INDUSTRIAL MINERALS AND METALS OF ILLINOIS


                              J. E. Lamar


                   _Illinois State Geological Survey
                         Educational Series 8_


STATE of ILLINOIS

    [Illustration: DEPARTMENT of REGISTRATION and EDUCATION]

1965

  ILLINOIS STATE
  GEOLOGICAL SURVEY
  John C. Frye, Chief
  URBANA, ILLINOIS

Printed by Authority of State of Illinois, Ch. 127, IRS, Par. 58.25.

                           (15M-4/65-8976) 10




              _INDUSTRIAL MINERALS AND METALS OF ILLINOIS_


                                                             J. E. Lamar


The mineral resources of Illinois include many rocks and minerals of
varied character and uses. From them are made an array of everyday
products whose sources may not even be recognized by the consumer. The
user of a glass bottle, for instance, rarely knows that it may have been
made from Illinois silica sand, nor is the driver of an automobile
generally aware that the Illinois concrete highway on which he is
driving probably was constructed from a mixture of cement, sand and
gravel, or crushed stone that may have come from Illinois pits or
quarries.

The significance of these rocks and minerals to the economy of Illinois
is great, although often unappreciated. Of the more than 600 million
dollar value of all Illinois mineral production in 1963, almost 200
million was from industrial minerals. The diversity and widespread
distribution of these mineral resources lend variety and balance to the
mineral industry of the state, and their production, processing, and
utilization afford direct and indirect employment to many people.

The term industrial minerals is used as a convenient group term for
nonmetallic minerals that are not fuels. In Illinois they include
limestone, dolomite, clay, shale, silica sand and other sands,
fluorspar, tripoli (amorphous silica), ganister, novaculite, sandstone,
feldspar-bearing sands, barite, gypsum, anhydrite, brines, greensand,
oil shale, marl, peat, humus, and tufa. The metallic minerals of
Illinois are galena (lead ore), sphalerite (zinc ore), pyrite, and
marcasite.

This booklet briefly and nontechnically discusses the foregoing
materials and some of the work the Illinois State Geological Survey does
in gathering information about their occurrence, and character and in
developing new uses.

The assistance of many Survey staff members and of many people in the
Illinois mineral industry in the preparation of this booklet is
acknowledged.




                               LIMESTONE


Limestone is a most versatile rock. Without it there would be no
portland cement for making concrete roads and buildings, no lime for
plastering and chemical use, no agricultural limestone for farms, and no
crushed limestone for driveways. A wide variety of industries, from
steel making to glass manufacturing, use limestone in one way or
another.

The early settlers of Illinois recognized the value of limestone and
quarried stone blocks and slabs for making foundations, chimneys, and
even houses. For mortar they used a mixture of sand and lime to hold the
blocks together. The lime was made by heating limestone red hot in
simple furnaces or kilns, the ruins of a few of which may still be seen.


                           Kinds of Limestone

Illinois has two principal varieties of limestone, referred to
technically as limestone and dolomite. “Limestone” may be used as a
general name for both varieties.

Limestone consists principally of crystalline particles of the mineral
calcite (fig. 1). This mineral is glassy in appearance and is composed
of calcium, carbon, and oxygen combined to form calcium carbonate—CaCO₃.
Dolomite is largely made of crystalline particles of the mineral
dolomite, which also has a glassy appearance and consists of calcium,
magnesium, carbon, and oxygen—CaMg(CO₃)₂. The crystalline particles of
limestone and dolomite vary in size. Some are coarse enough to be seen
easily, others are so small that they can be distinguished only with a
microscope.


                  Formation of Limestone and Dolomite

Almost all Illinois limestones were formed in seas that covered Illinois
millions of years ago. The many different limestone formations in
Illinois suggest that oceans covered all or part of the area several
times. Numerous kinds of shell fish, corals, and other marine animals
lived in these oceans and had shells and other hard parts made of
calcium carbonate. Through countless generations, these animal remains
accumulated on the ocean floor and gradually were compacted and cemented
into limestone (fig. 2).

Other Illinois limestones, however, were formed by the hardening of muds
composed mainly of calcium carbonate that accumulated on the floors of
the ancient seas. Still other limestones were formed of a combination of
animal remains and lime mud.

    [Illustration: Figure 1—Calcite crystals. Limestone is made up
    mainly of calcite crystals, but they are less perfectly formed and
    are crowded together.]

The coral reefs of the South Pacific Ocean have their counterparts in
Illinois. The ancient Illinois oceans contained extensive reefs that
were built up just as the modern reefs have been. In northern Illinois,
around Chicago for instance, a number of the ancient reefs are now the
site of stone quarries. In southwestern Illinois such reefs are a source
of petroleum.

The dolomites of Illinois probably were originally limestones, but,
either while the limestones were still beneath the sea or after the sea
had withdrawn, magnesium was exchanged for some of the calcium in the
limestones. If the exchange took place under the sea, the sea water was
the source of the magnesium. If it happened when the limestones were a
part of the land, the magnesium was brought in by water circulating
through the rock. Many of the marine animal fossils became difficult to
recognize after the change, and the texture and general appearance of
the rock also were altered. Some of it became noticeably porous.

    [Illustration: Figure 2—Limestone containing fossils. An Archimedes
    screw appears at lower left, lace-like bryozoa in the center, and
    fluted brachiopod shells at top and center.]


                     Uses of Limestone and Dolomite

Some of the major uses for Illinois limestone and dolomite are mentioned
below. Not every limestone or dolomite can be used for all purposes
because for each use the stone must fulfill special requirements of a
chemical or physical nature. For example, it must have high purity for
lime, resistance to wear and weather for roads and buildings, and a
pleasing appearance for decorative stone and marble.

_Lime._—When limestone or dolomite is heated to a high temperature, it
undergoes a change and carbon dioxide is liberated. The weight of the
gas set free is equal to somewhat less than half the weight of the rock
if the rock is pure. The solid product remaining after the gas has been
driven off is known as lime. The heating process is called burning.

Besides being used in making mortar and plaster, lime is valuable in
many other ways, especially in various chemical processes of modern
industry. Plants at Chicago and Quincy make lime from Illinois limestone
and dolomite.

    [Illustration: Figure 3—An Illinois dolomite quarry.]

_Cement._—Portland cement, sometimes called simply cement, is made at
LaSalle, Oglesby, and Dixon in northern Illinois from limestone and a
smaller, carefully measured amount of clay or shale. Cement also is made
at Joppa in southern Illinois. The blend of materials is thoroughly
ground and mixed, then heated at a high temperature in huge rotating
horizontal furnaces, called kilns, until it forms a clinker. After it
cools, the clinker is ground to a flourlike powder. This is basically
the cement that binds together the mixture of sand and limestone (or
dolomite), or of sand and gravel, to make the concrete from which roads,
bridges, buildings, and other structures are made.

_Aggregate._—The crushed stone used in making concrete is known as
concrete aggