modifications, and the
former has the usual tendency of compounds of this group to form
colloidal solutions, a tendency which extends to the element itself. The
metal resembles titanium in the eagerness with which it combines with
other elements, especially with oxygen, nitrogen, and carbon, whilst the
chloride closely resembles titanium tetrachloride in general properties,
and in the ease with which it forms addition and condensation products
with other substances.

_The Metal._--All the difficulties which attend the attempts to prepare
metallic titanium in the pure state have to be encountered in the
preparation of metallic zirconium. The attempts which have been made
have used the same methods, and obtained much the same kind of result as
those employed in the case of titanium.[456] The reduction of potassium
fluozirconate by metallic potassium, first employed by Berzelius, gives
an amorphous product of unknown metal-content; it certainly contains a
considerable percentage of oxygen. The monoxide is obtained when
zirconia is reduced by magnesium (Winkler’s method). The reduction of
the fluozirconates of potassium by means of sodium gives better results
if the reaction is carried out in presence of sodium chloride in a
sealed iron bomb; the product after careful washing contains 97-98 per
cent. of the metal. Reduction with aluminium leads to the formation of
alloys; Weiss and Neumann[457] have used these in the form of pencils as
electrodes between which they pass the electric arc _in vacuo_, and so
obtain an almost pure zirconium. The 97-98 per cent. amorphous product
obtained by the sodium reduction also yields the practically pure metal
when treated in this way (compare Titanium, p. 223). A very pure
zirconium has been obtained by Wedekind[458] by heating the oxide with
fine calcium turnings in an evacuated iron tube; the powdered product is
washed, in absence of air, and heated in an evacuated porcelain tube to
800°-1000°, at which temperature the powder sinters into lumps which
take a brilliant polish and contain 99·1 per cent. of the metal.
Attempts to prepare a purer product from this by the method of Weiss and
Neumann were unsuccessful.

  [456] For a detailed account of these, see Lewis, _Studien über das
  elementare Zirconium_, Stuttgart, 1912.

  [457] _Zeitsch. anorg. Chem._ 1909, ~65~, 248.

  [458] _Annalen_, 1913, ~395~, 149.

The amorphous metal is a dark powder, which when washed with water on
the filter paper passes through as a dark blue colloidal solution; it
burns readily when heated in the air. According to Wedekind and
Lewis,[459] amorphous zirconium is really the colloidal form of the
metal. The fused metal is very hard (7-8, Mohs’ scale--it scratches
quartz but not topaz) and very brittle; it has the density 6·4, and is
of a whitish colour, with good metallic lustre on freshly broken
surfaces. The atomic heat is abnormally high, being approximately 7·3;
the element is paramagnetic. The melting-point was given by Wedekind
and Lewis[460] as 2330°-2380°, but later work of the former author[461]
gives the much lower value of 1530°, which seems more probable in view
of the fact that the element cannot be employed for electric lamp
filaments (see p. 322).

  [459] _Ibid._ 1910, ~371~, 367.

  [460] Weiss and Neumann, _loc. cit._; also Wedekind, _loc. cit._

  [461] _Annalen_, 1913, ~395~, 149.

Metallic zirconium is highly resistant to acids; it is attacked only by
hydrofluoric acid and by aqua regia. In the compact form it burns in the
air only at very high temperatures, though when powdered it glows in the
air at a red heat, forming probably a mixture of lower oxides. It is
attacked by chlorine and by hydrogen chloride at a red heat, with
formation of the chloride; fused potash also oxidises it, with evolution
of hydrogen. When heated in a current of hydrogen at a red heat, it
forms the _hydride_, ZrH₂,[462] as a velvet-black powder, which burns
with an intense bluish flame in oxygen, forming the sesquioxide, Zr₂O₃.
When heated in nitrogen or ammonia, amorphous zirconium yields
_nitrides_, which are also obtained when any attempt is made to reduce
zirconium compounds to the metal in air. The most definite is the
compound Zr₂N₃,[462] which forms a bronze-coloured powder, resistant to
all mineral acids except hydrofluoric acid. Chlorine and bromine
transform this to the halide.

  [462] Wedekind and Lewis, _Annalen_, 1910, ~371~, 367.

The _hydroxide_ is of doubtful individuality, since on drying it loses
water progressively as the temperature is raised, no definite stable
compound being known; in this respect zirconium resembles titanium. When
heated to 100°, its composition corresponds approximately with that
required by the formula ZrO₂,H₂O, but the percentage of water varies
with the history of the specimen. When precipitated by alkalies in the
cold, it forms the so-called α or ortho modification, which, like the
analogous titanium compound, is readily soluble in dilute acids, and
glows when heated. By precipitation at the boiling point, the β form is
obtained; this is less soluble in acids, and does not glow when heated.
The differences between the two forms are by no means sharply marked;
they are rather the limiting forms of a continuously varying series than
distinct chemical individuals, and the properties of any hydroxide
precipitate depend very largely on the conditions under which it is
thrown down.

The hydroxide is insoluble in water, but can be obtained in colloidal
solution after it has been repeatedly heated with dilute acids, which
serve to break down the molecular complexes; it can be also readily
obtained in colloidal solution by dialysis of the nitrate, chloride, or
acetate. In these solutions it is positively charged; electrolytes
precipitate it with great ease. The gel has a very high power of forming
adsorption products. When thrown down from solution by soda or potash,
it carries down considerable quantities of alkali, to which it clings so
tenaciously that the most careful washing cannot entirely remove them.
If the gel be placed in contact with an ammoniacal solution of a cupric
compound, it removes the cuprammonium complex entirely from the
solution, becoming itself deep blue in colour, and leaving the liquid
quite clear and colourless. In colloidal solution it forms adsorption
compounds with negatively charged colloids, especially metals, the gels
obtained from such solutions containing both colloids.

In the presence of hydrogen peroxide, ammonia throws down an hydrated
peroxide, which is also obtained[463] by electrolysis of a brine
solution in which the hydroxide is suspended, oxidation being effected
by the sodium hypochlorite formed. This reaction is expressed by the
equation:

  Zr(OH)₄ + NaOCl = Zr(OOH)(OH)₃ + NaCl

  [463] Pissarjewski, _Zeitsch. anorg. Chem._ 1900, ~25~, 378.

It is an endothermic compound, and is very unstable, losing oxygen on
standing; by the action of acids it gives hydrogen peroxide. It
dissolves in alkalies containing hydrogen peroxide; from such solutions,
alcohol precipitates salts of the formula R´₄Zr₂O₁₁,9H₂O.

_Zirconium oxide_, ZrO₂, occurs in nature; it can be obtained in the
laboratory as a voluminous white powder by ignition of the hydroxide or
a suitable salt. The physical properties are described under the mineral
Baddeleyite (p. 75) and in Chapter XXI (p. 323), in which an account of
its technical applications is given. The melting-point is probably
about 2700°; at 3000° it begins to volatilise. It dissolves readily in
mineral acids, unless previously ignited very strongly; all specimens
dissolve easily in hydrofluoric acid, and are readily converted by
concentrated sulphuric acid into the sulphate.

When fused with metallic oxides or carbonates, it gives crystalline
_zirconates_, of which a large number have been prepared; the calcium
compound, CaZrO₃, is said to be isomorphous with perovskite, CaTiO₃.

A _suboxide_, ZrO, of somewhat doubtful individuality,[464] is said to
be obtained when the dioxide is reduced with magnesium; it forms a dry
black powder, which is not attacked by acids, and when heated glows,
forming the dioxide. A _sesquioxide_, Zr₂O₃, is obtained as a greenish
powder when the hydride is burnt in oxygen; when heated in the air, it
oxidises very slowly, forming the dioxide.

  [464] Wedekind and Teletow (_Annalen_, 1913, ~395~, 149) have recently
  denied the existence of this oxide.

An _oxysulphide_, ZrOS, is obtained when the anhydrous sulphate is
heated in a current of sulphuretted hydrogen; it is a bright yellow
powder, which ignites spontaneously in the air. No disulphide is known.
The _carbide_, ZrC, is obtained, according to Moissan and Lengfeld,[465]
when the oxide is heated with carbon in any proportions, excess of
carbon separating on cooling as graphite; the process is hastened by
addition of lime. It is a hard, dark-grey solid, and is a very good
conductor of electricity. When heated in oxygen or nitrogen, it reacts
readily, forming the oxide and nitride respectively; halogens attack it
at quite low temperatures (250°-400°), forming the halide compounds,
which are indeed best prepared in this way. Strong mineral acids, with
the exception of hydrochloric acid, attack it, and fused alkalies
dissolve it readily.

  [465] _Compt. rend._ 1896, ~122~, 651.

The _fluoride_, ZrF₄, is best obtained by the action of anhydrous
hydrofluoric acid on the chloride. It forms a white crystalline mass,
which readily sublimes, and is soluble in hydrofluoric acid; from the
solution it crystallises as the trihydrate, ZrF₄,3H₂O. The anhydrous
substance is very slightly soluble in water in the cold; when warmed, it
hydrolyses, forming the hydroxide. The solution in hydrofluoric acid
dissolves metallic carbonates and oxides, forming the numerous
_fluozirconates_ or _zirconofluorides_.

There are many types of these compounds, of which the potassium salt,
K₂ZrF₆, is the most important. The solubility of this salt increases
very rapidly with the temperature; 100 parts of water dissolve, at 15°,
1·41 parts, at 100°, 25 parts of the compound. It has been frequently
used for the purification of zirconium compounds, for the preparation of
the element, and for analytical determinations. Other potassium salts,
K₃ZrF₇ and KZrF₅,H₂O, are obtained by using a large excess of potassium
fluoride and zirconium fluoride respectively. The ammonium compounds are
analogous in composition to the potassium salts, but the sodium salt,
Na₅ZrF₉, is obtained from mixtures of the components in all proportions;
on account of its very low solubility, it can be obtained by double
decomposition of the potassium salt with sodium chloride. Of the salts
with divalent metals, the types R´´ZrF₆,_x_H₂O and R´´₂ZrF₈,_x_H₂O, are
the most common.

The _chloride_, ZrCl₄, is known, on account of the ease with which it
hydrolyses, in the anhydrous state only. It can be obtained by all the
usual methods, of which perhaps the action of chlorine on the carbide,
and of carbon tetrachloride, or a mixture of chlorine and sulphur
monochloride, on the oxide, are the most convenient; an interesting
method consists in heating the oxide with phosphorus pentachloride in a
closed tube at 190°. It forms a volatile white sublimate, which fumes
strongly in air, and reacts vigorously with water; it is soluble in
ether. It forms a series of addition compounds with ammonia and organic
bases, as well as with the chlorides of non-metallic elements; warmed
with phosphorus pentachloride, it forms a stable solid, 2ZrCl₄,PCl₅,
which melts at 240°, and boils at 345°. With organic compounds,
especially with esters, acids, and phenols, it forms a long series of
addition and condensation products, of which the compounds
ZrCl₄(C₆H₅·COOC₂H₅)₂ and ZrCl₂[O·C₆H₅·CHO]₂ may be taken as examples.
By addition of organic bases to a solution of the chloride in alcoholic
hydrogen chloride, double chlorides of the type (C₅H₅NH)₂ZrCl₆ are
obtained.

The _oxychloride_, ZrOCl₂,8H₂O, separates in characteristic tetragonal
prisms when the tetrachloride is dissolved in water or hydrochloric acid
of any concentration. It is readily soluble in water and alcohol, but
sparingly soluble in hydrochloric acid, from which therefore it is
generally recrystallised. According to Chauvenet,[466] it effloresces in
dry air, forming the hexahydrate, ZrOCl₂,6H₂O; when dried in a vacuum,
it forms the hydrate, ZrOCl₂,3¹⁄₂H₂O, whilst the dihydrate, ZrOCl₂,2H₂O,
is obtained by heating at 100°-105° in hydrogen chloride. When the
dihydrate is heated to 230°, it forms another basic chloride,
ZrOCl₂ZrO₂,[467] which is stable up to 600°; above this temperature, it
breaks up, forming the volatile tetrachloride, and leaving a residue of
the dioxide.

  [466] _Compt. rend._ 1912, ~154~, 821.

  [467] _Ibid._ 1234.

By repeated evaporation of the oxychloride with small quantities of
water, a ‘metazirconium chloride’ is obtained, which dissolves in water
to a colloidal solution, and on dialysis yields a colloidal solution of
‘metazirconic acid.’

The _bromide_, ZrBr₄, very closely resembles the chloride; when treated
with water it forms the oxybromide, which separates from solution
according to the conditions in various hydrated forms, of which the
commonest is the octohydrate, ZrOBr₂,8H₂O. The _iodide_, ZrI₄, is a very
reactive body, which closely resembles the preceding; it forms an
oxyiodide, ZrOI₂,8H₂O.

_Zirconyl chlorate_, ZrO(ClO₃)₂,6H₂O, is obtained from the sulphate by
double decomposition with barium chlorate; it forms very soluble
colourless needles. Alkali iodates or iodic acid throw down a voluminous
_oxyiodate_, very sparingly soluble, like the corresponding ceric and
thorium salts, in water and acids.

_The Sulphates._--When zirconium dioxide is dissolved in concentrated
sulphuric acid, and the excess of acid removed by heating to 400°, the
‘neutral’ sulphate, Zr(SO₄)₂, remains. The compound dissolves in dilute
sulphuric acid to form solutions which contain various ‘complexes’ as
shown by conductivity measurements, and the behaviour towards oxalic
acid. Whilst solutions of the nitrate or chloride give immediate
precipitates with this reagent, solutions of the ‘sulphate’ give no
precipitate, or at most a very gradual one; moreover, addition of
sulphuric acid or of alkali sulphates to other zirconium salts inhibits
the oxalate precipitation. These facts are explained by regarding the
‘neutral’ sulphate, Zr(SO₄)₂,4H₂O, as zirconylsulphuric acid,
ZrOSO₄,H₂SO₄,3H₂O, which in solution ionises to 2H^{.} and ZrOSO₄,SO₄´´.
This conclusion is confirmed by the fact that whilst in solutions of the
chloride in hydrochloric acid, zirconium goes on electrolysis to the
cathode, on addition of sulphuric acid to the solution it travels to the
anode. The anhydrous compound and the hydrate are extremely soluble in
water, but much less readily soluble in dilute sulphuric acid. Probably
in solution more complex salts are formed by further hydrolysis, for by
addition of concentrated alkali sulphate solution in the cold, double
salts of the formula Zr₂O₃(RSO₄)₂,8H₂O are obtained. When the solution
is kept for some time at 39°-40°, a basic sulphate, 4ZrO₂,3SO₃,14H₂O,
separates slowly. When concentrated solutions are boiled, a salt,
2ZrO₂,3SO₃,5H₂O, separates as a crystalline precipitate; in contact with
water it slowly hydrates itself to the compound 2ZrO₂,3SO₃,14H₂O; when
heated to 300°, it becomes anhydrous without further change. Various
other basic, acid and complex salts have also been described.

The _nitrate_, Zr(NO₃)₄,5H₂O, separates from concentrated solutions
of the oxide in nitric acid by evaporation over sulphuric acid
and sodium hydroxide; it is believed to be a zirconylnitric acid,
ZrO(NO₃)₂,2HNO₃,4H₂O by analogy with the sulphate. When its aqueous
solutions are warmed, basic salts separate. Kolbe[468] has described an
additive compound with antipyrine, Zr(NO₃)₄,6C₁₁H₁₂ON₂, which is soluble
in water, and melts at 217°-218°.

  [468] _Zeitsch. anorg. Chem._ 1913, ~83~, 143.

When phosphoric acid or a soluble phosphate is added to a solution of a
zirconium salt, _zirconium phosphates_ of doubtful composition are
thrown down; by fusion methods, various double phosphates have been
prepared. A _hypophosphate_, Zr(PO₃)₂,H₂O, has recently been obtained
by Hauser and Herzfeld[469] by precipitation. The same authors have
prepared a _hypophosphite_, which is sensitive to light. When
hypophosphorous acid, H₃PO₂, is added to a solution of zirconium
nitrate, a precipitate is obtained, which dissolves in excess of the
acid; by addition of alcohol to the clear solution, the hypophosphite,
Zr(H₂PO₂)₄,H₂O, is thrown down in colourless, highly refracting prisms,
which on exposure to sunlight for a short time become deep violet,
without further perceptible change.

  [469] _Zeitsch. anorg. Chem._ 1913, ~84~, 92.

_Zirconium carbonate_ has recently been obtained by Chauvenet.[470]
Addition of sodium carbonate precipitates a basic orthocarbonate,
ZrCO₄,ZrO₂,8H₂O, soluble in excess; when dried in vacuo, the precipitate
loses water, forming the dihydrate, ZrCO₄,ZrO₂,2H₂O. When the latter
compound is treated with carbon dioxide at a pressure of 30-40
atmospheres, the neutral orthocarbonate, ZrCO₄,2H₂O, is formed. When the
compounds are heated, other basic salts are obtained.

  [470] _Bull. Soc. Chim._ 1913 [iv.], ~13~, 454.

_Zirconyl oxalate_, ZrO,C₂O₄, is obtained in the hydrated form when
oxalic acid is added to a zirconium salt in the presence of hydrochloric
or acetic acid. It is a white powder, soluble in oxalic acid, and easily
hydrolysed by water. If an aqueous solution of oxalic acid be saturated
with zirconium hydroxide, an acid oxalate, ZrOH(HC₂O₄)₃,7H₂O, is
obtained on evaporation. Double oxalates are readily obtained by
dissolving zirconium hydroxide in solutions of alkali hydrogen oxalates,
the general form being Zr(C₂O₄R´)₄,xH₂O. The _tartrate_ precipitated
when tartaric acid is added to a zirconium salt in solution probably has
the cyclic structure,

  COOZr(OH)₃
  |
  CH--O
  |    \
  |     Zr(OH)₂
  |    /
  CH--O
  |
  COOZr(OH)₃

as shown by the great rise in the specific rotatory power of solutions
of alkali oxalates on addition of zirconium compounds. The precipitate
dissolves readily in alkalies, and various double alkali tartrates have
been prepared; the potassium salt, ZrO(C₄H₄O₆K)₂,3H₂O, is analogous to
the thorium alkali tartrates. The solubility in alkalies is of great
importance for the separation of iron and zirconium.


~Atomic Weight of Zirconium.~--The value of this constant is not very
accurately known. The International Committee has adopted the value
90·6, but there is some uncertainty as to the value of the decimal
fraction. Berzelius in 1825 employed the analytical sulphate method, and
found Zr = 88·47. The numbers of Hermann (1844), obtained by the
analysis of the oxychloride, 2ZrOCl₂,9H₂O,[471] were very discordant,
the mean giving the value 89·56. Marignac in 1860 analysed the potassium
salt, K₂ZrF₆; this he heated with strong sulphuric acid, the residue
being ignited until all the zirconium sulphate was transformed to oxide;
the weighed mixture was then freed from potassium sulphate, and the
residual oxide dried and weighed. From the three ratios K₂ZrF₆ : ZrO₂,
K₂ZrF₆ : H₂SO₄, and K₂SO₄ : ZrO₂, he obtained the mean values 90·02,
91·55, and 90·68 respectively. Weibull in 1881-1882 determined the
ratios Zr(SO₄)₂ : ZrO₂ and Zr(SeO₄)₂ : ZrO₂ by ignition of the sulphate
and selenate respectively; he obtained the values 89·55 and 90·81.

  [471] Chauvenet (_loc. cit._) could not confirm the existence of this
  hydrate.

Bailey carried out a series of analytical sulphate determinations in
1890, obtaining the mean value 90·656. Brauner criticises the method on
the ground that the preparation of the pure neutral anhydrous sulphate
is almost impossible; the sulphate heated to 400° is not yet anhydrous,
so that Bailey’s result, on this ground, is probably too low. Venable in
1898 analysed the oxychloride; he claimed to have obtained the compound
ZrOCl₂,3H₂O, by heating the crystallised salt at 100°-125° in hydrogen
chloride, a method which Chauvenet (_loc. cit._) has found to lead to
the dihydrate, ZrOCl₂,2H₂O. His value was 90·803.


~Detection and Estimation.~--The following reactions may be employed to
distinguish zirconium:

(1) The oxalate precipitated from neutral or faintly acid solution
dissolves readily in excess of oxalic acid; the oxalates of thorium and
of the rare earth elements are practically insoluble under these
conditions. The fluoride also dissolves in excess of hydrofluoric acid
or of alkali fluoride, behaviour characteristic of this element alone
among the group.

(2) By fusion with sodium carbonate in the oxidising flame, a bead is
obtained, which, when dissolved in boiling hydrochloric acid, forms a
solution which gives a voluminous precipitate on addition of disodium
hydrogen phosphate, if zirconium is present. Iron, aluminium, titanium,
thorium, and rare earths have no influence on the test.[472]

  [472] Biltz and Mecklenburg, _Zeitsch. angew. Chem._ 1912, ~25~, 2110.

(3) A solution of a zirconium salt in hydrochloric acid gives an orange
colouration with curcuma paper. Ferric and titanium salts, if present,
must be reduced by means of zinc before the test is applied.

The _estimation_ of zirconium is complicated by the difficulty of
separating it from the accompanying elements. The solubility of the
oxalate in oxalic acid allows of a rapid and easy separation from
thorium and rare earth elements, so that iron, aluminium, and chromium
only remain to be removed. Iron may be separated by the thiosulphate
method, or other processes mentioned on p. 338; when free from that
element, zirconium may be separated from aluminium and chromium by
precipitation with alkali iodate in presence of the least possible
excess of acid. The precipitates in the thiosulphate and iodate methods
may be washed, and ignited directly to the dioxide, which is weighed as
such; if the zirconium is left after separation in solution, it may be
precipitated with ammonia,[473] and after washing and drying, ignited
and weighed as dioxide.

  [473] On account of the adsorption of alkalies by the zirconium oxide
  gel, potash or soda is never used for this precipitation.


~Thorium~, Th = 232·4

The name Thoria (thorina) was proposed by Berzelius in 1817 for what
appeared to be a new earth, but which in 1824 was recognised as a basic
yttria phosphate. In 1828 a new mineral was discovered by Esmark near
Brevig in Norway; to the oxide isolated from this, Berzelius gave the
name thoria, from its resemblance to the substance he had obtained in
1817. The homogeneity of the new element was questioned by Bergmann in
1857, and also by Bahr in 1862, but the conclusions of those authors
have been shown to be quite unfounded.

Thorium occurs in traces in a large number of common minerals, and in
varying quantities in most of the uranium and rare earth minerals. Its
occurrence in monazite, and the distribution of the latter mineral, have
already been dealt with; the commercial treatment of monazite is
described in Chapter XVIII. The oxide forms the chief constituent in
Thorite, with its gem-variety Orangite, and the various secondary
minerals, and in the mineral Thorianite, in which the only other
important constituent is uranous oxide. The extraction from these
minerals is a comparatively simple matter. Decomposition is easily
effected by hydrochloric or sulphuric acid, thorianite dissolving easily
also in nitric acid; the solutions obtained, after appropriate treatment
to remove silica, excess of acid, etc., are treated with sulphuretted
hydrogen, to remove lead, bismuth, and similar foreign metals, and freed
from the rare earths by the carbonate, oxalate, or sulphate methods. The
last depends on the fact that thorium sulphate and its hydrates are much
less soluble than corresponding compounds of the rare earth elements;
the first two on the fact that thorium salts dissolve readily in excess
of alkali carbonates or oxalates, whilst the rare earth compounds are
much less easily soluble.

Thorium, like zirconium, forms only one series of salts, in which the
metal is tetravalent. The formula ThO was originally put forward by
Berzelius for the oxide, from its resemblance to the ceria and yttria
oxides, and its general occurrence with these. The true formula was
deduced, when the valency of zirconium had been decided by the vapour
density experiments of Troost and Deville, in 1857, from the isomorphism
of zircon and thorite, and the close relationship between the compounds
of the two elements, especially among the double fluorides, and was
confirmed by a determination of the specific heat of the metal by Nilson
in 1883.

In its chemical relations, the element resembles zirconium, though, as
is to be expected from the high atomic weight, it shows a much more
marked electropositive character, approaching in this respect the
elements of the yttrium group. The oxide has no longer acid properties,
and the neutral salts, though they hydrolyse readily and are therefore
acid to indicators in solution, may be recrystallised unchanged from
aqueous solution. The tendency to form double salts is still present,
though diminished; the oxalate is soluble in a large excess of alkali
oxalate, but not in oxalic acid, and the double fluorides are less
numerous and varied than those of zirconium and titanium. On the other
hand, it forms a well-crystallised and characteristic series of double
nitrates, R´₂Th(NO₃)₆, isomorphous with the analogous ceric salts. In
the behaviour of its sulphate it differs markedly from zirconium, and
closely approaches the rare earth elements. The hydroxide has the
characteristic tendency to form colloidal solutions and gels.

Thorium is peculiar, among the elements which have been considered,
through its property of giving characteristic radiations, and
disintegrating with formation of a whole family of new elements; or, as
it is commonly expressed, through its radioactive properties.[474] The
element has a half-life period of the order of 4 × 10¹⁰ years; in the
course of decay, it gives rise to mesothorium 1, which is rayless, but
decays to mesothorium 2, with its product radiothorium, both of which
give powerful radiations. Mesothorium 1 of course occurs in all
thorium-containing minerals, and may be separated from monazite by
addition of a barium compound during the sulphuric acid decomposition;
in consequence of the powerful radiating properties of its products, it
is itself of considerable importance, and proposals for extracting it
from monazite in the preparation of the thorium nitrate of commerce have
been put forward (see p. 276).

  [474] The nature of the present work allows only the briefest
  reference to be made to the exceedingly interesting phenomena which
  centre about this subject; for a more complete account, the student
  should consult Soddy, _The Chemistry of the Radio-Elements_, Part I,
  1911.

Mesothorium appears to be chemically identical with radium; since
monazite, like all other thorium-bearing minerals, contains uranium and
radium, the latter element is separated with the mesothorium, and
indeed, having a very much larger half-life period, constitutes by far
the greater part of such ‘mesothorium’ preparations. On account of the
great activity of the mesothorium products, the best preparations from
monazite, though estimated to contain only 1 per cent. of mesothorium to
99 per cent. radium, are said to be four times as active as pure radium
compounds. The chemical identity of the two products seems to preclude
any possibility of determining the physical properties and constants of
mesothorium.

The element radiothorium, which was discovered by Hahn in 1905, in the
mineral thorianite, is chemically identical with the parent element
thorium, but can be separated by means of the intermediate element,
mesothorium 1. The latter is readily separated by the sulphate
precipitation, and the radiothorium to which it gives rise may be
separated by precipitation with ammonia. Thorium is also chemically
identical with ionium, the parent of radium, and the thorium nitrate of
commerce therefore contains important quantities of ionium--important
that is, in view of the high radiating power of the latter element. The
study of these relationships constitutes one of the most important and
interesting fields in the province of radioactivity.

_The Metal._--Elementary thorium has not yet been obtained in the pure
state, owing to the ease with which it forms compounds and alloys with
all the common elements, and to its great affinity for oxygen; the high
melting-point also increases the difficulty of obtaining the pure metal.
Berzelius attempted to reduce the alkali double fluorides and double
chlorides with sodium or potassium; Nilson carried out the same reaction
in a closed iron cylinder, but his product still contained 20 per cent.
of thoria. Reduction of the oxide with magnesium is never complete, and
the carbon method gives only a mixture of carbide and metal.
Electrolytic methods give no better results, since the metal liberated
at the cathode always encloses oxide and other impurities. Moissan and
Hönigschmid in 1906, by heating the carefully purified anhydrous
chloride with sodium in a sealed glass tube from which air and moisture
had been removed, claim to have obtained a product containing only 3 per
cent. of the oxide. The element has recently been prepared in leaf form
by forcing the amorphous product into the bore of a copper tube,
hammering into sheets, and removing the copper by dilute nitric
acid.[475]

  [475] v. Bolton, _Zeitsch. Elektrochem._ 1908, ~14~, 768.

The amorphous impure metal is a dark grey powder, of specific gravity
11·3; the hammered and strongly heated leaf has the density 12·16. It
burns readily in air with great brilliance, and when finely powdered
ignites if crushed or rubbed. When heated in the electric furnace, it
melts, according to von Bolton,[476] at about 1450°; von Wartenburg[477]
found the melting-point to be about 1700°; the fused beads resemble
platinum in physical properties. It is somewhat resistant to acids,
dissolving easily only in aqua regia, and more slowly in fuming
hydrochloric acid. It combines directly when heated in sulphur or
halogens, and in nitrogen and hydrogen.

  [476] v. Bolton, _Zeitsch. Elektrochem._ 1908, ~14~, 768.

  [477] _Ibid._ 1909, ~15~, 866.

The _hydride_, ThH₄, is best obtained by heating the metal in hydrogen,
an energetic reaction taking place at a red heat. Winkler observed that
a mixture of the dioxide with magnesium absorbs hydrogen readily when
heated. The hydride is a stable greyish-black powder, not attacked by
water, but dissolving readily in hydrochloric acid, with evolution of
hydrogen. The _nitride_, Th₃N₄, is prepared by heating the metal in the
gas, or the carbide in a stream of ammonia. It is a brown powder,
decomposed by water with evolution of ammonia and formation of the
dioxide. The _azide_ has been used for purposes of detection and
estimation, since in boiling solution it is hydrolysed with separation
of the hydroxide; zirconium and ceric salts also show this reaction, but
the rare earth salts give no precipitate.

The _hydroxide_, Th(OH)₄,_x_H₂O, is precipitated from solutions of
thorium salts by alkalies or ammonia, as a gelatinous white precipitate,
insoluble in excess. It dissolves readily in mineral acids or in alkali
carbonates. Hydrogen peroxide and ammonia throw down an hydrated
_peroxide_, Th₂O₇; from neutral solutions hydrogen peroxide alone throws
down _peroxy-salts_, which contain acid groups. This peroxide may also
be obtained by the action of sodium hypochlorite or hydrogen peroxide on
the hydroxide, as in the case of the zirconium compound. It readily
gives up oxygen, passing into the more stable peroxide, ThO₃. Since in
neutral or faintly acid solutions zirconium and the rare earths give no
precipitate with hydrogen peroxide, the reaction is extremely useful in
the detection and estimation of thorium.

_Thorium dioxide_, ThO₂, is obtained by the ignition of the hydroxide or
of suitable salts as a white powder, of which the properties and
appearance depend largely on the method and temperature used in its
formation. Whilst the residue obtained by ignition of the nitrate is an
extraordinarily voluminous and light flaky mass, the sulphate yields a
dense thick powder; the nitrate was therefore always preferred in the
manufacture of incandescent mantles (_q.v._), as it was thought that the
oxide obtained from it was the most suitable for illumination. In the
crystalline form the oxide has been obtained in the laboratory by fusion
with borax and with potassium phosphate. The first method gives
tetragonal crystals, probably isomorphous with those of rutile and
cassiterite; the phosphate fusion is said to give cubic crystals (see p.
74). The oxide is insoluble in acids, but can be transformed into the
sulphate by evaporation with concentrated sulphuric acid, or fusion with
alkali bisulphate. It does not liberate carbon dioxide when fused with
alkali carbonates.

By repeated evaporation with small quantities of acids, thoria can be
transformed into a gel soluble in water (thorium meta-oxide). The sol is
an opalescent fluid, orange-red by transmitted light, and contains small
quantities of the acid employed. The hydroxide may also be obtained in
this form by carefully washing it, and boiling with small quantities of
acids, or with thorium or other salts, or even by long continued washing
with pure water; similarly, continued dialysis of thorium salts
eventually yields such gels. The colloid is positively charged, and
resembles the zirconium oxide gel in its relation to negatively charged
colloids. The gel is easily precipitated by electrolytes.

Ignited thorium oxide has found considerable application in recent years
a catalyst in the preparation of ketones by the contact method of
Sabatier and Senderens.[478] By passing mixtures of the vapours of
appropriate acids over the catalyst heated to the necessary temperature,
good yields of the required ketones are obtained.[479]

  [478] Cf. Senderens, _Ann. Chim. Phys._ 1913 [viii.], ~28~, 143.

  [479] Cf. Pickard and Kenyon, _Trans. Chem. Soc._ 1913, ~103~, 1923.

The _sulphide_, ThS₂, is obtained, together with the oxysulphide, ThOS,
according to Duboin,[480] by passing a current of sulphuretted hydrogen
over a mixture of thorium chloride with excess of sodium chloride, at a
red heat. The former forms large brown crystals, from which the small
orange-yellow crystals of the oxysulphide may be separated by means of a
sieve; the latter is purified by treatment with warm nitric acid, which
dissolves the sulphide very readily. The oxysulphide is also obtained
when the anhydrous sulphate is heated in sulphuretted hydrogen.[481]

  [480] _Compt. rend._ 1908, ~146~, 815.

  [481] Hauser, _Zeitsch. anorg. Chem._ 1907, ~53~, 74.

The _carbide_, ThC₂, is obtained by the action of carbon on the oxide in
the electric furnace; it is a yellow crystalline mass, decomposed slowly
by water, energetically by dilute acids in the cold, with evolution of a
complex mixture of hydrogen and hydrocarbons, in which many members of
the paraffin, olefine and acetylene series have been observed.[482]
Hydrogen constitutes over 50 per cent. of the mixture, the next most
important constituents being the acetylenic hydrocarbons, followed by
ethane.

  [482] Lebeau and Damiens, _Compt. rend._ 1913, ~156~, 1987.

_Thorium fluoride_, ThF₄, is obtained anhydrous by passing hydrogen
fluoride over the anhydrous chloride or bromide at a temperature of
350°-400°. The tetrahydrate, ThF₄,4H₂O, is precipitated by addition of
hydrofluoric acid to a solution of a thorium salt, or by the action of
the acid on the hydroxide. Hydrofluosilicic acid also throws down the
fluoride, even in the cold, from solutions of thorium salts. The
fluoride is insoluble in water and mineral acids, as well as in excess
of precipitant; this behaviour allows of a complete and easy separation
of thorium from titanium and zirconium. The rare earth fluorides are
also much more easily soluble in concentrated mineral acids than thorium
fluoride, so that this compound may also be used in the separation from
the rare earths. When heated in a stream of the acid to 800°, the
hydrated salt yields the oxyfluoride, ThOF₂; ignited in the air, it
leaves the dioxide. Precipitation with potassium fluoride gives the
_double fluoride_, KThF₅,H₂O, which may be obtained anhydrous by fusion
of the mixed fluorides; it is insoluble. An amorphous insoluble
compound, K₂ThF₆,4H₂O, is obtained by boiling the hydroxide with a
mixture of potassium hydrogen fluoride and hydrofluoric acid. Sodium and
ammonium fluorides throw down the simple fluoride.

_Thorium chloride_, ThCl₄, is obtained in the anhydrous form by all the
usual methods, the most convenient being perhaps the action of chlorine
and sulphur monochloride on the heated dioxide. It almost invariably
contains small quantities of oxychloride. When pure, it forms colourless
needles fairly stable in dry air; the impure product gradually darkens
in colour. It dissolves in water with considerable evolution of heat,
and is soluble also in alcohol and moist ether. It melts at about 820°,
and sublimes unchanged at somewhat higher temperatures; the vapour
begins to dissociate at about 1050°, the dissociation increasing rapidly
as the temperature rises. It resembles zirconium chloride in the ease
with which it forms additive compounds with ammonia and organic bases,
and addition and condensation products with organic oxygen-compounds;
many _double_ and _complex chlorides_ are also known, among which the
platinum compounds ThPtCl₈,12H₂O and Th₂Pt₃Cl₁₄,24H₂O, and the pyridine
salt (C₅H₅NH)₂ThCl₆ may be mentioned.

From aqueous solution the octohydrate, ThCl₄,8H₂O, separates at ordinary
temperatures; a heptahydrate and an enneahydrate have been described as
precipitated from the alcoholic solution by addition of water. The
_basic salts_, Th(OH)Cl₃,7H₂O and Th(OH)₂Cl₂,5H₂O, have been obtained
by addition of the hydroxide to alcoholic hydrogen chloride. The
_oxychloride_, ThOCl₂, may be obtained by the carefully regulated action
of carbon tetrachloride on the dioxide, according to the equation:

  ThO₂ + CCl₄ = ThOCl₂ + COCl₂

It is a colourless crystalline solid, which takes up moisture from the
air, forming the hexahydrate.

_Thorium bromide_, ThBr₄, is a volatile solid which boils at 725°; it
closely resembles the chloride. The _iodide_ and a _basic iodide_,
Th(OH)I₃,10H₂O, are known.

No cyanide of thorium is known, addition of potassium cyanide merely
causing separation of the hydroxide. A _ferrocyanide_, Th[Fe(CN)₆],4H₂O,
is thrown down as a white powder by potassium ferrocyanide; with
potassium ferricyanide no precipitate is obtained. The _platinocyanide_,
Th[Pt(CN)₄]₂,16H₂O, is obtained by double decomposition in
yellowish-brown prisms.

Among the halogen oxysalts, the _perchlorate_, _chlorate_, _bromate_,
and _iodate_ were prepared by Cleve. The iodate is of great importance
for purposes of detection and estimation, from the fact that, in
presence of a large excess of alkali iodate, it is insoluble in strong
nitric acid, whilst the analogous compounds of the rare earth elements
dissolve readily in that solvent.

The _sulphate_, Th(SO₄)₂, is obtained anhydrous by evaporating the
excess of acid from a solution of the dioxide in oil of vitriol, or by
heating the hydrates. It resembles the sulphates of the rare earth
elements, in that it dissolves in water at 0° to form a highly
supersaturated solution, from which the hydrated forms separate out
almost quantitatively when the temperature is allowed to rise. The
solubility relations of the various hydrates, on account of their
commercial importance, are somewhat fully treated in Chapter XVIII. A
dihydrate, Th(SO₄)₂,2H₂O, is obtained by keeping the tetrahydrate at
110°. The ennea- and octohydrates are isomorphous with the corresponding
thorium selenate hydrates, and the ennea- and tetrahydrates with the
analogous uranous sulphate hydrates. The hydrates yield the anhydrous
salt when heated to 400°; the anhydrous sulphate has already a
considerable dissociation tension (15 mm.) at 575°. By treatment with
excess of acid, and subsequent heating to 130° _in vacuo_, the _acid
sulphate_, Th(SO₄),H₂SO₄, is obtained. An insoluble _basic salt_,
ThOSO₄,2H₂O, is formed by continued boiling of the tetrahydrate in
dilute solution, or more quickly by heating the solution in a closed
tube to 120°-125°; a monohydrate, ThOSO₄,H₂O, is also known. Halla[483]
has recently obtained the hydrate, ThOSO₄,5H₂O, by boiling a solution of
the neutral sulphate with magnesium sulphate, and also by treating the
anhydrous sulphate with a little water in presence of magnesium
carbonate.

  [483] _Zeitsch. anorg. Chem._ 1912, ~79~, 260.

By precipitation with potassium sulphate the _double salt_,
Th(SO₄)₂,2K₂SO₄,2H₂O, is formed; this is soluble in water but insoluble
in potassium sulphate solution. The analogous sodium and ammonium salts
are soluble both in water and excess of the corresponding alkali
sulphate.

The _sulphite_, Th(SO₃)₂,H₂O, is obtained as a white amorphous
precipitate by warming a solution of a thorium salt with sulphurous
acid. Basic sulphites and double sulphites are also known; the
precipitates obtained by addition of alkali sulphite dissolve readily in
excess. The hydroxide is almost insoluble in sulphurous acid, behaviour
which distinguishes thorium (and zirconium) from all the trivalent
metals. No _thiosulphate_ is known, the hydroxide being thrown down from
boiling solution by addition of sodium thiosulphate: this method of
precipitation was formerly much used for purposes of estimation, but it
is more tedious and less accurate than the modern methods.

_Thorium nitrate_, Th(NO₃)₄,12H₂O, crystallises at ordinary temperatures
in large hygroscopic tablets, very soluble in water and alcohol. The
hydrates, Th(NO₃)₄,6H₂O and Th(NO₃)₄,5H₂O, have been obtained from hot
solution and from nitric acid solution respectively. Thorium is employed
in commerce almost entirely in the form of this salt, which is
dehydrated until it contains about 48 per cent. ThO₂, which
approximates to the formula Th(NO₃)₄,4H₂O; the commercial product,
however, is not a definite hydrate. Kolbe[484] has described the
additive product with antipyrine, 2Th(NO₃)₄,5C₁₁H₁₂ON₂, which melts at
168°. The extent to which thorium salts are hydrolysed in solution is
very considerable, as is evident from the fact that the nitrate may be
titrated with standard potash in presence of phenolphthalein as
indicator; the solution first becomes alkaline to this reagent when 3·5
molecules of potash have been added for each molecule of thorium nitrate
present.[485] Of the large number of _double nitrates_ which have been
prepared, the general types R´₂Th(NO₃)₆, where R´ = NH₄,K,Rb,Cs, and
R´´Th(NO₃)₆,8H₂O, where R´´ = Mg,Mn,Zn,Ni,Co, are the most important.

  [484] _Zeitsch. anorg. Chem._ 1913, ~83~, 143.

  [485] Halla, _loc. cit._

_Thorium phosphates._--The precipitates obtained by addition of
phosphoric acid or alkali phosphates to solutions of thorium salts are
gelatinous solids of doubtful composition; they dissolve in mineral
acids and in alkali carbonates, and their behaviour is of great
importance in the technical treatment of monazite. Various phosphates
and _double phosphates_ are obtained by fusion methods, but none of
these are important. The _phosphite_, Th(HPO₃)₂,3H₂O, and
_hypophosphite_, Th(H₂PO₂)₄, are insoluble solids obtained by double
decomposition. The _hypophosphate_ ThP₂O₆,11H₂O, is of great importance
for purposes of detection and estimation, since it is thrown down
quantitatively from strongly acid solutions; under these conditions the
rare earths remain in solution.

No neutral _carbonate_ of thorium is known. Alkali carbonates
precipitate a basic salt, which dissolves readily in excess; this fact
is of very great importance in the commercial extraction of thorium, the
sodium and ammonium double carbonates of the cerium elements being
almost insoluble in alkali carbonates. Addition of alcohol to the
solution throws down double carbonates, which can be washed with ice
water. The salts K₆Th(CO₃)₅,10H₂O, Na₆Th(CO₃)₅,12H₂O, and
(NH₄)₂Th(CO₃)₃,6H₂O have been obtained in this way; they dissolve
readily in water or dilute alkali carbonate, though on warming or
diluting the solution, the hydroxide separates. The thallium compound,
Tl₆Th(CO₃)₅, is sparingly soluble, and is thrown down from a solution of
the ammonium compound on addition of a thallium salt; it has been
proposed for the microchemical detection of thorium. The quantitative
separation of thorium by means of pure moist lead carbonate has been
proposed for the purpose of estimation (see p. 288).

_Thorium oxalate_, Th(C₂O₄)₂,6H₂O, is precipitated quantitatively by
means of oxalic acid, even in presence of considerable quantities of
mineral acids. It is less soluble in sulphuric acid than any of the rare
earth oxalates,[486] and is not attacked, as are the latter compounds,
by concentrated nitric acid. In hydrochloric acid the solubility first
increases rapidly with the concentration of the acid, and then suddenly
decreases; this behaviour is due to the formation of an _oxalochloride_,
3Th(C₂O₄)₂,ThCl₄,20H₂O. When the amorphous oxalate obtained by
precipitation is allowed to remain for a considerable time in contact
with acids, it forms characteristic tetragonal prisms of the more stable
form. The dihydrate, Th(C₂O₄)₂,2H₂O, is obtained when the hexahydrate is
dried over sulphuric acid, or heated to 100°. The salt dissolves easily
in excess of alkali oxalate, but is precipitated from the solutions by
mineral acids, a fact which allows of another means of separation from
zirconium, the double oxalates of which are much more stable towards
acids. The solubility of the oxalate in alkali oxalate allows of
separation from the rare earth elements, whilst its insolubility in
excess of oxalic acid can be used for the separation from zirconium.

  [486] Hauser and Wirth, _Zeitsch. anorg. Chem._ 1912, ~78~, 75.

The _formate_ and _acetate_ can be obtained in the form of neutral salts
by the action of the acids on the hydroxide; by double decomposition,
amorphous precipitates of basic salts are obtained. With _tartaric acid_
stable complex compounds are formed, as shown by the fact that alkalies
will not precipitate the hydroxide from a solution in presence of that
reagent, and by the elevation of the specific rotatory power. Many
_complex salts_ are known, the simplest having the composition
ThO(C₄H₄O₆R´)₂,8H₂O, where R´ = K,Na,NH₄; these are obtained by
dissolving thorium hydroxide in concentrated solutions of alkali
hydrogen tartrates. _Thorium acetylacetone_, Th(C₅H₇O₂)₄, is
precipitated by addition of ammonia to an aqueous solution of the
nitrate mixed with acetylacetone dissolved in ammonia; the solid is
recrystallised from alcohol, and melts at 171°.


~Atomic Weight of Thorium.~--The value adopted by the International
Committee (1914) is 232·4, but most of the determinations carried out
within the last thirty years show considerable discrepancies. The
earlier work of Berzelius (1829) and Chydenius (1861) led to very widely
varying results, and for the same reason little reliance can be placed
on the results of Delafontaine (1863) and Hermann (1864). In 1874 Cleve
determined the constant by ignition of the sulphate, obtaining the mean
values 234·03 and 233·97; the figure 234 based on these results was for
many years accepted as the true atomic weight. A series of
determinations carried out by Nilson in 1882 led to much lower results.
He employed the sulphate ennea- and octohydrates, first dehydrating
these, and then igniting to oxide, and showed that Cleve’s value must be
too high on account chiefly of the hygroscopic nature of the ignited
oxide, which increases in weight when kept; but his own values show
considerable discrepancies. The ratio Th(SO₄)₂,9H₂O-ThO₂ : ThO₂
(enneahydrate converted to oxide) gave the figure (corrected to vacuo)
232·51, whilst the ratio ThO₂ : 2SO₃ (anhydrous sulphate to oxide) gave
232·16; the ratio Th(SO₄)₂ : 9H₂O (hydrate to anhydrous salt) gave,
however, 233·75. The value obtained for the ratio ThO₂ : 2SO₃ for
anhydrous sulphate prepared from the octohydrate was 232·49 (corrected
to vacuo). Five years later, Krüss and Nilson prepared the anhydrous
sulphate from the pure octohydrate, and ignited this to the oxide. The
ratio ThO₂ : 2SO₃ gave as a mean of very concordant results the figure
232·49.

Brauner criticises these values on the ground that no details are given
as to the temperature required to obtain the anhydrous salt from the
hydrates, and that probably some traces of sulphate must be decomposed
at the temperatures required (450°-500°) to drive off all the water. The
results obtained from the enneahydrate are to a great extent invalidated
by the doubts as to the purity of the hydrate, completeness of
dehydration, etc., which arise from the discrepancies in the values
deduced from the three ratios. He accepts, however, the figure 232·49
obtained by Nilson and by Krüss and Nilson from material separated as
octohydrate, with some uncertainty as to the second decimal figure.

Brauner himself employed the oxalate method in 1898; the purified
hexahydrate was used, the percentage of thoria being determined by
ignition, and of (C₂O₃) by titration with permanganate. The ratio ThO₂ :
2C₂O₃ gave results varying from 232·21 to 232·29, but as the value rose
continuously as purification was carried further and further, he did not
feel justified in taking a mean value. In 1900 Urbain determined the
constant with material purified by the acetylacetone method. He prepared
the octohydrate, heated it for ten hours in a bath of sulphur vapour at
440°, and ignited the anhydrous salt so obtained at a white heat. The
ratio ThO₂ : 2SO₃ gave the result (corrected to vacuo) Th = 233·67.
Brauner criticises the value on the ground that the hydrated salt was
heated in a vessel open to the air, and that at the high temperature
obtained, traces of moisture gaining access to the sulphate caused
hydrolysis, with loss of sulphuric acid; this would cause the results to
be too high. In 1905 Meyer and Gumperz employed the same method, and
obtained values varying from 232·2 to 232·7, with the mean 232·47.
Finally Brauner carried out an extended investigation to disprove the
heterogeneity of thorium which had been ‘discovered’ by Baskerville
(1904), in the course of which he showed the atomic weight of the
element to lie between the limits 232·34 and 232·52.


~Detection of Thorium.~--The element is best detected in a mixture of
earths by the following reactions:

(1) Precipitation with hydrogen peroxide from warm, faintly acid
solution.

(2) Precipitation with sodium hypophosphate, Na₂H₂P₂O₆, in concentrated
hydrochloric acid solution. On boiling, a perceptible precipitate is
obtained if only traces of thorium are present; but ceric and zirconium
salts and titanium must be absent. The latter element gives no
precipitate under these conditions if hydrogen peroxide is present;
ceric salts may be decomposed by boiling. The possible presence of
zirconium renders it necessary to boil the hypophosphate precipitate
with nitric acid; on addition of oxalic acid to the clear solution,
thorium is precipitated, whilst zirconium remains in solution, and may
be detected.

(3) Potassium azide, KN₃, throws down thorium hydroxide from boiling
neutral or faintly acid solutions. Ceric salts if present must be
previously reduced; zirconium must be previously removed by oxalic acid.

(4) Precipitation may be effected with potassium iodate in strong nitric
acid solution. Here also ceric salts must be reduced before applying the
test. Zirconium also gives the test; the precipitate must therefore be
washed and warmed with oxalic acid, in which thorium iodate is
insoluble, whilst zirconium iodate is soluble.

The methods of estimating thorium are given in Chapter XVIII.




PART III

THE TECHNOLOGY OF THE ELEMENTS


CHAPTER XVII

THE INCANDESCENT MANTLE INDUSTRY--HISTORICAL AND GENERAL INTRODUCTION

The group of elements which we are considering can be divided, from the
point of view of technical application, into two classes. The first of
these contains one element only, titanium, which in its technology, as
in its chemistry, stands apart from the others; it will, accordingly, be
treated in a separate chapter. The second class contains the yttrium and
cerium metals, with zirconium and thorium; the technical importance of
these elements is due chiefly to the use of their oxides in
illumination, to a small extent in Nernst lamps, and to a much greater
extent in the so-called Incandescent Lighting. The manufacture of
incandescent mantles[487] is a large and ever-extending industry,
intimately bound up with the older process of coal-distillation, with
its innumerable ramifications; indeed, it may be said that but for the
ingenious invention of Dr. Auer, illumination by means of coal-gas would
to-day have been almost obsolete. The discovery which resulted in the
production of the familiar incandescent mantle of the present day may be
regarded as the culmination of a century’s effort to increase the value
of coal-gas as an illuminating agent. In the present chapter it is
proposed to outline the history of these endeavours, and to give a short
general account of Auer’s work and its results.

  [487] The term ‘incandescent mantle’ is not, perhaps, scientifically
  very desirable. It is used here, not only on account of its general
  acceptance, but also because there seems to be no brief and convenient
  term which might be used in its stead.

Soon after the introduction of gas as an illuminating agent it was
realised that the luminosity of the flame is dependent on the presence
of solid particles, which by the heat of combustion of the gas are
raised to a temperature at which they emit radiations of wave-lengths
corresponding to the ‘luminous rays’ of the spectrum. A non-luminous
flame of sufficiently high temperature, therefore, can be rendered
luminous by the introduction of suitable solids, and numberless
investigators have striven, during the past century, to discover the
most suitable method of increasing the luminosity of a flame in this
way. The luminosity of the ordinary ‘bats-wing’ or ‘flat’ flame, now so
rapidly going out of use, is due to the presence in the outer zone of
the flame of heated particles of carbon, produced by the
decomposition--or partial combustion--of ‘dense’ hydrocarbons, _i.e._ of
hydrocarbons having a high percentage of carbon. Ordinary coal-gas
consists largely of a mixture of hydrogen and methane, both of which
burn with practically non-luminous flames, with small quantities of
olefines, acetylenes, etc., to which the luminosity is chiefly due. It
would appear, then, that by the introduction of dense hydrocarbons, a
gas of poor illuminating power might be made much more valuable as a
source of light. On the other hand, it is also apparent that the same
end might be achieved by the introduction into a non-luminous or feebly
luminous flame of an altogether foreign substance, introduced as such,
and not continuously consumed, as is the carbon in the former method.
Both these directions of improvement have been followed; since, however,
the results achieved by the latter method have become recently of far
greater importance, the applications of the first method will be
dismissed quite briefly, and the history of the second will then be
treated somewhat fully.

The first important attempt to increase the illuminating power of gases
burning with feebly luminous flames was that of Faraday, who in the
course of an investigation into the causes of the variations in
luminosity of ‘portable gas,’ discovered benzene, or bicarburet of
hydrogen, as he called it, in 1826. In 1830 an engineer named Dunnovan
undertook to illuminate Dublin by means of water-gas[488] which he
‘carburised’ by addition of dense hydrocarbons. During the latter half
of the nineteenth century this method became of some importance. It has
been applied, in particular, to enrich the ‘natural gas’ of Ohio, North
America. The dense hydrocarbons necessary for this purpose are obtained
by the process known as ‘cracking.’ The viscous residues from the
distillation of the mineral oil of the district are allowed to drop into
a brick chamber, of which the walls are raised to a bright red heat, and
the dense hydrocarbons which are evolved are removed by a current of the
gas to be enriched. In this way a gas of relatively high illuminating
power is obtained.

  [488] Water-gas is a mixture of equal volumes of carbon monoxide and
  hydrogen, obtained by blowing steam through a glowing coke furnace. At
  intervals the steam is shut off, and air is blown through to raise the
  temperature of the coke.

In the year previous to that in which Faraday first carburised
water-gas, Berzelius had observed that thoria and zirconia, when heated
in a non-luminous flame, emit an intense white light. Similar behaviour
had long before been observed in the cases of magnesia, alumina, lime,
zinc oxide, etc. The first practical application of this property of the
oxides was that of Drummond, who in 1826 heated a pencil of lime in the
oxy-hydrogen flame and obtained the intense white light which has since
become so familiar as the Drummond or ‘lime-light.’ A further
development in this direction was due to du Motay and Maréchal, who in
1867 illuminated the Place de Tuileries and the Hôtel de Ville in Paris
by means of pencils of compressed zirconia--magnesia was also
used--heated by means of oil vapour and oxygen.

The use of non-luminous flames to secure illumination, by raising the
temperature of solids suspended in them to the point of incandescence
was proposed in 1839 by Cruickshank, who used a mantle of platinum wire,
covered with lime and rare earths, which he heated by means of
water-gas. In 1846 Gillard employed mantles of platinum wire, raised to
incandescence in the flame of burning hydrogen, which he obtained by
passing steam over heated iron wire; later he used water-gas (1848), his
lamps with this modification being employed in Paris and in
Philadelphia. Narbonne was later illuminated (1856-1865) by a similar
device, but permanent success could hardly be obtained in view of the
cost of the platinum mantles, which lasted only a few months. The same
mantle was proposed in 1882 by Lewis, the ordinary Bunsen flame being
suggested as the source of heat. In the same year Popp exhibited at the
Crystal Palace lamps in which a platinum mantle was raised to
incandescence by means of a flame of coal-gas and heated air. These
attempts, however, served only to show that no permanent advance could
be made in this direction.

A new development was made in 1880 by Clamond. He prepared a paste by
grinding up calcined and powdered magnesia with a concentrated solution
of magnesium acetate; by forcing this through a press he obtained a
ribbon which was then wound crosswise on a wooden shaper, dried
carefully, and ignited. In his later experiments twenty per cent. of
zirconia was added to the magnesia. The mantle was supported in a
platinum cage and heated in the flame of a mixture of coal-gas and
heated air. This mantle gave an intense light, but was too fragile for
extended use. In the following year, Lundgren patented a process by
which lime, magnesia, and zirconia, made into a paste by the addition of
gum, were forced through a press, and the resulting thread wound on a
graphite-covered shaper. The mantle so obtained was stable, and gave an
intense white light, but after having been heated for some time the
oxides crumbled to powder. A modification of this process was introduced
by Knöfler in 1894, in an attempt to use a cellulose solution containing
rare earth salts; this was forced through jets, and the cellulose
precipitated as a continuous thread from which the mantle was made. A
further modification of Knöfler’s process by Plaisetty in 1901 was
technically successful; but these developments must be taken up in a
later chapter (_vide_ p. 307).

In 1883 a process was patented by Fahnehjelm in Stockholm, by which for
the first time a cheap and stable mantle of considerable efficiency was
produced, and which, but for the advent of the Auer mantle, would
undoubtedly have been commercially successful. Fahnehjelm’s mantle
consisted of an arrangement of needles or lamellæ of magnesia, lime,
zirconia, etc., suspended over a burner. The plates and needles were
usually arranged in the form of a comb of suitable shape, and were found
to give an intense light, and to be long-lived. In later forms the combs
were made of rods of magnesia dipped into solutions of chromium salts.
The great disadvantage of this invention lay in the fact that the combs
required to be heated in the flame of water-gas, in order to secure a
good incandescence; had it been possible to attain a sufficiently high
temperature by the use of coal-gas, it is doubtful whether the Auer
mantle would have ever been evolved.

The more important attempts to secure arrangements by which the
radiations of heated solids could be used for illumination have now been
outlined and the ground cleared for the consideration of the work of
Baron von Welsbach. There remain yet to be mentioned, however, two
attempts which are of especial interest in view of that work. The first
is that of Frankenstein, who in 1849 made use of a ‘Light-multiplier’
obtained by impregnating gauze with a paste of chalk and magnesia ground
with water. The second is that of Edison, who proposed (1878) to utilise
the observations of Bahr and Bunsen (1864) and of Delafontaine (1874),
of the remarkable incandescence exhibited by the yttria and erbia
earths, and the terbia earths, respectively, when heated; he suggested
the employment of a mantle of platinum wire covered with zirconia and
the oxides of the rare earth metals, a proposal similar to that put
forward nearly forty years earlier by Cruickshank.

About the year 1880 Dr. Carl Auer began the study of the rare earth
elements. The chemical aspect of his work has already been dealt with
(_vide_ p. 168); but the results obtained by the technical application
of his observation that threads of cotton, impregnated with a solution
of salts of the elements, leave after ignition a coherent ash of oxide,
which glows brightly when heated, have been of far greater importance
than the purely scientific aspect, valuable though that is. A series of
experiments soon showed that a fabric of suitable shape, impregnated
with a solution of nitrates or acetates of the rare earth elements,
after being dried and drawn together at one end by means of a platinum
wire, can be ignited in a Bunsen flame in such a way as to leave a
coherent skeleton of the earth oxides, which can be formed and hardened
by suitable manipulation with a high temperature burner; the mantle so
prepared, when suspended from a lateral support in a Bunsen flame, gives
a light of considerable intensity, the colour varying with the oxides
employed from green to orange tints.

The earlier mantles, which were placed on the market about 1883,
consisted chiefly of oxides of lanthanum and zirconium, with smaller
quantities of the other oxides, selected according to the shade of light
desired. These mantles were protected by patents taken out in France in
1884, and in Germany in 1885 and the following years. The process[489]
was briefly the following: A vegetable fibre, of cylindrical form, woven
from threads of about 0·22 mm. diameter, is washed with dilute
hydrochloric acid, then with distilled water, and impregnated with a 30
per cent. solution of the selected salts. The fabric is then wrung out
and dried, and cut into suitable lengths, allowance being made for
subsequent shrinkage. One end of each cylinder is then drawn together by
means of a platinum wire, and the mantle hung from a side support over a
burner and incinerated. The head is then treated with a solution of
aluminium and magnesium nitrates (beryllium nitrate and the
corresponding phosphates are also specified) to strengthen it, and the
mantle dried, and ‘formed’ by means of a very hot flame. This first
patent protected several definite mixtures of salts, chosen so that the
mantle should emit light of a definite known tint. The chief oxides
employed were lanthana, yttria, magnesia, and zirconia. A German patent
granted in 1886[490] protects the use of thorium salts, and a long list
of salts of the elements with numerous acids; an important advance
mentioned in this specification is the process of collodinisation of the
finished mantle, by dipping in a solution of rubber in benzene or of
collodion (cellulose nitrate) in ether and alcohol, which renders the
product strong enough for transport. From 1885 to 1891 numerous
improvements were effected; asbestos threads were substituted for
platinum wire, central rods of magnesia replaced the lateral platinum
support, and various mixtures of oxides were tried. None of the
innumerable mixtures employed, however, was successful in establishing
the struggling industry on a firm basis in face of the vigorous
competition of the electric lamp, and it was not till 1891 that the
introduction of the final ‘Auer Mixture,’ which is in use at the present
day, gave the welcome assurance of a certain success to von Welsbach and
his assistants. The discovery of this mixture was a result of the
examination of a quantity of impure thoria; it was found that mantles
made from the nitrate gave a light which steadily decreased in intensity
as the impurities were removed. It needed only the observation that the
impurities consisted chiefly of cerium compounds to turn the long and
arduous investigation in the direction of final success, and our present
mantles, which consist approximately of 99 per cent. thoria and 1 per
cent. ceria, were placed on the market in 1891, the composition being
announced by patent in 1893.[491]

  [489] _Vide_ _D. R. P._ 39162. Granted September 23, 1885.

  [490] _D. R. P._ 41945.

  [491] _Vide_, _e.g._ Moeller, _E._ 124, 1893.

The effect of increasing or decreasing the ratio of the two oxides, and
the theories which have been advanced to account for the results, must
be referred to in a later chapter (_vide_ p. 294). It may be mentioned
here, however, that practically no other known mixture gives such
satisfactory results, though mantles have been manufactured of alumina
with small quantities of chromic oxide, and ‘inverted’ mantles made of
these oxides with zirconia have recently been advocated by Professor
Lewes,[492] an authority on gas lighting. Mixtures of alumina and
uranium oxide have also been patented, but no mantles appear to have
been manufactured according to the specifications. In this connection,
also, may be mentioned the various attempts to evade the Auer patents by
taking advantage of the ‘discovery’ of ‘new’ elements. One enterprising
firm, after having an account of a ‘new’ element, Lucium, inserted in a
well-known scientific periodical, put salts on the market, and proceeded
to manufacture mantles from what were proved by analysis to be cerium
compounds. Similar ‘new’ elements were Russium, Kosmium, and
Neo-kosmium, names which covered various mixtures of thorium and cerium
compounds with other salts.

  [492] _Vide_ _D. R. P._ 218333 of January 1910.

After the introduction in 1891 of the final Auer mixture, progress
became rapid. The original mantles, made from cotton, had many
disadvantages; thus after being in use for some time they were found to
shrink considerably, with marked decrease in strength and light-giving
power. Once the success of the new form of lighting was assured,
numberless investigations were undertaken to lengthen the life and
increase the efficiency of the mantles. The most important of these were
connected with the endeavour to replace cotton by some fabric which on
ignition would leave the oxide skeleton in a harder, more coherent and
more elastic condition. The first great advance in this connection was
the introduction of Ramie fibre by Buhlmann in 1898. Ramie, China-grass,
or grass-cloth, as it is sometimes termed, is a fabric made from the
fibres of the tschuma plant of the Yang-tse-kiang valley and other parts
of Asia; mantles made from it last longer and maintain their efficiency
much better than the earlier cotton mantles, which they have very
largely displaced. The use of artificial silk was patented by De Mare in
1894, but his process was unworkable; it was an effort to adapt to the
purposes of incandescent lighting the nitro-cellulose process introduced
by Chardonnet in 1890 for the manufacture of artificial silk. In 1897 De
Lery and in 1900 Plaisetty made further efforts in this direction, and
finally in 1902-1903 the latter worked out a process by which mantles
were made directly from the spun fabric. These mantles are superior in
every way to the earlier ramie or cotton kinds, and are rapidly coming
into general use, especially for lamps using high-pressure gas.
Numberless patents for the manufacture and improvement of this kind of
mantle have been taken out during the last ten years; the most important
of these will be dealt with in a later chapter.

Attempts have been made to secure greater strength and toughness in
mantles in other directions also. The use of metallic wires in the fibre
has been suggested; numerous patents deal with mantles ‘strengthened’ by
doubling the thread at intervals, and by special methods of weaving the
fibre. One method, which follows on the lines of Glamond and Lundgren,
proposes[493] the use of mantles made from various oxides mixed with
silica, the whole being worked into a paste by use of a gum or soap,
from which threads are prepared by pressure; mantles made from these
threads are said to be very strong and porous. Another patent[494]
protects the manufacture of ‘incandescence bodies’ made from plates or
combs prepared from a thread obtained in a rather similar way. A third
of these innumerable suggestions recommends a preliminary impregnation
of the fabric with an aluminium or magnesium salt,[495] from which the
oxide is precipitated on the fabric by a suitable means, impregnation
with the ordinary ‘lighting fluid’ being effected after drying. Quite an
early patent[496] proposes the impregnation of the prepared mantle,
either after or just before burning off, with an alcoholic solution of
an organic silicon compound, so that when the mantle is in use a
skeleton of silica is formed to ‘strengthen’ the oxide ash. No useful
purpose can be served by extending the list of these proposals; enough
has been said to indicate the various directions in which so many vain
attempts at improvement have been made.

  [493] Laigle, _D. R. P._ 216871 of December, 1909; see also _D. R. P._
  216877 and 219640.

  [494] Michaud and Delasson, _D. R. P._ 210640, June, 1909; see also
  _D. R. P._ 227257.

  [495] Zdanowich, _E._ 27755, 1908.

  [496] Jasper, _E._ 30145, 1897.

From the mechanical and physical side the recent developments have been
very marked. The introduction of the ‘inverted’ lamp was a tremendous
step forward, and paved the way to the second great improvement, the use
of ‘high-pressure’ gas, with which such successful results are being
obtained. The form of lamp now coming into use for street lighting gives
1500 candle-power per mantle, and usually carries three mantles; each
lamp thus develops 4500 candle-power. The purely mechanical devices
which are now used to secure ‘automatic’ lighting are rapidly bringing
this form of lamp into favour for street illumination. A full account of
these developments would be entirely beyond the scope of the present
work. In the following chapters, therefore, no complete treatment of the
incandescent lighting industry can be given; but whilst the chemical
aspect is treated at some length, many points of more purely technical
character, which are connected with this, have also been included.


CHAPTER XVIII

THE CHEMICAL TREATMENT OF MONAZITE

It has been stated in the previous chapter that the first Auer mantles
were made of mixtures of various rare earth oxides, the mixture of
thoria with 1 per cent. of ceria being first employed in October, 1891.
The impetus given to the mantle industry by the success of the new
mixture caused an immediate demand for thoria, which was at that time
extracted from thorite (see p. 43). A ‘thorite-fever’ broke out along
the coasts of Scandinavia, and the price of orangite rose to 600 marks
per kilogram (about £13 10_s._ per pound avoirdupois), sinking again
shortly to 80 marks[497] (about £1 16_s._ per lb). The discovery of the
monazite sands of the Carolinas and Brazil, which at the present rate of
consumption may be considered to be, for all practical purposes,
inexhaustible, placed the industry on a firm basis, and the pure
monazite, extracted from these deposits by the methods outlined in
Chapter VII, is now almost the sole source of the thorium nitrate of
commerce. Small quantities are obtained from thorianite, the separation
of the pure material presenting, in this case, very little difficulty by
reason of the solubility of the mineral in acids and the very high
percentage of thoria.

  [497] _Vide_ Böhm, ‘Die Thorium Industrie,’ _Chem. Ind._ 1906, ~29~,
  450 and 488.

The extraction of pure thorium compounds from monazite is a process of
very great technical difficulty. The percentage of thoria is small,
whereas that of the ceria oxides is high. The mineral is almost always
decomposed by heating with concentrated sulphuric acid, and when the
resulting pasty mass is taken up with water, a large amount of free
sulphuric acid must be present in order to hold the rare earth
phosphates in solution. For the separation of thoria from ceria and
yttria compounds in acid solution no processes were known until quite
recently. When it is remembered that the thorium nitrate used for the
manufacture of mantles must be of a degree of purity which very few
commercial products ever approach, some idea of the difficulties of the
extraction may be obtained.


~Decomposition of the Monazite.~--Two processes have been used for the
working up of monazite. The first of these consists in fusing the
mineral with soda, and extracting the sodium phosphate with water; the
earths may then be taken into solution with acid, and the separation
effected as outlined below. This method is very rarely used. A process
has been proposed, in which the monazite is fused with carbon in an
electric furnace; the cooled mass is treated with mineral acids, which
take the earths into solution free from phosphoric acid. No technical
application has so far been made of this proposal.

The method commonly used is that in which the sand is decomposed by
means of sulphuric acid. The charge usually employed, about two to three
hundred kilograms, requires from four to six hours’ heating, about twice
the weight of concentrated acid being needed. The operation is carried
out in cast-iron vessels, and an efficient draught must be maintained to
remove the acid fumes; the factories are usually isolated. The treatment
with sulphuric acid converts the phosphates chiefly into sulphates; when
the reaction is finished, the liquor fumes strongly and begins to
thicken, heating being stopped when a thick broth is obtained. The
cooled mass is extracted with water, care being taken to maintain a
degree of acidity sufficient to prevent any precipitation of the
phosphates.

It has been already stated in Part I (_vide_ p. 73) that a strongly
radioactive product. Radiothorium, has been obtained from the mineral
thorianite. This body is produced by the atomic degradation of thorium,
and an intermediate body, mesothorium, has been found to be formed
during the change. Mesothorium is a substance which, though it appears
to be chemically identical with radium, has an activity equal to three
hundred times that of radium, and when in equilibrium with its
degradation products the ‘rays’ it emits are very similar to those of
the latter element. Since mesothorium is a degradation-product of
thorium, it occurs in minute quantities in all thorium minerals, and by
reason of the possibility of using it as a substitute for radium, its
extraction becomes a matter of importance. Soddy[498] has shown that if
a barium compound be added to monazite before the treatment with
sulphuric acid, the mesothorium remains with the barium sulphate; this
is readily separated from the heavy unchanged grains of sand, and is
purified, and finally obtained as chloride by treating the solution with
hydrogen chloride. On recrystallisation of the barium chloride, the
active products are concentrated in the less soluble part, and it is
possible to prepare on the commercial scale a mixture which, though it
contains only 0·25 per cent. of mesothorium, has an activity equal to
that of pure radium bromide. This mixture contains 25 per cent. of
radium compounds, radium being present as an original constituent of
monazite; owing to the chemical identity of radium and mesothorium,[498]
the latter cannot be separated, but Soddy, by removal of much of the
barium compound in the laboratory, has obtained a product four times as
active as the pure radium salt.

  [498] _Proc. Chem. Soc._ 1910, ~26~, 336, and _E._ 25504, November,
  1910. See also Hahn, _Chem. Zeitg._ 1911, ~35~, 845.

It is probable that the treatment of monazite will in the future be
modified by the addition of barium sulphate before the sulphuric acid
decomposition, to allow of the commercial extraction of its mesothorium.


~Separation of Thorium.~--The separation of a crude thorium product from
the acid solution obtained after decomposition of the mineral can be
effected in two ways, both of which are based on the fact that thoria is
less basic than the oxides of the cerium and yttrium metals. In the
first, the rare earth elements, including thorium, are precipitated as
oxalates by the addition of oxalic acid to the acid solution. These are
again taken into solution by the action of hydrochloric acid on the
hydroxides, obtained by prolonged digestion of the oxalates with sodium
hydroxide; the acid solution is then treated carefully with sodium
hydroxide, or pure powdered magnesia, until about one-sixth of the bases
has been precipitated, the liquid being constantly stirred. Thorium
hydroxide being very weakly basic is precipitated before the other
hydroxides, and the precipitate obtained, after one or two repetitions,
contains most of the thorium originally present in the monazite. In the
second process, thorium is partially separated from the other metals by
adding gradually to the solution obtained after the treatment of the
mineral with sulphuric acid, the quantity of magnesia calculated to
precipitate a suitable fraction of the earths, with constant stirring;
this throws down a mixture of phosphates containing almost all the
thorium and some of the other elements. The slimy phosphate precipitate
is dissolved in hydrochloric acid, and the earths precipitated as
oxalates; the precipitate must be washed thoroughly in order to remove
phosphoric acid. It will be seen that these two methods differ only in
that in the first the phosphoric acid is removed before the
precipitation of thorium, whereas in the second the thorium is
precipitated as phosphate, and this transformed into oxalate.

Quite recently, methods have been proposed by which the thorium can be
separated in a fairly pure condition from the acid solution obtained
from the sulphuric acid treatment. Rosenheim, Meyer and Koppel[499]
protect the use of hydrofluosilicic acid (H₂SiF₆), and its salts, for
this purpose. The sodium salt, added to the hot acid liquid, produces a
quantitative separation of thorium silicofluoride; the precipitate is
washed by decantation, and treated with sulphuric acid, the thorium
sulphate being then purified directly by the sulphate method described
below. A second method proposes to make use of the insolubility of
thorium hypophosphate, ThP₂O₆,11H₂O, which was found by Kaufmann in 1899
to be insoluble in water, and in acids and alkalies. This method has
already been in use for some years for analytical work;[500] it appears
to be readily susceptible of adaptation for the technical
extraction,[501] the sodium hypophosphate, Na₂H₂P₂O₆,6H₂O required as
the precipitating agent being obtainable in large quantities by the
electrolytic oxidation of copper phosphide, employed as the anode in an
electrolytic cell.[502] This method also gives a thorium compound
sufficiently free from other earths to be subjected at once to the
refining process; the hypophosphate has in fact been suggested as a very
suitable compound for the impregnation of artificial silk mantles
directly. The thorium nitrate of commerce, however, is still prepared
almost entirely from the crude product obtained by one or other of the
two methods of fractional precipitation first described, so that it
becomes necessary to outline the method generally employed for
separating from this a compound pure enough to be suitable for the final
refining process.

  [499] _D. R. P._ 214886, October, 1909.

  [500] Rosenheim, _Chem. Zeitg._ 1912, ~36~, 821; also Koss, _ibid._
  686

  [501] Wirth, _Zeitsch. angew. Chem._ 1912, ~25~, 1678.

  [502] Rosenheim and Pinsker, _Ber._ 1910, ~43~, 2003.

The crude oxalate or hydroxide is thoroughly digested with a
concentrated solution of sodium carbonate. The carbonates of the cerium
elements are much less soluble in sodium carbonate solution than is
thorium carbonate. After thorough digestion the liquid is filtered from
the undissolved carbonates. The thorium is reprecipitated from the
filtrate, either as oxalate, by the addition of hydrochloric acid (if
the crude material was in the form of oxalate), or as hydroxide, by the
addition of sodium hydroxide. The process is again repeated, and a final
digestion is then made with ammonium carbonate; addition of an alkali to
the clear filtrate now gives thorium hydroxide sufficiently pure to be
used for the last refining.


~Purification of the Thorium Compounds.~--The object of this last stage
is to remove from the thorium compound small quantities of cerium and
yttrium salts which cannot be separated by the carbonate method. The
chief process is the sulphate crystallisation, the principles underlying
which have been thoroughly examined in the patient researches of Koppel
and Holtkamp.[503] Since the process is based on the solubilities of
the various thorium sulphate hydrates, it is necessary to consider these
in some detail.

  [503] _Zeitsch. anorg. Chem._ 1910, ~67~, 266.

The solubility-curve of thorium sulphate was examined by Demarçay and by
Roozeboom. Three important hydrates are known, viz. Th(SO₄)₂,9H₂O,
Th(SO₄)₂,8H₂O, and Th(SO₄)₂,4H₂O, other unstable intermediate compounds
being said to exist. From a study of the diagram it will be seen that
the hydrate with 8 molecules of water is labile, whilst the 9-hydrate
and the 4-hydrate have a transition temperature at 43°C., the transition
temperature of the 8-hydrate and the 4-hydrate being just below this.

[Illustration: FIG. 10]

Since the 8-hydrate is labile with regard to the 9-hydrate, and the
transition temperatures are so near, the former will be formed first as
a solution cools, and by reason of the great similarity of the
solubility-curves for the 9- and 8-hydrates the rate of change of this
to the 9-hydrate will be very slow. In practice, therefore, it is always
the 8-hydrate which is formed, and it is on the separation of this
compound that the success of the process depends. The anhydro-compound,
Th(SO₄)₂, which can be obtained by heating any of the hydrates to
300°-400°C., is very soluble at 0°, but slowly hydrates itself and
separates from the solution as the 8-hydrate, which has a very low
solubility. The sulphates of the cerium metals, compounds of which form
the chief impurities to be removed, are considerably more soluble, and
can be separated by repeated crystallisations.

The thorium hydroxide to be purified is dissolved in sulphuric acid, and
in the first form in which the method was employed, the thorium sulphate
obtained by evaporation of the solvent was heated until it became
anhydrous. This was dissolved to saturation at 0°, and the solution
raised to the boiling-point, the 4-hydrate being precipitated; this
treatment was repeated several times. It was pointed out by Bunsen, from
theoretical grounds, that this method could never yield a pure thorium
salt, and Krüss and Nilson accordingly introduced a modification. The
impure sulphate, after dehydration, as before, is dissolved at 0°, and
allowed to come to ordinary room temperature, 20°; the hydrate which
separates (the 8-hydrate) is collected and dried at high temperature and
the crystallisation repeated. This method gives a fairly pure salt after
three recrystallisations, but the process is very tedious, owing to the
time required for drying and heating the hydrate. For this reason the
method was further modified by Cleve and Witt. The crude sulphate is
boiled with ammonia, and the hydroxide obtained dissolved in
hydrochloric acid; addition of sulphuric acid to the concentrated
solution in the cold transforms the chloride into the sulphate, which
separates as the 8-hydrate at ordinary temperatures. Three repetitions
give a satisfactory product, and in this form the method is now much
used.

The work of Koppel and Holtkamp referred to above has placed the process
on a sound basis. These authors have examined the solubilities of the
various hydrates in presence of hydrochloric, nitric, and sulphuric
acids, and mixtures of these, at different temperatures. They find that
hydrochloric acid is to be preferred to nitric acid, in the process of
Cleve and Witt, as besides its lower price, its use involves less loss
than that of the latter acid; excess of hydrochloric acid is not
harmful within wide limits, whilst a slight excess of sulphuric acid
over the quantity required to form the sulphate is desirable, to secure
the greatest yield. Finally, the temperature at the addition of the
sulphuric acid must not be allowed to rise above 25°, for in the
presence of so much acid the transition temperature to the 4-hydrate,
normally 42°, is considerably lowered; it is necessary to avoid
separation of the 4-hydrate, which is a flocculent unworkable
precipitate.

Recently it has been proposed to carry out the purification by use of
alkyl hydrogen sulphates,[504] as it is stated that the differences of
solubilities of the alkyl sulphates of thorium and the cerium metals are
greater than in the case of the sulphates themselves. It is also claimed
that the presence of a small quantity of the alkyl sulphate in the
thorium nitrate which forms the final product has a good effect on the
quality of the mantles made from it.

  [504] Kreidl u. Heller, _D. R. P._ 233023, March, 1911; _F._ 414463,
  June, 1910.

Another process of purification which has found considerable commercial
application is the acetate crystallisation, thorium acetate being
considerably less soluble than the acetates of the cerium elements. The
impure hydroxide is dissolved in acetic acid and the solution evaporated
to dryness; repeated washing with small quantities of water removes the
cerium acetates, and a fairly pure salt is obtained. This is repeatedly
damped with nitric acid and heated to dryness, but even after this
treatment a certain amount of unchanged thorium acetate is usually
present.

In a second form of this method, due to Haber, the impure hydroxide is
dissolved in hydrochloric acid, and the acetate precipitated by addition
of sodium acetate. The precipitate is filtered off and re-dissolved in
acid, and the acetate again thrown down by means of sodium acetate. The
precipitate is then dissolved in nitric acid, and the solution
evaporated to dryness. In this form the method gives very good results,
even from a comparatively crude product; but the process is, of course,
considerably more expensive than the sulphate purification.

The high price of the necessary reagents, again, is a bar to the
technical application of the very simple and efficient process of
Wyrouboff and Verneuil. These authors suggest the precipitation of
thorium peroxide from a warm dilute neutral solution by means of
hydrogen peroxide, a process which is quantitative and yields a very
pure product. The last traces of the cerium metals can be completely
removed by a second precipitation. The cost of hydrogen peroxide is too
high, however, to allow its employment on such a large scale, and the
method has not, in consequence, come into general use.

The thorium nitrate obtained after purification by the sulphate method,
or by the less generally employed acetate method, is usually considered
sufficiently pure for technical purposes. Even now, however, it may
contain traces of sulphate, of iron, of alkalies, and of cerium metals.
If absolute purity is desired, the salt may be dissolved, and freed from
all impurities, except the cerium compounds, by precipitation with
ammonium oxalate and thorough washing; the oxalate may then be dissolved
in chromic acid, and potassium chromate solution added drop by drop; the
precipitated thorium chromate is nearly free from other rare earth
compounds, and repetition of the process will give a pure salt. The
separation from cerium metals may also be effected by the hydrogen
peroxide process. If the technical processes are carefully carried out,
however, a thorium nitrate of a very high degree of purity may be
obtained, and the laboratory purification need only be undertaken if
material is needed for very accurate quantitative work.


~Preparation of Thorium Nitrate from Mantle-ash.~--Since the ordinary
incandescent mantle, in use, consists only of the pure thoria and ceria,
with small quantities of alumina, lime, and magnesia, which have been
employed to strengthen the ‘head,’ the working-up of mantle-ash gives an
easy means of obtaining the nitrates, and high prices are accordingly
paid for the ash in quantity. At one period of great competition between
rival manufacturers, canvassers went from house to house in many large
towns buying up mantle residues, to be used for the extraction of the
thorium for ‘lighting-fluid.’

For this purpose, the oxides are treated with hot concentrated sulphuric
acid, the cooled residue dissolved in water, and the thorium and cerium
precipitated free from compounds of aluminium, magnesium, and calcium by
oxalic acid. If pure thorium nitrate, free from cerium, is required, the
oxalates are added to the last precipitate from the double carbonate
purification in the treatment of monazite (_vide supra_), and the
ordinary processes of refinement continued; more often, however, the
mixed nitrate for impregnation of the mantle-fabric is required, and
this is obtained by ignition of the oxalates and solution of the oxides
so obtained in nitric acid, more cerium nitrate being added if
necessary.


~Extraction of Cerium Nitrate.~--Since monazite is primarily a phosphate
of the cerium metals, the percentage of thoria being usually quite low
(_vide_ Monazite, Chapter VI), very large quantities of compounds of the
cerium group of elements are annually produced in the process of
extraction of thorium. There is at present a very limited demand for
these compounds (_vide_ Chapter XXI), no important uses having yet been
found for them. In the ordinary process of extraction of the thorium,
these elements remain as the sparingly soluble double carbonates, whilst
the thorium double carbonate is removed in solution. From the mixed
salts which contain 50-60 per cent. of the cerium compound, the cerium
nitrate required for the manufacture of mantles is prepared, but the
amount so used is a small fraction of the whole, and large quantities of
compounds of cerium and the allied elements are available as soon as
profitable uses can be found.

Three processes are in general use for the preparation of cerium nitrate
from the mixed carbonates; all of these are based on the fact that
cerium can become tetravalent, forming in this condition compounds which
can readily be separated from those of the allied elements, which can be
obtained only in the trivalent condition. When ceria is dissolved in hot
nitric acid, ceric nitrate, Ce(NO₃)₄, is formed, though the action of
nitric acid on cerous carbonate or oxalate gives rise to cerous nitrate.
Two of the three processes are based on this reaction, and for these the
mixed carbonates are dissolved in hydrochloric acid, freed from foreign
elements by precipitation with oxalic acid, and the oxalates ignited to
the oxides, which are then dissolved in the required quantity of nitric
acid. In the first process the cerium is precipitated from this solution
by merely pouring it into a large excess of very dilute nitric acid,
when a yellow basic ceric nitrate is precipitated; this is washed with
dilute nitric acid by decantation, dissolved in concentrated acid, and
purified by a second precipitation in the same way. In the second
process, separation is effected by addition to the nitric acid solution
of the calculated quantity of ammonium nitrate; the solution is
concentrated to incipient crystallisation, and on cooling the double
ceric ammonium nitrate, Ce(NO₃)₄,2NH₄NO₃, separates. This is collected,
washed with dilute nitric acid, and recrystallised until a pure salt is
obtained. The double nitrate can be readily decomposed by ignition,
leaving ceria, which is dissolved in nitric acid; the nitrate is
obtained by evaporation.

The third method, due to Drossbach, is based on the oxidation of cerium
salts in neutral solution by potassium permanganate. The mixed
carbonates are dissolved in hydrochloric acid, a further quantity of the
carbonates stirred in, to neutralise excess of acid, and a solution of
the required quantity of potassium permanganate added. The reaction is
said to proceed according to the equation:

  3Ce₂O₃ + 2KMnO₄ + H₂O = 6CeO₂ + 2KOH + 2MnO₂

The precipitated solid is separated, and dissolved in acid; the cerium
is then precipitated as the oxalate, which is transformed into nitrate
in the usual way. The solution contains the other elements of the cerium
group, which are precipitated by means of sodium hydroxide. The yield
obtained by this method is very good, practically the whole of the
cerium being separated without loss; whilst it has the further advantage
that the remaining elements of the group can be precipitated at once
after the separation.


~Analysis of a Monazite or Monazite Sand for Thorium.~--Since the
commercial value of a monazite sand or concentrate, or of the pure
mineral, depends, at present, entirely on the percentage of thoria, it
is important to have a rapid and reliable method of estimating this
constituent. The only reliable methods of quantitatively decomposing
the mineral, however, all involve acid treatment, and excess of acid
must always be present to prevent precipitation of phosphates. Until
recently, no way was known for estimating thorium in an acid solution,
and all the earlier methods therefore involved tedious processes for
complete removal of phosphoric acid, so that the salts could be obtained
in neutral solution. This was usually effected by precipitation of the
whole rare earth content with oxalic acid, and thorough washing of the
oxalates; these can then be dissolved directly in fuming nitric acid on
the water-bath, or ignited to the oxides, which may then be dissolved in
the same reagent. The solution of nitrates is evaporated to dryness, to
effect removal of the excess of acid, the nitrates dissolved in water,
and the thorium estimated in the neutral solution.

Among the earliest methods employed for the estimation in neutral
solution was the thiosulphate precipitation.[505] Thorium thiosulphate
is not known; when sodium thiosulphate is added to a neutral solution of
a thorium salt, a precipitate of thoria mixed with sulphur is obtained,
by hydrolysis of the potential thiosulphate, and decomposition of the
unstable thiosulphuric acid. The method, however, leaves much to be
desired; other earths are partly precipitated, and the separation of
thoria is not complete. For analytical purposes the precipitate obtained
is redissolved in hydrochloric acid, and a second precipitation with
thiosulphate effected. The filtrates from the two precipitations are
collected, and the whole earth-content precipitated from these with
ammonia; the hydroxides are dissolved in hydrochloric acid, and again
treated with thiosulphate to throw down any thoria which has escaped the
previous precipitations. The three precipitates of thoria are then
collected, dried, and ignited for weighing as pure thorium dioxide,
ThO₂.

  [505] Full accounts of this and the two following methods will be
  found in an important paper by Benz, _Zeitsch. angew. Chem._ 1902,
  ~15~, 297

Even more tedious and unsatisfactory is the method based on the
solubility of thorium oxalate in excess of ammonium oxalate in neutral
solution. The solution is boiled, ammonium oxalate added, and after some
moments a small quantity of ammonium acetate solution. On cooling, the
oxalates of the cerium metals separate, and can be collected; thoria is
precipitated from the filtrate by addition of ammonia. The process must
be repeated two or three times, the solution being allowed to stand for
one or two days each time, and finally the thoria must be precipitated
by thiosulphate to remove traces of the other bases before it can be
weighed. Benz (_loc. cit._) gives a complete account of this method, and
quotes numerous analyses carried out to test its accuracy.

Far more satisfactory than either of the above is the peroxide method
used by de Boisbaudran and Cleve, and later by Wyrouboff and
Verneuil.[506] Thorium is completely precipitated as a ‘peroxide salt’
(Th₂O₇,SO₃ or Th₂O₇,N₂O₅ respectively) from warm neutral solutions of
the sulphate or nitrate on addition of dilute hydrogen peroxide, a
second precipitation being necessary to free it from cerium compounds.
Wyrouboff and Verneuil state that the process is rendered difficult by
the fact that the peroxide cannot be converted into the dioxide by
heating, either alone or with acids, as decrepitation takes place and
may cause loss; they accordingly reduce the compound in presence of
hydrochloric acid by ammonium iodide, and precipitate thorium hydroxide
by ammonia. Benz (_loc. cit._) does not find this difficulty; he states
that small quantities of the peroxide dissolve easily in acids without
loss, and further finds that if an ammonium salt be added to the neutral
solution of the thorium compound before addition of hydrogen peroxide,
the precipitate forms much more readily and is very easily handled.
Borelli[507] states that the precipitated peroxide can be ignited
without loss to the dioxide, and weighed as this.

  [506] _Compt. rend._ 1898, ~126~, 340.

  [507] Abstract in _J. Soc. Chem. Ind._ 1909, ~28~, 625.

       *       *       *       *       *

The azoimide method of Dennis[508] is of interest rather than of use. He
finds that addition of potassium azoimide, N₃K, precipitates thoria
quantitatively from a neutral solution, the reaction being expressed by
the equation:

  Th(NO₃)₄ + 4N₃K + 2H₂O = 4KNO₃ + ThO₂ + 4N₃H

Cerium, however, if present, is always precipitated with the thorium,
and cannot be removed by re-precipitation; this fact, together with the
cost of the reagent and the difficulty of obtaining it pure, renders the
method quite useless for mineral analysis.

  [508] _Zeitsch. anorg. Chem._ 1897, ~13~, 412.

Numberless experiments have been made with organic acids in the hope
that an easy method of separation might be found, but though some useful
results have been obtained, precipitation has always to be effected in
neutral solution, so that all such processes involve the tedious
preliminary work of which an outline has been given above. Metzger[509]
finds that a quantitative separation of thorium can be effected from a
solution in 40 per cent. alcohol by use of fumaric acid; a second
precipitation is needed to secure the complete removal of the cerium
elements. Neish[510] uses meta-nitrobenzoic acid, which precipitates the
thorium salt from a boiling solution; cerium earths, if present, are
carried down in small quantities, and are removed by dissolving the
precipitate in dilute nitric acid, adding a further quantity of the
organic acid, and treating carefully with ammonia to almost complete
neutralisation. The compound obtained by this second precipitation is
the pure thorium salt. More recently, Smith and James[511] have shown
that sebacic acid gives a quantitative precipitation of the thorium
salt, from boiling neutral solution, as a voluminous granular
precipitate, readily filtered and washed; sebacic acid is very sparingly
soluble in cold water, but dissolves readily at 100°, and since, in
virtue of this property, it can be readily recovered after use, the
authors suggest it as a suitable reagent for the technical separation of
thorium from monazite. In all cases where thorium is precipitated as an
organic salt in quantitative analysis, the precipitate is dried and
ignited, and the residue weighed as the pure dioxide.

  [509] _J. Amer. Chem. Soc._ 1902, ~24~, 275 and 901.

  [510] _Ibid._ 1904, ~26~, 780.

  [511] _Ibid._ 1912, ~34~, 281.

An interesting method has been worked out by Giles.[512] If pure moist
lead carbonate be stirred into a neutral solution of rare earth
compounds, thoria is completely precipitated. Only the tetravalent
elements are separated by this method, so that if ceric compounds are
present, they must first be reduced by means of sulphuretted hydrogen or
sulphur dioxide; zirconium, if present, must afterwards be separated
from the thorium. One precipitation is said to ensure almost complete
separation from the trivalent elements. The precipitate is collected,
washed, and dissolved in hydrochloric acid; after filtering, if
necessary, the solution is saturated with sulphuretted hydrogen, to
ensure complete removal of the lead, and thorium hydroxide is then
precipitated by ammonia. The drawback to this method lies probably in
the fact that it is necessary to use absolutely pure lead carbonate, a
substance which, as the author’s elaborate process of purification seems
to show, could not be obtained very cheaply on a large scale.

  [512] _Chem. News_, 1905, ~92~, 1 and 30.

An account has recently been published[513] of a volumetric method for
the estimation of thorium. The mixed oxides are dissolved in
concentrated acetic acid, and the solution titrated with a standard
solution of ammonium molybdate. This reagent effects complete
precipitation of thorium, but does not react with compounds of the
cerium elements; excess of the molybdate is shown by a solution of
diphenyl carbazide, CO(NH·NH·C₆H₅)₂, used as an external indicator. The
carbazide, which is obtained by the action of phenyl hydrazine on urea,
has the property of producing definite, though evanescent, colourations
with compounds of many of the metallic elements; a drop of the working
solution, brought into contact with a drop of the carbazide solution,
shows a deep rose colouration when excess of ammonium molybdate is
present.[514]

  [513] Metzger and Zons, _J. Ind. Eng. Chem._ 1912, ~4~, 493.

  [514] Vide Skinner and Ruhemann, _Trans. Chem. Soc._ 1888, ~53~, 554;
  also Cazeneuve, _Compt. rend._ 1900, ~131~, 346.

The iodate process of Meyer and Speter[515] has the great advantage that
it is carried out in a strongly acid solution, so that here the tedious
purification from phosphoric acid is no longer necessary. After
decomposition of the mineral with sulphuric acid, the sulphates are
extracted with water, and a suitable quantity of nitric acid added; the
solution is then treated with a nitric acid solution of potassium
iodate, and the thorium iodate which separates is dissolved in
concentrated nitric acid, and re-precipitated to remove traces of the
cerium elements. The iodate, after washing, is dissolved in hydrochloric
acid, and reduced by sulphur dioxide; the hydroxide is then precipitated
by ammonia. Since zirconium is also thrown down under these conditions,
the hydroxide is dissolved in hydrochloric acid; pure thorium oxalate is
precipitated from this solution by oxalic acid, and is ignited and
weighed as oxide, in the usual manner. Since ceric iodate is also
insoluble in dilute nitric acid, it is necessary to reduce any ceric
compound which may be present before the iodate treatment by the usual
methods.

  [515] _Chem. Zeitg._ 1910, ~34~, 306. See also _Zeitsch. anorg. Chem._
  1911, ~71~, 65.

Another method which can be carried out in acid solution is based on the
insolubility of the hypophosphite, ThP₂O₆,11H₂O, in dilute acids.[516]
To the boiling acid solution, an aqueous solution of sodium
hypophosphate, Na₂H₂P₂O₆,6H₂O, is added drop by drop. The precipitate,
which contains any titanium and zirconium present in the original
solution, is best treated with a mixture of sulphuric and fuming nitric
acids; the phosphates produced by the oxidation are freed from nitric
acid by evaporation, dissolved in water, with addition of sulphuric
acid, and thorium precipitated as the oxalate, which is then ignited as
usual. This method has been suggested for the technical separation of
thorium from monazite (_vide_ p. 278). Since the precipitations by means
of sodium hypophosphate and potassium iodate can be carried out with
solutions obtained directly from the product of the action of sulphuric
acid on the mineral, these two methods are probably more suitable for
the rapid and accurate estimation of thorium for technical purposes than
any of the others mentioned.

  [516] Wirth, _Zeitsch. angew. Chem._ 1912, ~25~, 1678; see also Koss,
  _Chem. Zeitg._ 1912, ~36~, 686, and Rosenheim, _ibid._ p. 821.


CHAPTER XIX

THE MANUFACTURE OF MANTLES FROM COTTON AND RAMIE

The fabric chosen for the manufacture of the original Welsbach mantles
was a specially selected cotton, woven from threads of a specified
thickness. The oxide skeleton left after burning off the impregnated
fabric, however, showed many serious defects. Gradual shrinkage occurred
during use, so that the mantle was gradually withdrawn from the hottest
zone of the flame; the contraction also resulted in crumpling, which
caused the fragile fabric to fall to pieces. The light-giving power
showed a gradual but continuous diminution, so that after a hundred
hours, the decrease sometimes amounted to thirty per cent. of the
original intensity. Lastly, owing to the fragility due to the torsion
introduced by the twisting together of so many short fibres in the
spinning of the fabric, the life of these mantles was very short, and
their susceptibility to shock very great.

With the introduction of ramie, many of these defects disappeared.
Mantles made from this fibre do not shrink continuously, nor to any
considerable extent, so that crumpling does not occur; the decrease in
light-giving power is very much less than with the cotton mantles, the
life is much longer, and the resistance to shock very much greater.
Microscopic examination of the fibres, and of the ash left after burning
off, shows that these differences are traceable to differences in the
mechanical structure of the two fabrics. The cotton thread is spun from
a very large number of very short fibres, which are twisted together,
whereas the separate fibres of ramie are of much greater length, and the
torsion in the thread correspondingly less. In the case of artificial
silk, continuous filaments are spun into threads, and this simple fact
accounts for the very much greater durability and elasticity of mantles
made from this fabric.

The introduction of ramie for the manufacture of mantles effected no
great alterations in the processes employed in the case of cotton; the
treatment of artificial silk, however, requires a profoundly modified
method, and in consequence the preparation of this fabric and the
manufacture of mantles from it, are considered separately in another
chapter. Though the manufacture of artificial silk was first commenced
in France, it is in Germany that its adaptation to the incandescent
mantle industry has been successfully effected. Its adoption, however,
has not yet become general, and by far the greatest number of mantles
are to-day made from ramie. In the United States, where the early
working of the monazite deposits gave a great impetus to the industry,
the manufacturers still cling largely to the older methods, so that even
now quite a considerable number of mantles are made from cotton.

In the present chapter, a short account will be given of the methods
employed in the manufacture of mantles from ramie and cotton. The
general methods of preparation of cotton fabrics are well known. Ramie
is prepared from the tschuma plant, which is found in India, China, and
other parts of Asia. The fibre is obtained from the inner side of the
bark; this is dried, pressed into bales, and exported. After removal of
gum and resin, by heating with sodium hydroxide solution under pressure,
the fibre is bleached, dried and cleaned, and then combed and spun in
the usual manner.[517]

  [517] For the preparation and technical uses of Ramie, or Rhea, as it
  was formerly called, _vide_ Cross, ‘The Industrial Uses of Cellulose,’
  in the _Cantor Lectures of the Society of Arts_, 1897, ~vi~, p. 20.


~Washing.~--The influence of even small quantities of impurities on the
intensity of the light emitted by a mantle is remarkably great, and the
manufacture requires a degree of care and attention far beyond that
needed in ordinary technical processes. Washing of the fabric in
particular must be very thorough and careful, if a mantle of reasonable
quality is required. If the mineral content of the fabric, _i.e._ the
ash left on incineration, amount to more than 0·03 per cent. of the
total weight, the quality of the mantle is seriously affected.
Particularly is this the case if even the smallest traces of iron come
in contact with the fabric; on this account wooden implements must be
used as far as possible in the washing house, and all iron parts must be
carefully protected so that no water can drip from them on to the
material.

The fabric is used in the form of a long cylindrical tube usually of
about twice the diameter required for the base of the finished mantle.
Before the mineral impurities can be removed, this must be entirely
freed from grease. It is therefore washed thoroughly with a warm
solution of sodium carbonate, which removes all the hydrolysable fats.
If paraffin or other non-hydrolysable grease is present, the alkaline
wash must be followed by a soap wash. The fabric is now cleansed from
alkali and soap by running water, and mineral impurities are removed by
treatment with dilute hydrochloric acid (1-3 per cent.) at a temperature
of 50°-60°C.; it is finally washed free from acid with distilled water.
So susceptible is the mantle to traces of impurity that the use of
ordinary tap water, or even of a distilled water which has become
contaminated to the smallest degree, for this final washing, will
considerably lower its efficiency.

A centrifugal machine removes most of the water, and the drying is
completed by passage over wooden rollers, through a small chamber, of
which the air is kept at 30°-40°C. The dried fabric is now cut into
lengths ready for the next process.

[Illustration: FIG. 11]


~Impregnation.~--The composition of the ‘Lighting Fluid,’ as the
solution of salts used for impregnation is technically termed, varies
slightly according to the nature of the mantle required, and the
conditions of washing. It is of the greatest importance that the ratio
of thoria to ceria should be constant and definite; the usual
proportions are chosen so that the ratio of the oxides is 99 : 1. Fig.
11 shows at a glance to what a remarkable extent small variations in the
percentage of ceria affect the luminosity of the finished product.[518]
The thorium nitrate is made up with distilled water to a solution of
25-35 per cent. strength, and the calculated quantity of a standard
solution of cerium nitrate is added. It is usual to add to the mixture a
small quantity of another nitrate, which on ignition will leave an oxide
of which the function is to strengthen the skeleton of ash. Beryllium,
zirconium, magnesium, or aluminium nitrate is usually employed, in
quantity calculated to leave an amount of oxide constituting about 0·5
per cent. of the total oxides; for ramie fabrics, beryllium nitrate is
generally chosen.

  [518] Numberless theories have been advanced to account for the
  extraordinarily high light-emitting power of this particular mixture
  of thoria and ceria. An account of these would be beyond the province
  of the present work; the reader who desires to pursue the subject
  should consult the interesting work of Dr. H. W. Fischer, _Der
  Auerstrumpf_, Ahren’s _Sammlung_, 1906, vol. xi. _Vide_ also Lévy,
  _L’Éclairage à l’incandescence par le gaz_, Paris, 1910, Ch. II; and
  Foix, _Thèse présentée à la Faculté des Sciences de Paris_, Paris,
  Gauthier-Villars, 1910.

  The diagram is after Drossbach, _J. Gasbel_. 1898, 352.

After having been immersed for two to five minutes in the solution, the
separate lengths are freed from excess of the lighting fluid by means of
a small wringing machine. The pressure between the rollers must be
regulated very exactly, since on the amount of solution taken up by the
fabric will depend the mass of the oxide skeleton. The weight of oxides
left after ignition should be 0·5-0·6 gm. for a ‘normal’ upright mantle
of 9·5 cm. length, corresponding to 1·0-1·2 gms. of the nitrates, or,
for a 30 per cent. solution, to 3·3-4·0 gms. of solution. The weight of
the fabric before impregnation is approximately 5 gms. for cotton, 3
gms. for ramie, and 1·5 gms. for artificial silk. A cotton
mantle-fabric, therefore, must be allowed to retain rather less, a ramie
fabric rather more, than its own weight of solution, whilst an
artificial silk fabric must take up 2-2¹⁄₂ times its own weight of the
fluid. The weight of the oxide ash left from these quantities has been
found by experience to be most suitable; if the mass is greater than
this, the light-emission is diminished without a compensating gain in
strength; if it is less, the light-emission is indeed greater, but the
mantle becomes too fragile.

The impregnated fabric-lengths, after passing through the wringing
machine, are drawn singly on to glass forms which are arranged on
stands, and freed from moisture in a drying room by hot air, a
temperature of about 30°C. being maintained. Three to four hours are
required, under these conditions; if the drying be too rapid,
considerable shrinkage occurs, and the mantles obtained are then
extremely fragile.


~The Mantle Head.~--The normal upright mantle is supported from a
central rod of compressed magnesia--fused quartz has recently been
suggested[519]--by means of an asbestos thread. The thread in the older
patterns was supported by simply doubling over the fabric at the end
which was to become the head; more generally, however, a strip of tulle
or gauze is sewn to the head end before the impregnation. In order to
strengthen the head, it is treated before ‘finishing’ with a hardening
or ‘fixing’ fluid, which usually consists of a mixture of magnesium and
aluminium nitrates in aqueous solution; the following may be cited as a
typical mixture: Aluminium nitrate, 300 parts; magnesium nitrate, 300
parts; chromium nitrate, 3 parts; borax, 5 parts; distilled water, 1500
parts. In order to secure that this fluid is applied to the head only, a
little organic colouring matter is generally added, so that it may be
clearly seen. The solution is soaked on to the head from mechanically
held felt pads, which are kept at a convenient degree of saturation with
the fluid by means of an ingenious compressed-air device. The mantle is
then rapidly dried in a hot-air chamber.

  [519] _Vide_ _D. R. P._ 244959, March, 1912.

After the fixing and drying processes, the head is ‘finished.’ The
ordinary upright mantle is sewn together, at the end which has been
treated, with carefully selected asbestos threads, an opening of some
ten millimetres being left, and the asbestos is threaded diametrically
across this opening--these diametrical threads support the mantle on its
rod during use. These operations were formerly done by hand, when
mantles of good quality were required, but machine treatment is
gradually coming into extended use. Several mantles now on the market
are supported at the head by metal rings, made from thin sheets of iron
which have been plated with aluminium. In petroleum lamps, the mantle is
usually supported from both sides by means of asbestos threads.

In the case of fabrics from which ‘inverted’ mantles are to be made,
fixing is carried out as usual at one end, to a depth of about 1·5 cm.
After drying, a strip of about 0·5 cm. width is bent over and sewn down,
and through this double band an asbestos thread is drawn, by which the
mantle is secured to a magnesia ring. The lower end is drawn together in
the shape of a hemisphere, by means of threads drawn through the meshes
of the fabric; an opening of 6-8 mm. is sometimes left, but in the more
modern patterns the end is drawn almost completely together, and after
cutting is pressed out on a wooden shaper by a wooden mallet.

The product is now ready for burning off; if it is to be marked, it is
stamped at this stage with a solution of didymium nitrate and methylene
blue; the former being only faintly coloured, the organic dye is added
to give a definite impression. On ignition, the nitrate is converted
into the oxide, which is deeply coloured, and, of course, permanent.


~Burning off and Shaping.~--For the production of mantles of the best
quality, these processes are usually carried out by skilled operators,
each mantle being treated separately. Very frequently, however,
mechanical arrangements are employed. The great objection to machine
treatment of such a product lies in the fact that it must be identical
for every mantle; whereas it is exceedingly difficult to ensure that the
original fabric, and the processes of washing, impregnating, wringing,
and drying have been absolutely uniform. The operation of shaping and
hardening is a very delicate one, and on the care with which it is
carried out, the quality of the mantle finally depends. Until quite
recently, only the cheaper kinds of mantles were machine-treated; but as
the uniformity of the fabric becomes more assured, and the earlier
operations more exact, employment of machines at this stage will
undoubtedly increase.

The prepared fabric is shaped on a suitable form, and removed by a
holder, which supports it from the asbestos thread; a flame is then
applied to the head. The burning-off proceeds readily, once started;
when the upper half has been incinerated, the flame is removed. The
weight of the unburnt portion prevents too rapid contraction taking
place at first; when the flame is removed, the glow spreads slowly
downwards, and the shrinking is thus kept as uniform as possible. The
operation must be carried out under a ventilating hood. The organic
material of the fabric is completely oxidised, and the nitrates are
converted into oxides, which retain the exact shape of the original
fibres. The skeleton now undergoes the process of shaping and hardening,
for which a ‘radial’ blowpipe flame is used. The burnt-off product is
placed over this; the gas is supplied at an initial pressure of only a
few inches of water, which is increased towards the end of the
operation. The process commences at the head, the mantle being slowly
lifted and rotated so that it is shaped and hardened along the whole
length. By this means the oxide skeleton is not only suitably shaped,
but is rendered considerably more elastic and resistant. For inverted
mantles, of course, specially shaped burners are required. The eyes of
the operators must be protected from the glare by shades of green glass.
Recently the processes of burning-off and hardening have been carried
out by means of the same burner.

Where machines are employed, the prepared fabrics are burnt off on wire
shapers, usually in rows of ten; mechanical arrangements for continuous
ignition and motion and, in the hardening, for continuous elevation of
the ash-skeleton, are in use, but the finished mantles maintain a
uniform good quality only when the structure of the fabric and the
earlier processes have been absolutely uniform.


~Collodinisation.~--The burnt-off mantle is now ready for use, but is
far too fragile for transport. A method has therefore to be found by
which the finished product can be protected for a time without detriment
to its use for illumination. Mantles of artificial silk, particularly
those for use in high-pressure lamps, are sometimes sent out without
having undergone the final processes of burning off and shaping, which,
in this case, must be carried out on the consumer’s burner. ‘Inverted’
mantles also were formerly sent out after impregnation and drying. In
this condition, of course, the mantles are readily packed and
transported, and there is the additional advantage that the duty on the
unburnt product is very much less than that on the finished mantle.

One of the earliest of Auer’s patents (_vide supra_, p. 271) protected
the process of collodinisation, which is now extensively employed. The
oxide skeleton is dipped into a solution of collodion (the mixed lower
nitro-derivatives of cellulose, or cellulose nitrates) in a mixture of
alcohol and ether, to which, to prevent shrinkage on drying, a little
camphor is added. On account of the inflammability of the mixture, the
ethyl alcohol and ether are occasionally replaced by a mixture of methyl
alcohol and acetone, but with this less volatile mixture, drying of
course is slower. After dipping, the solvents are removed in a current
of air, leaving the mantle coated with an exceedingly thin film of
collodion, which increases enormously its power of resisting shock and
vibration. This film is not removed until the mantle is placed on the
consumer’s burner, when on the application of a match it ignites
instantly and burns away, leaving the oxide skeleton in the condition to
which it was brought in the final stage of hardening and shaping in the
factory. The process is now used for almost all kinds of mantles, having
been successfully applied in Germany in recent years to those made from
artificial silk. The addition of small quantities of various inorganic
salts, _e.g._ nitrates of zirconium, magnesium, platinum, thorium, etc.,
to the collodion solution, has been proposed; these salts make the
collodinised product extremely resistant, but have a very harmful effect
on the oxide ash when the collodion has been burnt off.

The collodinised mantles are cut to length on a trimming machine, and
are then ready for packing.

       *       *       *       *       *

The present chapter may be concluded with a bare mention of a few
disconnected details, selected from the great mass of proposals,
suggestions, and developments which have sprung up round the
incandescent mantle industry.[520]

  [520] For a complete account of the mechanical developments, the
  reader is referred to the monograph ‘Beleuchtung und Lichtmessung,’ by
  Dr. Börnstein, in Dammer’s _Chemische Technologie der Neuzeit_,
  Stuttgart, 1910-11, ii. 243-266.

With regard to the composition of mantles, numerous proposals have been
made. It is stated that thoria with 0·25 per cent. of uranic oxide, UO₃,
gives a light almost equal to that of the Auer mantle. Zirconia with
0·40 per cent. of vanadium, in the form of the pentoxide, is said to
give a splendid white light; the vanadium oxide slowly volatilises, but
addition of an equivalent proportion of silica is said to prevent this.
Langhans claims to have obtained a product equal in light-giving power
to the Auer mantle, by using as impregnating fluid a solution of
colloidal silica, obtained by the addition of nitric acid to a solution
of sodium silicate, to which suitable quantities of rare earth nitrates
are added. Bodies obtained by the use of very similar solutions give
skeletons which are coming into extended employment for gas radiators.
The ‘Sunlight’ mantles use a mixture of thoria (50 per cent.), alumina
(40 per cent.), and chromium sesquioxide (10 per cent.).

A direction of development in which some success has been attained is
the introduction of self-lighting devices. The catalytic action of
finely divided metals has been proposed in innumerable patents,[521] but
these devices are unreliable, and it seems doubtful if chemical methods
will ever be successfully applied to the problem. For the lighting of
streets, shops, etc., the ‘by-pass’ system is employed; a tiny jet of
gas burns continuously from a pin-hole nozzle, which is momentarily
increased, when the main supply is turned on, to such an extent that the
gas issuing from the burner is ignited.[522]

  [521] _Vide_, _e.g._ _D. R. P._ 158974 and 253550; _F._ 417934.

  [522] For automatic regulators for self-lighting, _vide_ _J. Gasbel_.
  1910, ~53~, 490.

An account of the innumerable forms of lamps and burners which have been
introduced in the last twenty years would fill several volumes. The
theoretical grounds on which improvements in this direction are based
are outlined in an able article by Dr. H. Bunte, a recognised authority
on incandescent lighting, which appeared recently;[523] for an account
of some of the lamps which have been successfully applied, the reader is
referred to a recent French publication.[524]

  [523] _J. Gasbel._ 1911, ~54~, 469; _vide_ also Pickering, _J.
  Gaslighting_, 1911, ~113~, 156.

  [524] _L’Éclairage à l’incandescence par le gaz_, Lévy, Part I. Ch.
  III.


CHAPTER XX

ARTIFICIAL SILK--ITS PRODUCTION AND USE IN THE MANTLE INDUSTRY

The history of the artificial silk industry, since its foundation about
the year 1890, illustrates curiously the rapidity with which isolated
facts, of apparently merely academic interest, are seized upon and
adapted to the needs of modern civilisation. It is during this period,
especially, that the bonds between science and industry, in a dozen
different directions, have been drawn so close that to-day it is in many
cases impossible to differentiate the two. The pure science of to-day is
the technology of to-morrow--and not always even of to-morrow, but of
to-day. But we have moved even beyond this; the industrial needs of the
day are creating and extending our science at a rate which shows how
relatively poor a stimulus has been the mere desire for knowledge. Such
has been the history of the artificial silk industry. No sooner had
Chardonnet shown that the preparation of a new fabric was not only
possible but profitable, than a thousand aspects of the problem were
taken up. Patents were taken out on all sides--the majority, as usual,
valueless, one or two of great importance. Companies were formed,
factories built, machines invented; numberless applications were
proposed, mostly again worthless, whilst patient research and
innumerable experiments have carried one or two suggestions to a
successful place in practice. Among these has been the adaptation of
artificial silk to the manufacture of mantles, which will be outlined in
the present chapter. Before taking up this question, however, a short
account of the manufacture of the fabric itself must be given.


~Chardonnet Process.~--In the Chardonnet process, an account of which
was published about 1890, continuous fibres are obtained by forcing
through tiny jets a viscous solution of collodion, or nitrocellulose, as
it has been misnamed, in a mixture of ethyl alcohol and ether. In the
original form of the process, the solution was forced into water, which,
by removing the alcohol and ether, caused an instantaneous coagulation
of the surface, so that a filament was obtained which could be wound
directly on to a spool. More generally, however, the jets deliver the
solution into a chamber through which warm air is passed; this is
equally effective in removing the solvents and causing surface
coagulation, and the filaments are woven directly into threads of ten to
forty strands, according to the purpose for which the fabric is
required, fifteen to twenty being used for silk from which mantles are
to be made. On account of its inflammability, the thread is denitrated
by means of a solution of ammonium sulphide.

The raw material for the process is cellulose, usually in the form of
cotton. Treatment of this with a suitable mixture of concentrated
sulphuric and nitric acids replaces some of the hydroxyl groups by the
‘nitrate radicle,’ NO₃, a mixture of various nitrates of cellulose being
formed, in which the so-called tetra-, penta-, and hexa-nitrates
predominate.[525] The product, cellulose nitrate or collodion, very
closely resembles the original cellulose in appearance and structure. It
is washed thoroughly to free it from traces of acid--which render it
liable to explode spontaneously--and after drying, dissolved in the
minimum quantity of the mixed solvents;[526] the solution is filtered
from insoluble impurities through wads of cotton, pressures of thirty to
sixty atmospheres being required. This filtration purifies and
thoroughly mixes the solution, so that perfect uniformity is obtained in
the product. The glass jets through which the solution is now forced,
under a pressure of forty to fifty atmospheres, have a diameter of 0·08
mm., but the threads obtained contract on the removal of the solvents,
so that fibres of 0·01-0·02 mm. are formed.

  [525] The cellulose esters are usually named as if they were derived
  from a compound C₁₂H₂₀O₁₀, the formula for cellulose being
  (C₆H₁₀O₅)_{_n_}. Thus the formation of the ‘hexa-nitrate’ would be
  represented--

  C₁₂H₂₀O₁₀ + 6HNO₃ = C₁₂H₁₄O₄(NO₃)₆ + 6H₂O.

  [526] In the Lehner process, in which collodion is also used, larger
  quantities of solvent are employed, so that much more dilute solutions
  are obtained; these require low pressures to form the thread, which is
  then hardened chemically.

Chardonnet probably began his work about 1885. It is interesting to
observe that an Englishman, Swan, had proposed in 1883 to use a solution
of collodion in acetic acid, fabrics prepared by his process being shown
at the London Exhibition of 1884.[527]

  [527] _Vide_ Böhm, _Zeitsch. angew. Chem._ 1912, ~25~, 657. There is
  no account of this process in the English patent literature.


~The Pauly or Cuprammonium Process.~[528]--It has long been known that a
solution of copper hydroxide in ammonia solution--Schweitzer’s
reagent--will dissolve cellulose. The use of this solvent for the
production of artificial silk was proposed about 1900, and the method
has become a serious rival of the older Chardonnet process. The solvent
is prepared on a large scale by passing air through an ammonia solution
to which copper turnings have been added. After addition of the
cellulose, and filtration, the solution is forced through tiny jets into
a bath of dilute acid, which removes the copper and precipitates the
cellulose again.

  [528] A full account of this and of the other processes employed in
  the manufacture of artificial silk will be found in the work of Piest,
  _Die Zellulose_, Stuttgart, 1910.

The solution of cellulose by Schweitzer’s reagent is undoubtedly a
chemical action. Cellulose is to be regarded as a polyhydric alcohol,
with one or several atoms of hydrogen of the hydroxyl groups replaceable
by metals. According to Piest (_loc. cit._) a ‘Cupramine base’ is formed
by the replacement of this hydrogen by copper and the amino-group, NH₂.
The action of sodium hydroxide on cellulose, however, is generally
regarded rather as an additive reaction, the product, ‘alkali
cellulose,’ being usually written C₆H₁₀O₅,NaOH. A careful chemical
investigation alone can reveal the actual nature of the compound formed;
such an investigation, apart from its scientific interest, might yield
results of considerable technical importance.


~The Viscose Process.~--Shortly after the introduction of the Chardonnet
process, patents were taken out which protected a very cheap and simple
method of dissolving cellulose,[529] which had been discovered by two
well-known English authorities. Cross and Bevan. They found that
mercerisation, _i.e._ the action of the sodium hydroxide on cellulose,
produces a swollen, transparent mass, which very readily takes up carbon
disulphide. When exposed to the action of this liquid for three or four
hours, at ordinary temperatures, the mass swells further, gelatinising
and becoming soluble in water. On treatment with water, a yellowish,
extremely slimy solution is obtained, from which cellulose is
precipitated on prolonged standing, by heating, or by oxidation. The
substance is apparently a cellulose xanthate, and may be written
NaS·CS·O·C₆H₉O₄,NaOH.[530] On account of the extremely viscous nature of
the aqueous solution, Cross and Bevan gave it the name Viscoid.

  [529] _Vide_, _e.g._ Cross, Bevan, and Beadle, _D. R. P._ 70999,
  granted September, 1893.

  [530] _Vide_ Beltzer, _Zeitsch. angew. Chem._ 1908, ~21~, 1731.

During the last few years this method of dissolving cellulose has been
employed in the manufacture of artificial silk, under the name ‘Viscose
Process.’ The product obtained is very suitable for the manufacture of
incandescent mantles, and is considerably cheaper than either the
Chardonnet or Pauly silk.


~The Acetate Process.~--Quite recently numerous experiments have been
carried out with the object of finding methods for employing the
cellulose esters of organic acids in the preparation of fabrics. The
acetate, which is generally used, gives solutions from which fibres can
be obtained which are comparable to natural silk in strength, and which
have the further advantage of being non-inflammable, and far less
readily affected by water than artificial fabrics obtained by the above
methods. It is prepared by treating cellulose with dilute acid, by which
the so-called ‘hydrocellulose’ is obtained; this is treated with a
mixture of glacial acetic acid and acetyl chloride, and the whole, after
addition of a little concentrated sulphuric acid, warmed to 65°-70°C.
As early as 1894, Cross and Bevan[531] had patented a process for this
preparation by the action of acetyl chloride in the cold on an intimate
mixture of cellulose and zinc chloride.

  [531] _E._ 9676, 1894.

From the solution obtained, the acetates are precipitated by water,
washed and dried. The mixture of esters dissolves in chloroform,
nitromethane, acetic acid, phenol, pyridine, etc., and is
re-precipitated by addition of alcohol, benzene, or ligroin (petroleum
ether). On account of its non-inflammable character, cellulose acetate,
as the product is called, is being used instead of the nitrate in the
manufacture of celluloid; it is also used for non-inflammable
cinematograph films. Fibres can be obtained by forcing the solutions
through jets, and removing the solvent, as in the above processes; these
are spun into threads which are coming into increasing use, on account
of their extremely low conducting power, for the insulation of very fine
electric leads. The product is at present too expensive, however, for
use in the textile industries, or for the manufacture of mantles.

A solvent which had at one time some technical importance is zinc
chloride.[532] The concentrated aqueous solution of the salt will take
up cellulose in considerable quantity; and the solution has been used in
the preparation of carbon filaments for glow lamps.

  [532] Gulbrandsen, _Prog. Age_, 1912, ~30~, 77; Wynne and Powell, _E._
  16805, December, 1884.

The fabrics prepared by the processes which have been mentioned above
are of great technical value. In lustre they far surpass natural silk,
and they take dyes very well, but owing to the ease with which they
tear, they cannot be woven alone for textiles, but are always used in
‘mixed’ materials. The acetate silk, which approaches the natural fibre
in strength, is not much less expensive. Whilst the price of natural
silk is roughly 35 francs per kilo. (approx. 13_s._ 3_d._ per lb.), the
costs of production of the artificial fabrics are--Chardonnet 15 frs.,
Pauly 12 frs., Viscose 7 frs. per kilo. (respectively 5_s._ 8_d._, 4_s._
6_d._, and 2_s._ 8_d._ per lb.). Artificial silk, however, has uses
distinct from the natural fibre, and is at present a competitor with it
in one or two small fields only. Thus the production of natural silk is
ten times that of artificial silk (50,000,000 kilos. per annum to
5,000,000 kilos.) in spite of the difference in price.

Artificial silk is very susceptible to the action of water, which
weakens it very considerably. Its resistance is said to be greatly
increased by the action of formaldehyde; the fabric is plunged into a
bath containing an aqueous solution of the aldehyde, to which a little
lactic acid has been added. The chemistry of the change is discussed at
length by Beltzer (_loc. cit._).

The threads of artificial silk far surpass in lightness those spun from
vegetable fibres. A thread of twenty strands weighing one pound
avoirdupois would be more than twenty miles long. At the same time the
filaments have not the irregular tubular structure of vegetable fibres,
but are solid cylinders. The fact that the filaments are continuous, so
that there is relatively little torsion in threads spun from them, gives
artificial silk its great advantage over the natural vegetable fibres
for the manufacture of mantles. For this purpose the Pauly or
Cuprammonium silk is most suitable, though Viscose silk is almost as
good; the fibre obtained by the Chardonnet process is not quite so
useful in this direction.


~The Manufacture of Mantles from Artificial Silk.~--Whilst the fabrics
made by the various processes outlined above are more expensive than the
cotton and ramie formerly exclusively used in the mantle industry, they
have the advantage, in addition to the fact that they produce better and
more lasting mantles, that they do not need the laborious and
troublesome process of washing which is so essential in the case of the
vegetable fibres. From the nature of the methods used in its
manufacture, artificial silk can contain no mineral residue; hence the
fabric is immediately ready for impregnation.

As early as the year 1892 Schlumberger and Sinibaldi proposed the use of
Chardonnet silk for the manufacture of mantles; but their patent, a
Belgian one,[533] attracted little attention, although they stated
clearly that the denitrated silk will readily take up the lighting
fluid. Ignorance of this fact deferred the successful application of
this fibre for ten years. In 1894 De Mare suggested the preparation of
mantles by addition of the necessary salts to the collodion solution
before squirting; in the following year Knöfler used the same process,
recommending in addition the use of ammonium sulphide to denitrate the
impregnated threads. These two attempts, which were found to be
unworkable, owing to the difficulty of obtaining a homogeneous product
before squirting, were merely efforts to compete against the Auer
monopoly, resting on Welsbach’s patents, which covered impregnation of
any natural fibre. In Knöfler’s process,[534] the salts were dissolved
in alcohol and added to the collodion solution, which was then forced
through jets into water, to which ammonia was added to prevent removal
of the nitrates in solution; the threads were then denitrated with
ammonium sulphide. The ammonia treatment of course converts the nitrates
into the insoluble hydroxides, a departure which was followed in most of
the numerous patents inspired by Knöfler’s process.

  [533] _Vide_ Böhm, _Zeitsch. angew. Chem._ 1912, ~25~, 657. Apparently
  this patent was not taken up; no account of it has been found in the
  published patents of the Belgian Government.

  [534] _E._ 11038, 1895, granted July, 1895.

The first indications of the method which ultimately led to success are
to be found in a patent taken out by Plaisetty, in 1901.[535] The
specification protects the addition of thorium and cerium hydroxides to
the cuprammonium solution of cellulose, but apparently without any
inkling of the results that were to follow, and more or less
incidentally, he includes in this patent the impregnation of the
finished fabric and the subsequent treatment with ammonia. In the
following year he applied for a German patent,[536] which was granted in
May, 1903, in which he definitely protects the impregnation of the
finished fabric, and the ammonia treatment, the fabric being then washed
and dried, and burnt off as usual.

  [535] _E._ 20747, 1901.

  [536] _D. R. P._ 141244.


~Impregnation.~--Since the filaments from which artificial silk is
obtained are solid and rod-like in form, as opposed to the tubular
structure of cotton and ramie filaments, it is rather surprising that
the fabric should take up the lighting fluid in the necessary quantity
(_vide_ p. 295). It is found that a 50 per cent. solution of nitrates
gives the best results, the impregnation requiring half an hour; a warm
bath is usually employed. It is usual to add to the bath a quantity of
thorium hydroxide, since the thorium nitrate of commerce generally
contains nitric acid, which has a bad effect on the fabric.[537] The
excess of solution is removed by means of a glass or porcelain
centrifuge, not, as with cotton or ramie mantles, by use of a wringer;
drying must be carried out very slowly. The fabric is not cut into
lengths before impregnation, as in the case of cotton or ramie, but is
immersed in the lighting fluid in long strips.

  [537] _Vide_ Buhlmann, _D. R. P._ 188427, 1907; also _E._ 6828, 1907.


~‘Fixing.’~--If the dried fabric, impregnated with the necessary salts,
be finished and burnt off in the usual way, the oxide skeleton is
extremely fragile, and soon falls to powder. The reason for this lies
probably in the explosive decomposition of the nitrates, the weight of
organic matter relative to the salts being very much less than in the
case of ramie or cotton fabrics (_vide_ p. 295). The additional ammonia
bath advocated by Knöfler (_vide supra_) was therefore adopted by
Plaisetty, and the nitrates in the dried impregnated fabric are
converted into hydroxides by this treatment. For this process, to which
the name ‘Fixing’ has been given by Böhm, numberless alternative
proposals have been made. Plaisetty’s ammonia fixing gives a mantle
which, after burning off, is exceedingly elastic and strong, but it is
nevertheless open to serious objections. Thus the nitrates may be to
some extent dissolved out by the fixing bath before precipitation of the
hydroxides has occurred; to remedy this, the impregnated fabric must be
very thoroughly dried before fixing, and in this case, apart from the
trouble involved, the acid of the commercial nitrates will attack the
fabric unless addition of thorium hydroxide has been made to the
impregnating fluid. Again, the conversion into oxides is not complete,
the outer layer first formed preventing free diffusion of the alkaline
fluid. Finally, since ammonium nitrate is formed in the reaction, a very
thorough washing is necessary to remove this salt.

It would be impossible to mention all of the numberless proposals which
have been put forward for fixing; nor are the great majority worthy of
mention.[538] One of the most important was that of Albrecht,[539] by
which hydrogen peroxide is substituted for ammonia. This reagent, as is
well known, precipitates from solutions of thorium salts ‘peroxide’
compounds (_vide_ p. 255); from the fabric impregnated with the nitrate,
free nitric acid is liberated in the reaction. Since the peroxide is
soluble in nitric acid, two baths are used, the fabric being allowed to
remain for a short time only in the first, which becomes strongly acid,
and being then transferred to the second, in which the precipitation is
completed. The burning off of the product so treated proceeds quite
quietly, and leaves a very hard and elastic skeleton. The method,
however, has the grave disadvantage that cerium salts are not
precipitated under these conditions, but escape into the solution. To
remedy this, arbitrary additions of cerium nitrate are made to the
fixing bath, but the percentage of cerium, and therefore the lighting
power (_vide_ p. 293) of mantles fixed by this method is liable to
fluctuate. A modification has been introduced[540] in which various
salts are added to the hydrogen peroxide fixing baths to prevent the
withdrawal of the cerium salt; these are chiefly acetates of the alkali
metals and allied compounds. The fabric requires washing after this
treatment.

  [538] A large number of patents are mentioned by Böhm, _J. Gasbel._
  1909, ~52~, 855.

  [539] _D. R. P._ 188427, September, 1907; _E._ 15295, 1907.

  [540] _Vide_ _E._ 2240, 1908. Cerofirm Gesellschaft, by Brit. Cerofirm
  Co.

A rather similar proposal[541] substitutes for ammonia an alkaline
solution of hydrogen peroxide, obtained by dissolving sodium peroxide in
water. After saturation for a minute or so, the fabric is wrung out and
dried, there being no necessity, according to the patent, for any
subsequent washing. The same company in an earlier patent[542] suggest a
fixing bath of ‘an alkali or amine with an acid which can form insoluble
basic double salts with the earth metals,’ the said salt to be
precipitated on the fibre, whilst an alkali or amine nitrate goes into
solution; acetic and benzoic acids and phenol are mentioned. Apparently
this process did not give satisfactory results. The same may be said of
the hypochlorite method of Visseaux.[543] Equally interesting, and
doubtless equally practical is the proposal[544] to treat the dried
impregnated mantle with ozone in a closed chamber, which possibly
inspired the even more original suggestion[545] that the fabric be first
washed in ozonised water, dried, then impregnated and dried, and finally
treated with ozone. According to yet another French patent,[546] the
impregnated and dried fabric is to be treated with an alcoholic solution
of hydrofluoric acid, which will give a viscous insoluble mixture of
thorium and cerium fluorides in the fabric, and at the same time will
remove any impurities. Another patent[547] proposes the use of hydrazine
and similar bases, cerium nitrate being added to the fixing solution.

  [541] _D._ 247940, June, 1912; _F._ 430417, August, 1911.

  [542] _F._ 403433, September, 1909, Bruno Co.

  [543] _F._ 408807, February, 1910.

  [544] _F._ 414700, June, 1910.

  [545] _F._ 422643, January, 1911.

  [546] _F._ 426156, April, 1911.

  [547] _E._ 11904, 1909.

Innumerable suggestions have been made for fixing by means of ammonia
gas, or vapours of organic bases.[548] An English patent granted in
February 1910[549] protects ‘various improvements,’ which consist in
carrying out a preliminary treatment with steam, impregnation with the
solution of nitrates, conversion of nitrates into oxides either by steam
carrying ammonia, pyridine, etc., or by the action of these vapours
without steam in a vacuum, all in one chamber, which can be exhausted or
filled with various solutions or vapours as required.

  [548] _Vide_, _e.g._ _D. R. P._ 199615 of June, 1908.

  [549] _E._ 25549, 1908.

More recently, the use of organic salts of thorium and cerium for
impregnation has been proposed by Dr. F. W. Wirth;[550] in fixing the
impregnated fabric with hydrogen peroxide, the cerium is not removed in
solution, since the weak organic acid formed will not dissolve cerium
peroxide. The same author has also suggested[551] the addition to the
fixing bath of substances which absorb oxygen from the air--_e.g._
sodium hydrogen sulphite, resorcinol, tannin--to prevent removal of
cerium. He has also advocated impregnation with amorphous salts,[552]
which will obviate the necessity for any subsequent fixing treatment,
the hypophosphites and double compounds with ammonium oxalate being
specified. Attempts have been made to achieve the same end by other
methods. Thus Silbermann[553] has proposed a preliminary treatment with
alkalies (mercerisation); the fabric is treated with concentrated sodium
hydroxide solution in absence of air for half an hour, pressed through
rollers, and plunged into the impregnating solution. Two years
previously a patent was taken out by Drossbach[554] to protect the use
of colloidal solutions of the hydroxides. To a boiling suspension of
well-washed, freshly-precipitated thorium hydroxide, a solution of a
small quantity of the nitrate is gradually added; after half an hour a
colloidal solution is obtained, which, after the addition of the
required quantity of cerium nitrate, and dilution to a suitable extent,
is employed directly for impregnation. The patent states that this
solution is more readily absorbed than the ordinary nitrate solution,
but the statement has been questioned.

  [550] _Chem. Zeitg._ 1911, ~35~, 752.

  [551] _Zeitsch. angew. Chem._ 1912, ~25~, 922.

  [552] _Chem. Zeitg._ 1911, ~35~, 752.

  [553] _Chem. Zeitg._ 1911, ~35~, 1037.

  [554] _D. R. P._ 212842, August, 1909; _vide_ also Kreidl and Heller,
  _E._ 17862, 1909, and _D. R. P._ 228203, 1910.

Artificial silk itself is of a colloidal nature, the solidification of
the filaments of cellulose during the manufacture being rather in the
nature of a coagulation than of a precipitation; it is to this fact that
the fibres owe their solid, rod-like structure, and it is probably this
circumstance also which determines the very intimate absorption of the
hydroxides or peroxides produced by fixing. It is well known that
colloidal substances under some circumstances possess the property of
clinging tenaciously to foreign bodies, exhibiting the so-called
phenomenon of adsorption. The strength and elasticity of the oxide
skeleton, obtained when the fixed and dried fabric is subjected to the
operation of burning off, are presumably to be referred to such a
relation between the cellulose of the fibres and the insoluble thorium
and cerium compounds, precipitated by one of the methods of fixing
described.


~The Final Stages.~--The treatment of the fabric after impregnation and
fixing differs only slightly from that used for the impregnated ramie
and cotton products. The dried strips are cut into suitable lengths, and
the head is drawn together with asbestos and threaded across. No tulle
or gauze is required, the end being simply turned down before threading.
After the ordinary strengthening process for the head (_vide_ p. 296)
the process of manufacture was, until recently, finished, the goods
being sent out in the unburnt condition, on account of the difficulties
of collodinisation. These have now been overcome, so that the mantles
are burnt off and collodinised as usual. Burning off and shaping are now
frequently effected in one operation by machine; the nature of the
methods by which the fibre is made produces a uniform fabric, and if the
earlier processes are carefully carried out, a uniform product is
obtained, which is therefore suitable for machine treatment.


CHAPTER XXI

OTHER TECHNOLOGICAL USES OF THE CERIUM AND YTTRIUM ELEMENTS, ZIRCONIUM
AND THORIUM

The technical uses of the members of this group of the elements we are
considering, apart from the employment in the manufacture of
incandescent mantles, are at present very restricted. Innumerable
proposals for the employment of the compounds of cerium and the allied
metals, which are obtained in such large quantities as by-products in
the thorium industry, have been put forward, but the actual extent to
which they are utilised is so small that only an insignificant fraction
of the available quantities is annually required. In the metallic form,
a limited application is found for various alloys, _e.g._ the so-called
pyrophoric alloys, misch metal, and the magnesium and aluminium alloys.
Various compounds of the elements, as well as some alloys, have been
suggested for use in arc-lamp electrodes, and the use of the metals
themselves, as well as of various salts, for the manufacture of
flashlight powders, is protected by several patents. Investigations have
been made to determine the value of the oxides and sulphates as
catalysts in the contact process for the manufacture of sulphuric acid,
and one patent states that the yield obtained is equal to that given by
platinised asbestos. Cerium salts have been proposed for tanning, and in
the preparation of enamels; cerium sodium sulphate is used in the
catalytic oxidation of aniline to aniline black. The oxalate has a very
slight use in medicine. The oxidising power of ceric salts is of some
use in photography; ceric sulphate in acid solution is also said to be
an efficient oxidising agent for aromatic hydrocarbons. On account of
the deep colour of the higher oxide of praseodymium, didymium salts
find a limited application for marking textiles.

Compounds of the yttrium group have at present no technical importance.
They were formerly used to some extent for the manufacture of filaments
for Nernst lamps, but with the introduction of metal filament lamps in
electric lighting, the demand for Nernst lamps and consequently for the
yttria oxides, has to a very great extent died away.

Zirconium and its compounds, on the other hand, promise to become of
some technical importance. The metal received considerable attention in
the earlier stages of experimental work on metallic filaments for
electric lighting, but it has been shown that its melting-point is not
sufficiently high to allow of extended use in this direction. The
carbide has been proposed for the same purpose, but is even less
suitable; this compound, however, on account of its great hardness, is
likely to find employment as an abrasive, and in glass-cutting. The
oxide, which occurs in nature in an impure form as the mineral
Baddeleyite (_q.v._), is employed in the manufacture of ‘Siloxide’ glass
and of enamels, as a pigment and polishing agent, and in various forms
of lamps, _e.g._ the Nernst and Bleriot lamps, the Drummond light, etc.
Far more important, however, is its use for fire-resistant crucibles,
furnace linings and supports, etc., for which its refractory nature
renders it particularly suitable. On account of its high specific
gravity and non-poisonous character, it has been proposed for use in the
Röntgen ray examination of the human body. Quite recently, metallic
zirconium has been employed in metallurgy; addition of small quantities,
in the form of suitable alloys, is said to secure sound castings, with
increased strength and resistance to acids.


THE CERIUM GROUP


~Pyrophoric Alloys.~--It has long been known that the metals of the
cerium group possess the property, when scratched or struck, of throwing
off glowing particles; this power of emitting sparks is not lost when
the metals are alloyed, so long as the percentage of foreign metal is
not allowed to become too high. In a patent[555] protecting the use of
various ‘pyrophoric alloys,’ as these spark-giving alloys are called,
Auer states that the pure metals do not show this property, which only
appears when foreign metals are present; he accordingly patents alloys
of the cerium metals with iron, specifying particularly the alloy with
30 per cent. of the latter element. Auer’s statement has been
contradicted,[556] and it seems to be generally accepted that
misch-metal[557] of ordinary technical purity has the property of
sparking when scratched. This alloy of the cerium metals, however, is
far too soft to be useful for the purpose, and the addition of some
foreign element is required to obtain the strength, hardness, and
brittleness necessary in the various forms of ‘lighters.’ Besides the
addition of iron, the use of tin, lead, zinc, cadmium, silicon, etc.,
has been patented.[558]

  [555] _E._ 16853, 1903; _D. R. P._ 154807.

  [556] _Vide_ Böhm, _Chem. Zeitg._ 1910, ~34~, 361.

  [557] The crude mixture of cerium, lanthanum, neodymium, praseodymium,
  samarium, etc., with small quantities of iron and other metals,
  obtained by reduction of the earth-compounds formed as by-products in
  the thorium industry, is technically known as ‘misch-metal.’

  [558] _F._ 439058, March, 1912.

Various forms of these lighting devices are manufactured;[559] in all of
these the sparks produced by scratching the pyrophoric alloy with
hardened steel, by means of some simple mechanical device, is caused to
ignite a fragment of tinder, or a wick supplied with a suitable liquid,
_e.g._ methyl alcohol, benzene, or petrol. In the numberless forms of
cigarette-lighters at present before a somewhat indifferent public, the
friction is obtained by means of a toothed wheel, actuated by a spring
which is released when the device is opened. Many forms of gas lighter
are also on the market, but the demand for them is very small. Many
attempts have been made to adapt the device to the ignition of the Davy
miners’ lamp, but none have been successful, since it is impossible to
prevent the sparks flying through the gauze. Much work has also been
spent in efforts to utilise the pyrophoric alloys for the automatic
ignition of incandescent gas-lamps, but these have been equally
unsuccessful, so that it may be said that important technical
applications of this interesting property have still to be made.

  [559] _Vide_ Böhm, _Chem. Zeitg._ 1910, ~34~, 377; also Kellermann,
  _Die Ceritmetalle und ihre pyrophoren Legierungen_, Wilhelm Knapp,
  Halle, 1912, pp. 94 _et seq._

Auer prepared his alloys by addition of iron, or other heavy metal, to
the fused mixture of cerium metals obtained in the electrolytic
apparatus employed for the production of the latter. They can, however,
be prepared by fusing together the required quantities of foreign metal
and misch-metal, the latter being obtained by processes other than those
of electrolysis usually employed. The rare metals were obtained by the
earlier chemists in a very impure state by reduction of the halogen or
double halogen compounds with sodium or potassium. More recently[560]
much purer products have been obtained--especially in the case of
zirconium--by the action of metallic calcium, in the form of powder, on
the oxides. Another method,[561] which has been employed in the
preparation of metallic filaments for lamps, consists in heating the
oxides with powdered magnesium in an atmosphere of hydrogen or nitrogen;
by this means, hydrides or nitrides are obtained, which on heating
decompose into the gas and the metal.

  [560] _Vide_ Kuzel and Wedekind, _E._ 23215, 1909.

  [561] _Electrodon Gesellschaft_, _D. R. P._ 154691, September, 1904.

The ease with which misch-metal and its alloys with iron and other
elements throw off glowing particles when struck is due to the low
ignition temperature of cerium, and the energy with which it combines
with oxygen. When such alloys are scratched, small fragments are struck
off, which are raised to the ignition temperature by the heat of
friction. It is generally accepted, however, that this explanation is by
no means a complete one, and the existence of a pyrophoric suboxide was
suggested.[562] The theory was advanced that the pyrophoric properties
of the alloys were due primarily to the formation on the surface of a
film of this sub-oxide, and the partial oxidation of cerium alloys,
protected by patent (_loc. cit._), was said to cause a marked increase
in the ease with which sparks could be obtained. In this connection, an
experiment of Hirsch, who has thoroughly investigated the properties of
metallic cerium,[563] is of interest. He found that when the element is
warmed in a sealed glass bottle, a black powder forms on the surface,
which, when the bottle is opened, ignites spontaneously. It is probable
that this black sub-oxide plays an important part in the production of
sparks from the ordinary pyrophoric alloys.

  [562] _Vide_ _F._ 407117, December, 1909.

  [563] _Met. Chem. Eng._ 1911, ~9~, 543.


~Other Alloys.~--On account of the great affinity of the cerium metals
for oxygen, misch-metal has been suggested as a reducing agent,[564] as
have also the alloys of cerium and magnesium;[565] the formation of the
latter is endothermic, so that they act much more vigorously than either
metal separately. The alloys of cerium with tin and aluminium have been
thoroughly investigated from the stand-point of metallography, by
Vogel.[566] It has recently been claimed that the addition of very small
quantities of cerium to aluminium has a very marked effect, the rare
earth metal acting as a purifying agent,[567] and greatly improving the
properties of the aluminium. The cerium may be introduced as fluoride,
either to the electrolytic bath in which the aluminium is being
prepared, or to the latter metal, after preparation, in the fused state.
The most favourable effect is said to be produced by 0·2 per cent. of
cerium.

  [564] _Vide_, _e.g._ Escales, _D. R. P._ 145820, October, 1903.

  [565] Hirsch, _loc. cit._

  [566] _Zeitsch. anorg. Chem._ 1911, ~72~, 319; 1912, ~75~, 41.

  [567] Borchers and Barth, _D. R. P._ 246484, May, 1912.


~Applications of the Cerium Compounds.~[568]--One of the earliest
investigations in which cerium compounds were examined with a view to
technical employment, was that of Kruis,[569] who made experiments on
the comparative value of the salts of different metals as catalysts in
the manufacture of aniline black. He showed that a solution of aniline
with an oxidising agent (potassium chlorate or chromate) develops no
colouration unless a salt of a heavy metal is present. In the case of
the fabric impregnated with the solution, the only metals of which
compounds were found suitable for producing a colour were copper, which
was then generally used for the purpose, and cerium, iron, and
manganese. Of these, cerium, used in the form of the double sulphate,
was found to be by far the most suitable, and moreover to have the
advantage that only small quantities are required; the price was at that
time too high to allow of its use, but it has since been employed.[570]
Cerium compounds have also been proposed as mordants for alizarin,[571]
but they do not appear ever to have come into general use.

  [568] An account of the various suggestions for the technical
  employment of the rare earth elements, by Dr. Max Speter, will be
  found in Dammer, _Die Chemische Technologie der Neuzeit_, Stuttgart,
  1910, vol. i. pp. 500-504.

  [569] _Dingl. Polyt. J._ 1874, ~212~, 347.

  [570] _Vide_ Buhrig, _Dingl. Polyt. J._ 1879, ~231~, 77; and _Abstr.
  Chem. Soc._ 1879, ~36~, 683.

  [571] _Vide_ Witt, _Chem. Ind._ 1896, ~19~, 156.

In photography, ceric sulphate has been employed for some time for the
purpose of ‘reducing,’ _i.e._ removing silver from over-developed
negatives.[572] It is said to act very evenly and rapidly, the small
quantity of free sulphuric acid required to hold the salt in solution
having no bad effect. More recently, cerium salts have been proposed for
use in colour photography.[573] An emulsion is obtained by adding salts
of iron, uranium, or cerium to a colloidal solution of albumen in
ammonia, borax solution, or sodium carbonate solution; this is spread on
the paper or negative, and is said to be readily sensitive to light.

  [572] _Vide_ Lumière, _Bull. Soc. franc. Photog._ (2) 1900, ~16~, 103.
  Also _E._ 470, 1900.

  [573] Fateau, _E._ 20740, 1907.

The crude mixture of cerous sulphate with basic sulphates of other
elements of the cerium group, which has been patented for use as a
catalyst in the contact process for the manufacture of sulphuric
acid,[574] is prepared from the earth compounds obtained as by-products
in the treatment of monazite. These are converted into the sulphates,
and, after evaporation of the excess of sulphuric acid, heated for
several hours at a low red heat (300°-600°C.). The porous mass is then
broken, and is ready for use. It is stated that a nearly quantitative
yield of sulphur trioxide is obtained, and that the mixture acts more
efficiently than pure cerous sulphate. The reaction is said to depend on
the continuous formation and decomposition of the ceric salt, thus:

  Ce₂(SO₄)₃ + SO₂ + O₂ = 2Ce(SO₄)₂
             2Ce(SO₄)₂ = Ce₂(SO₄)₃ + SO₃ + O

  [574] Hölbling, _D. R. P._ 142144 and _F._ 326321 of May, 1903.

This process does not appear to have come into general use.

A general patent had been taken out in 1901, protecting the use of
oxides of the rare earth elements for ‘high-temperature catalysis’ in
the manufacture of sulphuric acid,[575] but the oxides do not seem to be
very efficient.[576]

  [575] Meister, Lucius and Brüning, _E._ 1385, 1901.

  [576] _Vide_ Plüddemann, Dissertation, _Beitrag zur Aufklärung des
  Schwefelsäurekontaktprozesses_, Berlin, 1907.

It has also been proposed to utilise the oxidising power of ceric salts
in acid solution[577] for the preparation of aldehydes, quinones, etc.,
from aromatic hydrocarbons, for which purpose they are claimed to be
more efficient than chromates. By the use of a crude cerium dioxide
(60-70 per cent. CeO₂) obtained by the ignition of the by-products of
the thorium industry, good yields were obtained of benzaldehyde,
naphthaquinone and anthraquinone from toluene, naphthalene and
anthracene respectively.

  [577] Meister, Lucius and Brüning, _D. R. P._ 158609, March, 1905.

Garelli[578] has examined the action of cerium salts in tanning; he
states that with neutral solutions, effects very similar to those
produced by aluminium salts are obtained, but Eitner, who has also
examined the question,[579] is of opinion that the cost of isolating and
purifying the salts from the monazite residues renders their employment
for this purpose impossible.

  [578] _J. Soc. Chem. Ind._ 1912, ~31~, 830.

  [579] _Ibid._ 1911, ~30~, 1128.

The fluoride, silicofluoride, and dioxide have also been proposed for
the preparation of enamels,[580] but do not give satisfactory results.

  [580] Rickmann and Rappe, _D. R. P._ 99165, September, 1898; also _D.
  R. P._ 203773, October, 1908.

Several patents protect the use of rare earth compounds for flashlight
powders. For most of the mixtures covered, it is claimed that the usual
defects of fumes, slow firing, etc., are absent. The usual recipes[581]
are for magnesium or aluminium powder with chromates, nitrates, or
similar salts of thorium, cerium, etc.; in one case[582] the rare earth
metals, alloyed with barium, silicon, uranium, or titanium, are to be
used with ‘an oxidising agent which leaves a non-volatile residue.’ None
of these mixtures appears to have been successful.

  [581] _Vide_, _e.g._ _E._ 14692, 1908; _D. R. P._ 158215.

  [582] _F._ 403722, October, 1909.

Cerium compounds have also been proposed for use in arc-lamp electrodes;
it is claimed that they give a very intense light, one patent[583]
adding that the presence of cerium peroxide and a little fluorspar
causes the arc to burn evenly and quietly. In another, the use of a
mixture of tungstates or molybdates of the alkaline earths, with
fluorides of the rare earth elements is protected;[584] the use of
pyrophoric alloys, either entirely, or for the core of the electrode,
has also been suggested.[585]

  [583] _E._ 414707, June, 1910.

  [584] _F._ 431040, August, 1911; also _E._ 21374, 1909.

  [585] _E._ 8150, 1909.


~The Nernst Lamp.~--The first efforts which were made for the employment
of electricity in illumination endeavoured to utilize the heat produced,
when a current traverses a very thin metallic filament, to raise the
conductor to incandescence. Numerous efforts were made to adapt platinum
to this purpose, but its melting-point was finally admitted to be too
low; at length it was found possible to produce carbon filaments, and
the well-known carbon lamps came into use. Numerous attempts were made
to effect improvements;[586] one plan was to coat the carbon filament,
after its production, with a skin of metallic conductor, and zirconium
and thorium were among the metals proposed in this connection.[587] The
first really important advance, however, was effected by Nernst, who
took up the study of ‘conductors of the second order,’ and within a few
months succeeded in adapting these to the purposes of illumination
(1897-1898). The Nernst lamps gave a very intense white light with
considerably less consumption of electricity than the carbon filament
lamps; they enjoyed a very considerable vogue for some years, but have
been almost entirely displaced by the cheaper metal filament lamps,
which were occupying the attention of Auer von Welsbach at the time
Nernst perfected his invention.[588]

  [586] The reader is recommended to consult the _Jahresberichte über
  die Leistungen der Chemischen Technologie_ of Fischer, Section
  ‘Beleuchtung,’ for the years 1898-1901 inclusive, from which some idea
  may be obtained of the innumerable proposals and suggestions, usually
  protected by patent, which were put forward at this time.

  [587] _Vide_, _e.g._ _D. R. P._ 153959.

  [588] _Vide_ _E._ 1535, 13116 and 17580, 1898.

In his first patent,[589] Nernst proposed the use of a rod of magnesia
or zirconia as filament; these oxides, which belong to his class of
conductors of the second order, are non-conductors at ordinary
temperatures, but their resistance decreases as the temperature rises,
so that at high temperatures they will conduct electricity at the
ordinary voltage. The preliminary heating was at first effected by means
of a Bunsen burner, but a later patent[590] of the same year protects a
method of heating by means of a platinum spiral in an auxiliary circuit,
which is automatically cut out when the current in the main circuit,
bearing the filament, attains its required strength. In the following
year[591] it was found that filaments composed of mixtures of oxides
were far more suitable than the earlier magnesia or zirconia rods;
yttria, thoria, and zirconia were the chief oxides used, small
quantities of ceria being occasionally introduced. With these filaments,
the increase of conductivity with temperature is far more rapid than
with the pure oxides; the preliminary heating required, therefore, is
less and the light obtained more intense. The filaments used were in the
form of rods or spirals obtained by compressing the powdered oxides.

  [589] _E._ 19424, 1897.

  [590] _E._ 23470, 1897.

  [591] _E._ 6135, 1898.

The Nernst filaments differ markedly from those of the ordinary electric
glow lamp in that they are not conductors in the ordinary sense (or of
the first order, as Nernst puts it) but electrolytes, the passage of the
current being actually attended by an electro-chemical change in the
filament.[592] The oxide is ionised; the ions of the metals travel to
the cathode or negative pole, where the liberated atoms of metal
instantly recombine with the oxygen of the air, whilst oxygen ions
travel to the anode, from which the gas is liberated. There is thus a
gradual redistribution, resulting in accumulation of oxide at the
cathode with a corresponding loss at the anode, which is balanced, after
some time, by diffusion, so that equilibrium is attained. In consequence
of this redistribution the filament glows more brightly at the anode,
where it is thinnest, than at the cathode.

  [592] _Vide_ Nernst, _Zeitsch. Elektrochem._ 1899, ~6~, 41.


INDUSTRIAL APPLICATIONS OF ZIRCONIUM

It has already been mentioned that zirconium received considerable
attention as a suitable substance for the preparation of metallic
filaments during the early stages of their development. Whilst at the
present time this element has been abandoned for the purpose,[593]
several zirconium lamps were at one time on the market, and a brief
mention of some of the work done in this direction may not be out of
place.

  [593] _Vide_ Baumhauer, _Zeitsch. angew. Chem._ 1910, ~23~, 2065.

One of the general methods for the preparation of the metallic filaments
may be illustrated by a patent taken out in 1902 by Sander,[594] for the
preparation of filaments of zirconium, with or without addition of
zirconium carbide. The metal, or a compound which on heating will yield
the metal and a volatile substance which can be removed, is prepared in
a finely divided condition, and made into a paste with some organic
binding material; the paste is then forced through a tiny aperture, and
the resulting thread is shaped and heated to a high temperature _in
vacuo_ or in an inert atmosphere. If an organic substance be used to
form a paste with metallic zirconium, the final process of heating
results in the formation of the carbide; the same compound is also
obtained by another process protected by Sander (_loc. cit._) in which
the hydride of zirconium, prepared by the reduction of the oxide by
means of powdered magnesium in an atmosphere of hydrogen, is mixed with
a cellulose solution, and the liquid treated as in the manufacture of
artificial silk, the threads obtained being then heated to remove all
organic matter as far as possible.

  [594] _D. R. P._ 133701, July, 1902.

The carbide is also probably obtained by the process of the British
Thomson-Houston Company, in which advantage is taken of the fact that
zirconium oxalate is a pasty gelatinous substance, which can be forced
through a die to form a thread without addition of any agglutinating
agent. The oxalate, precipitated by addition of ammonium oxalate to a
solution of a zirconium salt, is mixed with finely divided carbon, and
the threads obtained from the pasty mass heated to a very high
temperature in a furnace.[595] Zirconium oxalate is also proposed as a
binding material for powdered tungsten, in the preparation of filaments
from that metal.[596]

  [595] _E._ 5415, 1908.

  [596] _E._ 10590, 1908.

The compounds of zirconium and thorium with elements of group VB,
according to two German patents,[597] are suitable for the preparation
of metallic filaments in much the same way. Thorium, titanium, and
zirconium are also among the metals which, it is claimed, can be
obtained in the pure fused state by heating in an electric arc _in
vacuo_, so that filaments can be drawn directly.[598]

  [597] _D. R. P._ 153958 and 154299, September, 1904.

  [598] _Ibid._ 169928, April, 1906.

Metallic zirconium and its alloys have recently been employed in
metallurgy. The pure metal can be obtained by the calcium reduction of
Kuzel and Wedekind (_vide_ p. 316); zirconia is not reduced by powdered
aluminium (Goldschmidt’s process), but alloys of zirconium and iron can
be easily obtained by the reduction of mixtures of the two oxides by
this method. Alloys can be obtained containing up to 35 per cent. of
zirconium; this ferro-zircon, as it is called, has been used to some
extent recently in place of ferro-titanium (_vide infra_) for the
purification of steels.[599] Addition of small quantities of zirconium
to steels, brass, copper, etc., is said to secure sound castings, and to
increase considerably the strength and resistance to acids of the metal.

  [599] _Vide_ Weiss, _E._ 29376, 1910, and Lesmüller, _D. R. P._
  231002, February, 1911.


~The Technical Uses of Zirconia.~--Since the discovery of Baddeleyite,
the natural oxide of zirconium (_vide_ p. 75), which occurs in large
quantities in Brazil, many proposals have been brought forward for the
employment of this compound. Its application to the manufacture of
glasses and enamels will be referred to in the next chapter. Patents
have been taken out protecting its use for the preparation of white
pigments,[600] as a toilet-powder,[601] and as a polishing powder,[602]
for it is extremely stable towards chemical reagents, very voluminous,
and at the same time very hard. It has long been employed for coating
the lime and magnesia pencils used in the Drummond or ‘lime’ light; and
recently it has been employed for the headlights of automobiles, in the
Blériot lamp,[603] in which a rod of zirconia is heated in a blowpipe
flame fed with oil vapour and oxygen.

  [600] _D. R. P._ 235495.

  [601] _Ibid._ 237624.

  [602] _Ibid._ 230757.

  [603] _Ibid._ 174313, September, 1906.

By far the most important property of the oxide, from the technical
point of view, is the ease with which it resists high temperatures. The
natural oxide can be freed, to a very large extent, from the iron oxide
which it encloses, by the prolonged action of hydrochloric acid;
experiments were carried out on the material so obtained by
Simonis,[604] who showed that by prolonged heating at a high
temperature, the remaining impurities, chiefly ferric oxide and silica,
could be volatilised, leaving the zirconia unchanged. Riecke[605] showed
that whilst the oxide is very suitable for the manufacture of highly
resistant crucibles, its use is restricted by the fact that it is easily
reduced by carbon at high temperatures, forming the carbide.

  [604] _Sprechsaal_, 1908, ~41~ (1), 210.

  [605] _Ibid._ 214.

Weiss and Lehmann have carried out exhaustive experiments on the
preparation of crucibles of zirconia.[606] They worked first with
mixtures of zirconia and magnesia, with phosphoric acid as a binding
material; the best results were obtained with a mixture of 90 per cent.
zirconia and 10 per cent. magnesia, which gave extraordinarily resistant
crucibles. Prolonged heating at temperatures over 1900°C. eliminated
all the phosphoric acid by volatilisation; the crucibles could then be
heated in the blowpipe flame and plunged immediately into cold water
without cracking or breaking, and were not affected by fused sodium
hydroxide or potassium hydrogen sulphate. Crucibles were also made with
the addition of potassium and sodium salts, and were found to answer
very well; platinum could be melted in them to a mobile liquid. Similar
crucibles are already on the market.

  [606] _Zeitsch. anorg. Chem._ 1910, ~65~, 218.

As early as 1904 the use of zirconia was suggested for coating
muffles, retorts, and tubes which are required to withstand high
temperatures.[607] In 1906 it was proposed[608] for the manufacture of
crucibles in which rock-crystal (quartz) is fused for the preparation of
quartz-glass, since zirconia is not attacked by molten silica. It
promises to be of the greatest use in all cases where a very refractory
material, stable towards the ordinary chemical reagents, is required.

  [607] Pufahl, _D. R. P._ 156756.

  [608] Heræus Co., _D. R. P._ 179570.


CHAPTER XXII

THE INDUSTRIAL APPLICATIONS OF TITANIUM AND ITS COMPOUNDS

Though probably at least as plentiful in nature as most of the common
metals, titanium has always, until quite recently, been regarded as one
of the rare elements. Of its chemistry, very little indeed was known,
and it is improbable, even now, that the pure element has been isolated.
It had no technical value; indeed, its commonest ore, ilmenite or
titaniferous iron ore, was sedulously avoided by manufacturers, who
considered that even very small percentages of the element rendered an
iron ore valueless because unsuitable for working in blast furnaces.
Towards the end of the last century, one or two metallurgists had
demonstrated that ilmenite, under the proper working conditions, would
yield a pig iron of very good quality when smelted in the blast furnace,
but it was left for the long and arduous researches of Kossi to show
that the element is possessed of properties which render it very
valuable for metallurgical purposes. Since the successful culmination of
his work in the first few years of the present century, titanium has
attained considerable importance in the treatment of special steels for
rails, car wheels, crushing machinery, etc. At present, titaniferous
iron ores are being worked on a large scale, and many titanium compounds
are coming into use for technical purposes.

The titanium minerals of commercial importance are rutile and ilmenite
(_vide_ Part I. pp. 57 and 77). The former, the pure titanium dioxide,
is of fairly wide distribution, but ilmenite occurs in far greater
quantities, forming deposits of enormous dimensions, especially in
America, as, _e.g._ in New York Co. and Quebec. Owing to its high
melting-point and relatively low specific gravity, metallic titanium
can only be incorporated with molten steels with the greatest
difficulty, and for this reason alloys of titanium and iron, known
technically as ferro-titanium, are usually employed for the treatment of
steels. For the preparation of ferro-titanium, ilmenite of good quality
is as suitable as rutile, and, of course, far cheaper; hence the latter
is only employed for the preparation of titanium salts for use in
colouring and mordanting, and for titanium compounds for arc-lamp
electrodes, etc.

Various processes are employed for the manufacture of ferro-titanium
from ilmenite. In cases in which a considerable percentage of carbon is
not undesirable, for instance, where the alloy is required for the
treatment of cast iron or of high-carbon steel, the mineral is reduced
directly with carbon in an electric furnace; the ferro-titanium so
obtained usually contains from six to eight per cent. of carbon. For
pure iron-titanium alloys, the process worked out by Rossi[609] is used
in America almost entirely. Ilmenite is charged into a bath of molten
aluminium, heated electrically; the mineral is at once attacked, with
formation of iron, in which the titanium dissolves as reduction
proceeds. This process may also be used for reduction of rutile, if
scrap iron is added to the aluminium bath, to allow of the formation of
the required alloy. In Germany, the Goldschmidt or ‘thermite’ reaction
is largely employed; powdered ilmenite is intimately mixed with the
calculated quantity of aluminium powder, reduction being started as
usual by means of a fuse of magnesium ribbon imbedded in a small
quantity of barium peroxide.

  [609] _Elect. chem. Ind._ 1903, ~1~, 523.

Quite recently, the question of the separation of titanium compounds
from ilmenite used for the manufacture of pig iron has attracted
considerable attention. It has been already mentioned (_vide supra_)
that titaniferous iron ores have been shown to be perfectly amenable to
blast-furnace treatment, the old and deeply rooted idea that
titanium-bearing slags are stiff and troublesome being entirely contrary
to facts, when suitable conditions are observed;[610] moreover, it is
shown that the pig iron obtained is of unusually good quality. Rossi
has suggested[611] that if sufficient carbon be added to reduce all the
silica and oxides of iron, with enough lime to slag off the titanium
dioxide as calcium titanate, the latter can be used as a source of
titanium compounds or alloys, whilst a ferro-silicon will be obtained as
pig metal; the temperature must be carefully adjusted to ensure
reduction of the silica without loss of titanium dioxide. Another
patent[612] proposes the reduction of the ore in an electric furnace,
and the treatment of the crude ferro-titanium in a converter with a
blast of air or nitrogen; the titanium nitride formed is then driven out
of the metal by a blast of superheated steam--any ammonia or cyanogen
formed being collected--and removed, the iron remaining being
‘Bessemerised’ directly in the same converter; the titanium nitride can
be used as a manure, or for the manufacture of ammonia or nitric acid
(_vide infra_). The removal of iron as the volatile carbonyl has also
been suggested,[613] the titanium being subsequently transformed into
the nitride.

  [610] _Vide_, _e.g._ _Iron Age_, 1909, ~84~, 1149 and 1223.

  [611] _E._ 3582, 1901.

  [612] Sinding-Larsen and Willumsen, _D. R. P._ 220544, April, 1910.

  [613] Sinding-Larsen, _E._ 17632, 1910.


~Employment of the Element in Metallurgy.~--It has been already
mentioned that titanium itself is quite unsuitable for direct
incorporation with steel. Besides the relatively low specific gravity
(5·2), which would render mixing very difficult, the very high
melting-point (given by Weiss and Kayser[614] as 2350°) would prevent
uniform dissemination. The element is therefore generally used in the
form of a ferro-titanium of low titanium content, 10-15 per cent. being
the proportion usually employed. The addition should be made at the end
of the Bessemer process, and after the addition of the required
quantities of manganese and silicon alloys; the calculated quantity of
ferro-titanium is added as the steel runs from the converter into the
ladle. A suitable proportion is said to be one-half per cent. of alloy,
so that the actual proportion of titanium to steel is somewhere about
1·5-1·8 lb. per ton. Six or eight minutes should be allowed after the
addition, for the titaniferous slag to come to the surface.

  [614] _Zeitsch. anorg. Chem._ 1910, ~65~, 345.

Although low percentage ferro-titanium is usually employed, it has been
stated that high-percentage alloys, and even the element itself, are
immediately taken up by steel if aluminium be added at the same time.
Thus Venator[615] states that if titanium and aluminium be added
together to the bath, both elements are immediately taken up, the
reaction being very rapid and complete; the effects produced by the
titanium are in no way influenced by the presence of the aluminium.
Goldschmidt[616] proposes the use of ferro-titanium containing 24-25 per
cent. of the element, with 3 per cent. of aluminium; this dissolves very
readily, is very effective, and moreover, can be very easily prepared by
the alumino-thermic reaction.

  [615] _Stahl Eisen_, 1910, ~30~, 650.

  [616] _D. R. P._ 235461, June, 1911.

In some cases, where it is desired to treat a steel both with silicon
and with titanium, ferro-alloys containing both of these elements may be
employed. By reduction of ilmenite or rutile with carbon in an electric
furnace, in presence of silica, Becket[617] obtains alloys of high
titanium and silicon content, which are said to dissolve very easily in
molten steels and to produce improved effects. The Titanium Alloy
Manufacturing Company have also patented[618] the preparation of
titanium-silicon alloys, with or without addition of iron or copper, by
the reduction of a mixture of rutile and quartz.

  [617] _U. S. P._ 940665 and 941553 of November, 1909.

  [618] _F._ 407858, January, 1910.

Recently the use of ferro-titanium in the manufacture of pig iron has
attracted attention. For this purpose, alloys of very low
titanium-content (0·1-1·0 per cent.) are employed. Addition of very
small amounts of such alloys to the molten metal before casting is said
to have a marked cleansing effect,[619] resulting in much better and
stronger castings.

  [619] _Vide_ Slocum, _Chem. Eng._ 1911, ~13~, 257.

Whilst it is very generally agreed that the addition of titanium results
in the production of much stronger and more durable products, the
question of the precise effect obtained is by no means definitely
settled. The experimental work, whilst pointing on the whole to the
superiority of titanium-treated steel, is by no means conclusive; in
some cases, indeed, it is conflicting. Thus the micro-photographs
obtained by von Maltitz[620] and Venator[621] show that the
titanium-treated steel has a far cleaner fracture and far more
homogeneous structure than steels not so treated; on the other hand, the
micro-photographs of Treuheit[622] show practically no improvement in
structure for the titanium steel. The exhaustive tests of the first two
authors, again, and the experiments of numerous railways in the use of
titanium steel rails,[623] demonstrate clearly that the treatment
results in improvement in strength and durability of the product; but
the work of Otto[624] proves equally clearly that his products did not
differ markedly, whether titanium-treated or not, and he is of opinion
that the rail tests were not sufficiently prolonged or searching to be
considered conclusive. It is nevertheless to be considered certain that
the use of titanium does cause a marked improvement in the quality of
the steels obtained, and especially in the durability of rails. The
negative results obtained by some authors may be explained, firstly, on
the ground that no tests are conclusive unless carried out with steel
from the one bath, one half of which has been treated with titanium, and
the other half not so treated; secondly, that the ferro-titanium must be
incorporated with the metal, and must not be suffered to be taken up by
the slag, and so lost; and thirdly, that the bath must be allowed to
remain for some minutes after treatment, in order that the reaction may
be complete, and the titanium-bearing slag allowed to rise to the
surface. When these conditions are carefully observed, experiment shows
that marked improvement in the quality of the steels produced is
effected.

  [620] _Stahl Eisen_, 1910, ~29~, 1593.

  [621] _Ibid._ 1910, ~30~, 650.

  [622] _Ibid._ 1910, ~30~, 1192.

  [623] _Vide_ Dudley, _J. Ind. Eng. Chem._ 1910, ~2~, 299; also _Cass.
  Mag._ 1911, ~40~, 483.

  [624] _Vide_ abstract in _Stahl Eisen_, 1912, ~32~, 1497.

As to the actual nature of the effect produced, it is generally believed
that titanium acts merely as a cleansing agent, freeing the metal from
occluded or combined gases, and removing blow-holes, so producing a
denser and more homogeneous structure, with consequent improvement in
properties. The added titanium is usually found entirely in the slag,
so that it appears certain that it does not alloy, but merely purifies.
It certainly acts as a powerful and rapid deoxidiser, removing the last
traces of the gas which have escaped the action of the manganese,
silicon, etc., with which steels are now generally treated. Many
authorities, on the ground of analyses, and of the known affinity of
titanium for nitrogen, believe that it very largely reduces the
nitrogen-content,[625] which is so harmful; this, however, is still an
open question.[626] It is stated that if excess of titanium is used, so
that small quantities--0·05-0·20 per cent.--remain in the finished
steel, the toughness and durability are further increased;[627] but as a
rule, manufacturers prefer to work with smaller quantities, so that no
free titanium remains in the product.

  [625] _Vide_ von Maltitz, _loc. cit._

  [626] _Vide_ Venator, _loc. cit._

  [627] _Vide_ _Bull. Imp. Inst._ 1911, ~9~, 134.

       *       *       *       *       *

The preparation of alloys of titanium with almost all the commoner
metals is protected by patent, but few of these are of technical
importance. Small quantities of titanium are said to improve very
considerably the properties of copper and its alloys, the brasses,
bronzes, etc., especially in castings. The addition is usually made in
the form of an appropriate titanium alloy, prepared by reduction of the
mixed oxides with carbon in an electric furnace, or treatment of the
mixed oxides, together with the alloying metal, with aluminium under
similar conditions.[628] The titanium-silver alloys obtained in this
way[629] are said to improve greatly the structure of silver, by
preventing the familiar ‘spitting’ as the fused metal cools.

  [628] _Vide_ Rossi, _U. S. P._ 986505, March, 1911; 935863, October,
  1909, etc.

  [629] Rossi, _U. S. P._ 1024476 and 1025426, August, 1912.

An interesting process, which has been patented by Rossi,[630] recalls
the method of formation of cementation steels. He has found that if a
metal be loosely covered with its alloy with titanium, in a finely
powdered condition, and the whole heated, the titanium diffuses into the
metal, to a depth and concentration which vary with the temperature and
the time of heating. He suggests that in this way a metallic body may be
toughened and strengthened at any desired point, _e.g._ steel for
armour-plate at the surface. Whether the process will be of any
technical value or not can only be shown by experiment.

  [630] _U. S. P._ 986504, March, 1911.


~Application to Arc-lamp Electrodes.~--During the last fifteen years,
innumerable efforts have been made to adapt titanium and its compounds
to the manufacture of arc-lamp electrodes, or pencils.[631] The
spark-spectrum of titanium is very rich in lines, and in respect of
light efficiency, the element is very suitable for the purpose; the
experimental difficulties, however, have been very great, and though
electrodes containing titanium compounds have been on the market for
some years, the problem cannot be said to have been satisfactorily
solved. The best pencils contain titanium carbide, but successful
attempts have been made to use the oxide. As early as 1904, Weedon[632]
proposed an electrode prepared by heating 7 parts (1 mol.) of the
dioxide with 1 part of carbon to 1500°-2000°C.; the ‘sub-oxide’ produced
was powdered, worked up into a paste with a suitable binding material,
and forced through a nozzle. The sticks so obtained, after drying and
baking in the usual manner, were said to give satisfactory results, but
consumption is very rapid, and troublesome deposits of the dioxide are
formed at the end of the electrode. The dioxide, which alone is a very
bad conductor, enters directly into the composition of the so-called
‘magnetite’ pencils, which are best made[633] by fusing together
magnetite, rutile, and chromite, in suitable proportions, with a little
potassium fluoride, powdering the brittle mass, and using this to form a
paste from which the pencils may be obtained as usual. These electrodes
are said to give a very efficient and fairly steady arc. They have the
disadvantage that tiny glowing particles are thrown off, which soon
render the globes opaque; the addition of sulphur[634] to the powder
during manufacture is said greatly to diminish this inconvenience.
Pencils made in a similar manner from powdered ferro-titanium[635] do
not appear to have come into use.

  [631] _Vide_, _e.g._ Ladoff, _J. Ind. Eng. Chem._ 1909, ~1~, 711.

  [632] _E._ 26921, 1904.

  [633] _E._ 2027, 1909.

  [634] _E._ 18965, 1909.

  [635] _U. S. P._ 840634, January, 1907.

The carbide alone is a good conductor, and gives a very satisfactory
light,[636] but electrodes made from this compound without additions
have several disadvantages. The life is short, and the arc soon becomes
flickering and unsteady. A deposit of the badly conducting dioxide
gradually accumulates on the anode, and once the current has been
interrupted, this deposit renders it very difficult to strike the arc
again. These disadvantages are largely overcome by a series of
improvements recently patented in Germany by the Allgemeine
Elektrizitäts Gesellschaft of Berlin. Addition of small quantities--4·5
per cent.--of chromium carbide increases the length of life;[637] the
unsteadiness and flickering are greatly diminished by incorporation of
powdered coke, cryolite and fluorspar,[638] or better, of the
titanofluoride of calcium or cerium,[639] whilst the addition of finely
divided sulphur (or selenium or tellurium)[640] greatly reduces the
disadvantage due to the throwing off of incandescent particles. The
British Thomson-Houston Company patents a similar electrode,[641] in
which a carbon-mixture is used instead of coke, and the electrode is
manufactured with a carbon shell. For this purpose, the paste prepared
from the powdered mixture may be filled into a hollow carbon rod, or the
lightly baked pencil may be coated with pitch and heated to a high
temperature. The use of a mixture of cerium fluoride and tungstate, with
carbon and cryolite, is also said to prevent flickering.[642]

  [636] Weedon, _Trans. Amer. El. chem. Soc._ 1911, ~16~, 217.

  [637] _D. R. P._ 231231, February, 1911.

  [638] _Ibid._ 233125, March, 1911.

  [639] _Ibid._ 251837, October, 1912.

  [640] _Ibid._ 234466, May, 1911.

  [641] _E._ 6500, 1912.

  [642] Guay, _U. S. P._ 1039522, September, 1912.

In arc lamps in which pencils containing titanium compounds are used,
the anode is generally made of copper, and is placed below the cathode,
the reverse being the case where carbon electrodes are employed. The
copper is inactive, and contributes nothing to the light; if the anode
be of suitable dimensions, it wears away very slowly, whereas the
cathode, containing the titanium compound, is rapidly consumed. In lamps
in which carbon electrodes are used, the light is emitted chiefly from
the extremities of the electrodes, the path of the arc being
comparatively non-luminous; the light has the familiar reddish-yellow
colour characteristic of the earlier forms of arc lamps. Where titanium
pencils are employed, however, the light is emitted almost entirely from
the arc itself, the electrodes contributing very little, and is of a
pure white colour, very different from that of the carbon lamp.

Attempts have been made to employ titanium in the manufacture of metal
filaments for glow lamps. The metal would be very suitable for this
purpose, by reason of its high melting-point and low conductivity, but
the difficulty of obtaining it in the pure state, and the remarkable
susceptibility of the filament to traces of impurity, have so far proved
insuperable. For the sake of illustration, a proposal put forward in
1908 may be briefly referred to.[643] Pure titanium dioxide is heated in
a stream of ammonia; the nitride obtained is decomposed at 1200° _in
vacuo_, and after cooling, the metal is powdered and made into a paste
with a solution of albumen in ammonia. The threads obtained from this in
the usual manner are heated to 1200° in an electric furnace; the carbon
deposited from the albumen forms the cyanide by reaction with the trace
of nitride which has escaped decomposition, or which has been formed by
further action of ammonia. The cyanide is volatile, and can be removed
at high temperatures _in vacuo_, leaving a sintered filament of the
metal. So susceptible is the filament to impurity, that the trace of
carbon deposited from the vapour of the oil of the pump which diffuses
into the vacuum is sufficient to render it so fragile as to be
useless.[644]

  [643] Trenzen and Pope, _E._ 14852, 1908.

  [644] _Vide_ _Bull. Imp. Inst._ 1911, ~9~, 134.


~Titanium Compounds in Dyeing and Colouring.~--The use of titanium
compounds as mordants in the dyeing of leather and textile goods has
been known for a considerable time.[645] As early as 1896, a patent was
taken out by Barnes[646] for the treatment of prepared animal skins by
immersion in a bath of a titanium salt. Subsequent boiling or steaming
causes hydrolysis, with precipitation in the skin of hydrated titanium
dioxide, which forms lasting dye-lakes when the fabric is immersed in
the dye-bath. Whilst this treatment has been found satisfactory with
some classes of leather goods,[647] more delicate kinds are liable to be
injured by the mineral acid set free, and numerous patents protecting
the preparation and employment of organic salts of the element have been
taken out by Dreher.[648] The same investigator[649] has discovered that
excellent results can be obtained in the cold by the addition of various
‘Hülfsalze,’ which are chiefly acetates or formates of the alkaline
earth metals, chromium, or aluminium, or basic salts of the last two.
Double decomposition of these with the titanium salt forms basic or
highly hydrolysed salts of the latter, so that the hydrated oxide or a
basic compound is formed on the fabric.

  [645] A good account of some of the earlier work in this connection is
  given by Erban, _Chem. Zeitg._ 1906, ~30~, 145.

  [646] _E._ 5712, 1896.

  [647] _Vide_ Dreher, _D. R. P._ 142464, June, 1903.

  [648] _Vide_ _E._ 22629 and 23188 of 1901, 14921 and 27597 of 1902,
  and 5211 of 1903.

  [649] _Vide_ _D. R. P._ 139059 and 139060 of February, 1903, and
  139838 of March, 1903.

The titanium salts specified in these patents are salts of the element
in the tetravalent condition, prepared from rutile by the action of
strong mineral acids. As early as 1902, the technical preparation of
salts of trivalent titanium for reducing purposes was patented by Spence
and Spence, of Manchester.[650] The process is an electrolytic one, and
is effected in a cell divided into two compartments by a porous
partition, one electrode being introduced into each compartment; an
electromotive force of 3-4 volts is required. A 20-25 per cent. titanium
tetrachloride solution is introduced into the cathode compartment, and
dilute hydrochloric acid into the anode compartment; on electrolysing,
chlorine is evolved at the anode, and may be utilised as usual in the
preparation of bleaching powder, etc., whilst the tetrachloride in the
cathode compartment is reduced to trichloride. The solution is then
concentrated at 65°-70°C. under reduced pressure, and the crystalline
trichloride separated. In the preparation of the corresponding sulphate,
sodium sulphate must be present in the cathode compartment, and a double
salt is obtained; the process is carried out in lead-lined cells, in
presence of excess of sulphuric acid. The preparation of the
sesquioxide, Ti₂O₃, free from compounds of aluminium and iron, was also
suggested by Dreher[651] by reduction of the acid solution of the impure
or mixed salts with zinc or sodium amalgam, and approximate
neutralisation; the sesquioxide differs from the dioxide in that it
separates while the solution is still somewhat acid, which the hydrated
oxides of iron and aluminium will not do. Dreher suggested that the
strong reducing properties of the sesquioxide and its salts should make
these valuable for bleaching, colour-printing, and similar purposes.

  [650] _E._ 16238 and 18108 of 1902.

  [651] _E._ 1835, 1903.

More recently[652] the reduction of titanium salts by means of aluminium
powder has been suggested; in the case of the sulphate, the aluminium
salt formed may be partly eliminated as alum, in the ordinary way, if
desired, but it is claimed that its effect is beneficial rather than
harmful. The preparation of organic double basic salts of trivalent
titanium,[653] which hydrolyse very readily, suggested the use of such
compounds as mordants and for reducing purposes. These salts may be
prepared fairly easily[654] by adding concentrated solutions of the
appropriate potassium, sodium, or ammonium salts in excess to
concentrated solutions of the trichloride, in absence of air. The double
salts separate, and are washed and dried; in this condition they are
fairly stable, but the solutions hydrolyse at once on merely warming,
with separation of the hydrated sesquioxide. On this account, and also
because of the strong reducing action, these compounds are likely to
prove valuable as mordants, and for other purposes.

  [652] Spence, Craig, and Spence, _E._ 13260, 1911.

  [653] Stähler and Bachran, _Ber._ 1911, ~44~, 2912.

  [654] Kunheim and Co. and Stähler, _D. R. P._ 284251, June, 1912.

Titanium compounds have frequently been suggested for the preparation of
colouring-matters; the ferrocyanide has a fine green colour, and is used
to some extent in place of arsenical pigments for the preparation of
coloured wall-papers, whilst the dioxide is of some value for tinting
artificial teeth, porcelain tiles, etc. Yellow and reddish-yellow
pigments are produced from rutile and ilmenite by various methods. A
fine covering paint is said to be obtained by a process[655] in which
ilmenite is powdered and roasted to 500°C.; the cooled product is
crushed with water, and after one or two washings to remove soluble
compounds, yields a very finely divided orange-yellow suspension, the
precise shade of which varies with the duration and temperature of the
roasting. The product is at once thrown down from the suspension, by
addition of a small quantity of a salt solution, and so can easily be
obtained in the solid state. In another process,[656] the pulverised
ilmenite is warmed with concentrated sulphuric acid, in which it
dissolves with great development of heat; the excess of acid is removed
by evaporation and the mass calcined to decompose the sulphates. It is
stated that different shades may be obtained by carrying out the last
operation in an atmosphere of sulphur dioxide or other gas.

  [655] Farup, _E._ 3649, 1910; _F._ 412563, May, 1910.

  [656] _E._ 10368, 1911.

In connection with the colouring properties of the oxides of titanium,
it is interesting to note that the blue colour of sapphires is probably
due to the presence of compounds of trivalent titanium; Verneuil[657]
has succeeded in preparing artificial sapphires in all respects
identical with the natural stones by fusing alumina with small
quantities of titanium dioxide and ferric oxide in the flame of the
oxyhydrogen blowpipe, which effects the reduction.

  [657] _Compt. rend._ 1910, ~150~, 185.


~Other Uses of Titanium Compounds.~--Owing to the high price of the tin
dioxide which is largely employed for the preparation of enamels and
opaque glasses, innumerable suggestions have been made for the
employment of the oxides of titanium and zirconium in this
direction.[658] A critical examination of the question has been made by
Grünwald;[659] he finds that the opacity consequent on addition of these
compounds increases with the amount of clay used, within limits, and
concludes that the effect is due to displacement of alumina by the
oxides, with formation of silicates of titanium and zirconium, which
dissolve in the melt. He states that the results obtained from the use
of these oxides are not comparable with those given when stannic oxide
is employed, and that therefore the former oxides are of little use for
this purpose.

  [658] _Vide_, _e.g._ _D. R. P._ 189364, 218316, 115016, 207001; _F._
  438908, etc.

  [659] _Sprechsaal_, 1911, ~44~, 72.

These two oxides find employment to a small extent in the manufacture of
‘Siloxide’ quartz glass.[660] Quantities up to 1·5 per cent., added to
the molten silica, reduce the difficulty of working the material.
Exhaustive tests carried out by Thomas[661] indicate that the vessels
made from this material are, on the whole, to be preferred to ordinary
quartz glass, resisting high temperature better, and showing less
tendency to become crystalline and therefore brittle when maintained for
considerable times at high temperature.

  [660] Wolf-Burckhardt and Borchers, _F._ 432786, October, 1911.

  [661] _Chem. Zeitg._ 1912, ~86~, 25.

       *       *       *       *       *

Much work has been carried out during the last few years with the object
of utilising titanium compounds for the ‘fixation’ of nitrogen.

The metal combines very vigorously with the gas at about 800°C. (_vide_
p. 224), forming the nitride. If the gas, or air, be passed over a
heated mixture of the dioxide with powdered coke, formation of the
cyanonitride occurs at comparatively low temperatures (1100°-1300°C.) if
a small quantity of an alkali salt be present,[662] the action being
apparently catalytic; if excess of carbon is used, considerable
quantities of the cyanide may be formed. Numerous experiments carried
out by the chemists of the Badische Anilin- und Soda-Fabrik have shown
that at high temperatures, the action of water and a suitable oxidising
agent, or in the presence of metallic compounds, the action of steam
alone, will liberate considerable quantities of ammonia from both these
derivatives,[663] whilst in the presence of platinum compounds, if air
be pumped in, the higher oxides of nitrogen are formed. One or two
examples may be given:

  (1) Ti₂N₂ + 4NaOH + H₂O + 2CuO = 2NH₃ + Cu₂O + 2Na₂TiO₃--autoclave at
  180°C.

  (2) 2Ti₂N₂ + 2H₂SO₄ + 6H₂O + O₂ = 4TiO₂ + 2(NH₄)₂SO₄--autoclave at
  120°-140°C.

  (3) Ti₂N₂ + 3H₂O = Ti₂O₃ + 2NH₃--steam at 500°-600°C.

  [662] _Vide_ Bosch, _U. S. P._ 957842, May, 1910.

  [663] _Vide_, _e.g._ _D. R. P._ 202563 and 203748 of March, 1907;
  204204 and 204475 of November, 1908; _E._ 2414, 1908; _F._ 387002 of
  June, 1908; _U. S. P._ 957843 of May, 1910, gives a résumé of all the
  processes.

In the second case, the oxygen is derived from air pumped into the
apparatus, and ferrous sulphate is used as a catalyst. In the third
case, a metallic salt, oxide, or hydroxide is required as a catalyst.

In view of the success of the cyanamide method for the fixation of
atmospheric nitrogen, these processes, though of considerable
theoretical interest, do not seem likely to become of practical
importance.

       *       *       *       *       *

One or two minor uses have been suggested for titanium dioxide. Small
quantities are fused with bauxite, silica, and ferric oxide in the
preparation of abrasives,[664] whilst a mixture with carbon is suggested
as a refractory body for linings, crucibles, etc., surface heating of
this forming a layer of highly resistant carbide.[665] An interesting
American patent protects the use of the dioxide for the preparation of
phosphorus pentoxide from bone-ash or natural calcium phosphate.[666]
The pulverised mixture of the phosphate and oxide is introduced at the
upper end of an inclined rotating furnace, by means of a hopper and
screw feed; fuel is fed in at the lower end, and an outlet is provided
for the periodic removal of the calcium titanate, etc., formed. The
silica and alumina of the impure phosphate, together with the titanium
dioxide introduced, displace the phosphorus pentoxide, which, being
volatile, escapes continuously through a special pipe; there is left a
mixture of silicate, aluminate and titanate of calcium, which may be
used as a source of titanium compounds.

  [664] Saunders, _U. S. P._ 954766, 954777, and 954778.

  [665] Becket, _U. S. P._ 1038827, September, 1912.

  [666] Peacock, _U. S. P._ 995897, June, 1911.


~Estimation of the Element.~--Owing to the difficulties of the
separation from the acidic oxides, silica, zirconia, and the pentoxides
of columbium and tantalum, and from the basic oxides, alumina and the
oxides of iron and tin, the estimation of titanium in a mineral or a
steel is usually a difficult and tedious process. Gravimetric as well as
volumetric methods are employed. In the former, the element is isolated
and weighed in the form of the dioxide; in the latter, standard
solutions of suitable oxidising agents are employed, advantage being
taken of the ease with which the element can be transformed from the
trivalent to the tetravalent condition.

The mineral or steel in which the element is to be estimated is usually
fused with sodium hydrogen sulphate, which forms the sulphate. If
thorium, uranium or rare earths are present, treatment in the cold with
hydrofluoric acid is often more suitable; the acidic oxides are taken
into solution, leaving the more positive elements in the form of the
insoluble fluorides. Trautmann finds that steels or ferro-titaniums of
high silicon content are attacked only very slightly by fused sodium
bisulphate; he recommends[667] ignition to the oxides, evaporation with
hydrofluoric acid to remove silicon as the volatile tetrafluoride, and
fusion of the residue with bisulphate.

  [667] _Zeitsch. angew. Chem._ 1911, ~24~, 877.

The bisulphate melt, after cooling, is leached with water, and the whole
boiled under a reflux condenser for several hours; this treatment should
throw down the oxides of titanium, columbium and tantalum, leaving
zirconium and aluminium in the form of the sulphates in the acid
solution; the addition of ammonia may be necessary to effect complete
hydrolysis. The acidic oxides may also be precipitated if the solution
be diluted and treated with excess of acetic acid before boiling. In
both cases, a considerable quantity of iron is thrown down. The
precipitated oxides are dissolved in the cold by dilute sulphuric acid
to which hydrogen peroxide has been added.

For volumetric estimation, separation from iron is not generally
necessary. If gravimetric methods are to be employed, separation may be
effected in several ways. Titanium dioxide may be precipitated in a
fairly pure condition by reducing the solution with sulphur dioxide, and
boiling until the titanium sulphate has been completely hydrolysed.
According to Barneby and Isham,[668] this method gives low results;
these authors prefer to remove iron completely from the solution, and
then effect complete hydrolysis by addition of ammonium acetate and
acetic acid to the boiling solution. For this purpose, they dissolve
the mixed oxides in hydrochloric acid, and remove ferric chloride by
ether extraction. Bornemann and Schirmeister[669] precipitate titanium
dioxide completely by means of ammonia, holding iron in solution as
ferrocyanide; for this purpose, iron is completely reduced to the
ferrous state by means of sodium hydrogen sulphite, and solutions of
potassium cyanide and ammonia are added together to the warm liquid,
which is afterwards heated nearly to the boiling-point to effect the
precipitation.

  [668] _J. Amer. Chem. Soc._ 1910, ~32~, 957.

  [669] _Metallurgie_, 1910, ~7~, 723.

Iron may also be removed by the ordinary methods, if some reagent be
previously added to hold titanium in solution. For this purpose,
tartaric acid and its salts are commonly used; none of the ordinary
precipitants will throw down the element if this reagent be present.
After addition of ammonium tartrate, iron is removed by means of
ammonium sulphide. After filtering, tartaric acid may be removed by
means of potassium permanganate, the manganese dioxide formed being
reduced with sulphur dioxide. According to Thornton,[670] evaporation
with a mixture of sulphuric and nitric acids is a more convenient method
of destroying the organic acid; titanium dioxide is then thrown down by
diluting and boiling in the usual way.

  [670] _Amer. J. Sci._ [iv.], 1912, ~34~, 214.

Bourion[671] describes a method of separating the oxides by the action
of a mixture of hydrogen chloride and sulphur monochloride at a suitable
temperature. The ferric chloride which is formed sublimes, leaving
titanium dioxide unattacked.

  [671] _Compt. rend._ 1912, ~154~, 1229.

For volumetric estimation of small quantities of titanium in solution,
colorimetric methods are generally employed. Addition of hydrogen
peroxide to such a solution gives an intense reddish-yellow colouration,
which is compared with the colourations obtained with solutions
containing known quantities of the element. Wells[672] finds that under
suitable conditions, an accuracy of about 2 per cent. is to be expected
with this method. Lehner and Crawford[673] find that in concentrated
sulphuric acid solution, thymol gives a red colouration which is at
least twenty-five times as intense as the colour given by hydrogen
peroxide, and they accordingly propose thymol as a suitable reagent for
the colorimetric estimation. Fenton[674] has shown that a very intense
colouration is obtained when a solution of a titanium salt is treated
with dihydroxymaleic acid; this reaction has been shown by Mellor[675]
to be well adapted for the colorimetric estimation and for the
estimation of titanium and vanadium together in a solution.

  [672] _Zeitsch. anorg. Chem._ 1911, ~70~, 395.

  [673] _J. Soc. Chem. Ind._ 1912, ~31~, 956.

  [674] _Trans. Chem. Soc._ 1908, ~93~, 1064.

  [675] _Abstr. Chem. Soc._ 1913, ~104~, ii. 627.

The volumetric methods for the estimation of larger quantities require
complete reduction to the trivalent condition. This is best effected by
means of zinc and hydrochloric acid, or, where potassium permanganate is
to be used, by zinc and sulphuric acid. Precautions must be taken to
ensure that reduction is complete; an apparatus suitable for rapid
estimations has recently been described by Shimer and Shimer.[676] Where
potassium permanganate is employed (Pisani’s method), the iron must be
estimated separately by means of a standard solution of titanium
trichloride. Knecht and Hibbert[677] titrate directly, after reduction,
with a standard solution of a ferric salt, using potassium thiocyanate
as indicator; here no correction has to be applied for iron originally
present in the solution. The same advantage attaches also to the method
of titration by means of methylene blue,[678] a dye reduced to the
colourless leuco-base by salts of trivalent titanium, but not affected
by ferrous salts.

  [676] _J. Soc. Chem. Ind._ 1912, ~31~, 955.

  [677] _Ber._ 1903, ~36~, 1549.

  [678] See Hibbert, _J. Soc. Chem. Ind._ 1909, ~28~, 190.




INDEX


  Absorption Spectra, ~148~

  Acetate process, ~304~

  Acetylacetone derivatives, ~135~

  Actinium, 100

  Aenigmatite, 8, 55

  Aeschynite, 8, ~65~

  Aldebaranium, 205

  Allanite, 8, 36, ~39~, 91

  Alshedite, 54

  Alvite, 8, 59

  Anatase, 8, ~78~

  Ancylite, 8, 81

  Anderbergite, 8, 49

  Annerödite, 9, 61

  Arc spectra, ~151~

  Arfvedsonite, 9, 51

  Arizonite, 9, 59

  Arrhenite, 9, 70

  Astrophyllite, 9, 55

  Auer mantles, history of, ~270~

  Auerbachite, 9, 31

  Auerlite, 9, 51


  Baddeleyite, 10, ~75~

  Bagrationite, 10, 45

  Bastnäsite, 10, 81

  Beckelite, 10, 51

  Benitoite, 10, 55

  Beryl, 102

  Blomstrandine, 10, ~68~

  Blomstrandite, 10, 71

  Bodenite, 11, 42, 45

  Bragite, 63

  Brasilite, 76

  Britholite, 11, 51

  Bröggerite, 11, 73

  Brookite, 11, ~79~

  Bucklandite, 42


  Calciothorite, 11, 49

  Calcite, 2, 38

  Cappelenite, 11, 51

  Carbides of rare earth group, ~120~

  Carbonates of rare earth group, ~130~

  Caryocerite, 12, 51

  Cassiopeium, 205

  Cassiterite, 3, 45, 46, 77

  Castelnaudite, 12, 88

  Cataplejite, 12, 51

  Cathode luminescence, ~151~

  Celtium, ~207~

  Ceria, 111, 117, 118, ~161~

  Ceric compounds, ~160~

  Cerite, 1, ~30~

  Cerium, atomic weight of, ~164~
    compounds, applications of, ~317~
    detection of, ~165~
    estimation of, ~166~
    group, history of, ~168~
      separation of, ~169~
    intermediate oxide of, 162
    metallic, ~115~
    nitrate, extraction from monazite of, ~284~
    separation of, ~156~

  Cerous compounds, ~158~

  Chalcolamprite, 12, 70

  Chardonnet process, ~302~

  Chlorides of rare earth group, ~121~

  Chromates of rare earth group, ~129~

  Churchite, 12, 80

  Clamond mantles, ~268~

  Cleveite, 13, 73

  Cordylite, 13, ~80~

  Cossyrite, 13

  Cryptolite, ~84~

  Cuprammonium process, ~303~

  Crytolite, 13, 49


  Davidite, 13, 59

  Delorenzite, 13, ~56~

  Derbylite, 13, ~59~

  Drummond light, ~267~

  Dysanalyte, 14, 71

  Dysprosium, ~199~
    history of, 195
    separation of, 196


  Edwardsite, 84

  Elpidite, 14, 45

  Endeiolite, 14, 70

  Equivalent weight determination, ~153~

  Erbium, atomic weight of, ~202~
    detection of, ~203~
    group, 199
    history of, 194, ~201~
    salts of, ~202~
    separation of, 196

  Erdmannite, 14, 45

  Eremite, 84

  Erikite, 14, 51

  Ethylsulphates of rare earth group, ~127~

  Eucolyte, 14, ~50~

  Eucolyte-Titanite, 54

  Eucrasite, 15, 49

  Eudialite, 15, ~50~

  Europium, atomic weight of, 188
    compounds of, ~188~
    history of, 185

  Euxenite, 15, 66, ~68~

  Eytlandite, 60


  Fahnehjelm mantles, ~269~

  Fergusonite, 15, 38, ~63~, 90

  Ferrocyanides of rare earth group, ~123~

  Ferro-titanium, ~326~

  Florencite, 15, 51

  Fluocerite, 15, 89

  Fluorides of rare earth group, ~120~

  Fluorspar, 2, 89, 102

  Formates of rare earth group, ~133~

  Freyalite, 16, 49


  Gadolinite, 1, 16, ~33~, 91

  Gadolinium, atomic weight of, 190
    compounds of, ~190~
    detection of, 191
    history of, 184, ~189~

  Geikielite, 16, 59

  Gorceixite, 16, 88

  Greenovite, 54

  Gröthite, 26, 54

  Guarinite, 16, 51

  Gummite, 73


  Hainite, 16, 70

  Halogen oxy-salts of rare earth group, ~123~

  Harmatite, 10, 81

  Helium ratio, 104, ~106~

  Hellandite, 16, ~42~

  Hiortdahlite, 17, ~51~

  Hjelmite, 17, 64

  Holmium, compounds of, ~201~
    history of, ~195~
    separation of, 196

  Homilite, 17, 51

  Hussakite, 17, 87

  Hydrides of rare earth groups, ~116~

  Hydrotitanite, 17, 59

  Hydroxides of rare earth groups, ~116~


  Illuminating power of gas, 266
    of mantles, ~294~

  Ilmenite, 17, ~57~, 90

  Ilmenorutile, 17, 71


  Johnstrupite, 17, 55


  Kainosite, 18, 45

  Karyocerite, 12

  Kataplejite, 12, 51

  Keilhauite, 18, ~52~

  Kischtimite, 18, ~81~

  Knopite, 18, 59

  Kochelite, 18, 64

  Koppite, 18, 64


  Lanthanite, 18, ~79~

  Lanthanum, atomic weight of, ~173~
    compounds of, ~172~
    detection of, 173
    metallic, ~115~, 171
    separation of, ~170~

  Lavenite, 19, 51

  Lead, 105, ~107~

  Lederite, 54

  Leucosphenite, 19, 55

  Leucoxene, 55

  Lewisite, 19, 59

  Lighting devices, ~315~

  Ligurite, 54

  Loranskite, 19, 64

  Lorenzenite, 19, 55

  Lutecium, ~205~


  Mackintoshite, 19, 79

  Magnetic susceptibility, ~152~

  Malacone, 19, 49

  Mauzeliite, 20, 59

  Melanocerite, 20, 51

  Menaccannite, ~57~

  Mengite, 84

  Mesothorium, ~252~, 276

  Metals of rare earth group, ~114~

  Michaelsonite, ~14~

  Microlite, 20, 64

  Misch metal, ~115~, ~315~

  Molengraafite, 20, 55

  Monazite, 4, 20, ~82~
    sands, 83, ~90~
    technical treatment of, ~276~

  Mosandrite, 20, 55

  Muromontite, 20, 42, 45


  Naegite, 31, 45, ~49~

  Narsarsukite, 21, 55

  Neodymium, atomic weight of, ~179~
    detection of, ~180~
    metallic, 115, 177
    oxides, ~177~
    salts, ~178~

  Neoytterbium, 206

  Neptunite, 21, 55

  Nernst lamp, ~320~

  Nitrates of rare earth group, ~128~

  Nitrides of rare earth group, ~116~

  Nivenite, 21, 73

  Nohlite, 21, 64


  Octahedrite, 8, ~78~

  Oerstedite, 21, ~49~

  Oisanite, 78

  Orangite, 21, ~45~

  Organic salts of rare earth group, ~133~

  Orthite, 8, ~39~

  Oxalates of rare earth group, ~131~

  Oxides of rare earth group, 115, ~117~


  Parisite, 21, ~80~

  Pauly process, ~303~

  Perovskite, 14, 22, 59

  Peroxides of rare earth group, ~117~

  Pertitanates, ~235~

  Phosphates of rare earth group, ~129~

  Phthalates of rare earth group, ~134~

  Picroilmenite, 16, ~59~

  Pictite, 54

  Pilbarite, 22, ~49~

  Pitchblende, 22, ~72~

  Platinocyanides of rare earth group,  ~123~

  Platinum mantles, ~268~

  Plumboniobite, 22, ~62~

  Polonium, 99

  Polycrase, 22, ~66~

  Praseodymium--
    atomic weight of, ~175~
    compounds of, ~174~
    detection of, 176
    history of, 168
    metallic, ~115~, 174
    separation of, ~170~

  Priorite, 22, ~66~

  Pseudobrookite, 22, 59

  Pyrochlore, 23, 71

  Pyromorphite, 101

  Pyrophanite, 23, 59

  Pyrophoric alloys, ~314~


  Radioactivity, ~99~

  Radiothorium, 74, 99, ~253~

  Ramie, mantles of, ~291~

  Rare earth mixtures, examination of, ~147~

  Rare earths--
    extraction of, from minerals, ~147~
    and periodic classification, ~135~

  Retzian, 23, 88

  Rhabdophane, 23, 88

  Rhönite, 23, 55

  Rinkite, 23, 55

  Risörite, 23, 38, ~69~, 102

  Rogersite, 24, 64

  Rosenbuschite, 24, 55

  Rowlandite, 24, 55

  Rutile, 24, 45, ~77~, 90


  Samarium, atomic weight of, 182
    detection of, 183
    history of, ~168~
    metallic, ~115~, 181
    salts of, ~182~
    separation of, ~171~

  Samarskite, 24, 38, ~60~, 91

  Scandium, atomic weight of, ~217~
    chemical relations of, ~214~
    compounds of, ~215~
    detection of, ~218~
    history of, 194, ~213~
    occurrence of, ~3~
    separation of, ~186~

  Schorlomite, 24, 55

  Scovillite, 23, 88

  Selenates of rare earth group, ~128~

  Selenites of rare earth group, ~128~

  Semelene, 54

  Senaite, 24, 59

  Silicofluorides of rare earth group, ~121~

  Sipylite, 24, 39, ~63~

  Spark spectra, ~150~

  Sphene, 26, ~52~, 90, 107

  Steenstrupine, 25, 51

  Strüverite, 25, 71

  Sulphates of rare earth group, ~124~

  Sulphides of rare earth group, ~119~

  Sulphites of rare earth group, ~127~

  Synchisite, ~81~


  Tachyaphaltite, 25, 49

  Tautolite, 42

  Tengerite, 25, 81

  Terbium, atomic weight of, ~192~
    detection of, 193
    group, chemical relations of, ~185~
      history of, ~184~
      separation of, ~186~
    history of, ~184~, 191
    salts of, 192

  Thalénite, 25, ~43~, 102

  Thiosulphates of rare earth group, ~127~

  Thorianite, 25, ~73~, 107, 251

  Thorite, 25, ~45~, 108, 251

  Thorium, atomic weight of, ~262~
    chemical relations of, ~251~
    compounds of, ~254~
    detection of, ~263~
    estimation of, ~285~
    extraction of, 251, ~275~, 283
    group relations of, ~220~
    metallic, ~253~
    radiochemistry of, ~252~
    separation of, ~277~
    sulphate purification of, ~279~

  Thorogummite, 26, 49

  Thortveitite, 26, ~44~

  Thulium, history of, ~194~, 203
    individuality of, ~204~
    salts of, ~204~
    separation of, 196

  Titanates, ~234~

  Titaniferous ironstone, ~57~

  Titanite, 26, ~52~, 90

  Titanium, atomic weight of, ~236~
    compounds for fixation of nitrogen, ~337~
    compounds of, in dyeing, ~333~
    compounds of divalent, ~225~
    compounds of trivalent, ~226~
    compounds of tetravalent, ~230~
    cyanonitride, ~224~
    detection of, ~236~
    electrodes, ~331~
    estimation of, ~338~
    group relations of, ~219~
    metallic, ~223~
    occurrence and extraction of, ~222~
    olivine, 26, 55
    steels, ~329~
    uses of, in metallurgy, ~327~, ~330~

  Tritomite, 26, 51

  Tscheffkinite, 26, 55

  Tungsten, 1, 31

  Turnerite, 83

  Tyrite, 63

  Tysonite, 26, 89


  Uhligite, 27, 59

  Uraninite, 29, ~52~

  Uranosphærite, 73

  Urano-tantalite, 60


  Vasite, 42

  Vietenghfiote, 27, 64

  Viscose process, ~304~


  Warwickite, 27, 59

  Weibyite, 27, 81

  Wiikite, 27, ~70~

  Wöhlerite, 28, 70

  Wolframite, 2, ~214~


  Xenotime, 28, 45, ~86~, 90, 207


  Ytterbia, 1, 206

  Ytterbite, 1, 33

  Ytterbium, atomic weight of, 206
    detection of, 207
    history of, ~194~, 205
    salts of, ~206~
    separation of, 196, ~205~

  Yttria, 1, 35, ~111~, 209

  Yttrialite, 28, ~34~, 45

  Yttrium, atomic weight of, ~211~
    detection of, ~212~
    group, history of, ~194~
      separation of, ~195~
    history of, 194, ~208~
    salts of, ~210~
    separation of, ~196~, 205

  Yttrocerite, 28, ~88~

  Ythrocrasite, 28, 56

  Ythrofluirite, 28, ~89~

  Yttrofluorite, 28, ~89~

  Yttrogarnet, 28, 45

  Yttrogummite, 28, 49

  Yttroilmenite, 60

  Yttrotantalite, 29, 62

  Yttrotitanite, 18, ~52~


  Zircon, 29, 38, 45, ~47~, 90, 107

  Zirconia, uses of, ~323~

  Zirconium, atomic weight of, ~249~
    compounds of, ~249~
    detection of, ~242~
    estimation of, ~250~
    extraction of, ~239~
    group, relations of, ~219~, 240
    history of, ~238~
    industrial applications of, ~321~

  Zirkelite, 29, 79


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  Transcriber’s Notes


  Inconsistent and unusual spelling and hyphenation (including those of
  proper and geographical names) have been retained, except as mentioned
  below.

  Depending on the hard- and software used and their settings, not all
  elements may display as intended.

  Lists of elements were printed both with and without separating commas
  (for example, R´´ = Ca, Fe´´, Be and R´ = NH₄,K,Rb,Cs); these have not
  been standardised.

  Pages 34, weighing 200 lb.: later (page 41) referred to as weighing
  300 lb.

  Some tables appear to use nm, others Å; this has not been
  standardised.

  Page 181, table: 5923·35 may be an error; it is out of sequence.

  Page 200, table: 379·5 may be an error; it is out of sequence.

  Page 236, (NH₄)₂O₂,TiO₃,H₂O₂: as printed in the source document; the
  final O₂ is probably an error.


  Changes:

  Footnotes have been moved to under the paragraph in which they are
  referenced; illustrations have been moved out of text paragraphs.

  Some obvious minor typographical and punctuation errors have been
  corrected silently.

  Moh’s scale has been changed to Mohs’ scale, Guèrin and Guérin to
  Guérin. Where there was a space between the number and the percent
  sign, or between the degree sign and the C, this has been deleted for
  the sake of consistency.

  Page vii: Blomstandine changed to Blomstrandine.

  Page 20, Monazite: Yttr = 1 4; changed to Yttr = 1-4;

  Page 26: Osterby changed to Österby.

  Page 46: Struverite changed to Strüverite as elsewhere.

  Page 87: Kraus and Heitinger changed to Kraus and Reitinger.

  Page 155: Footnote anchor [194] was missing in the source document,
  and has been inserted at the end of the paragraph.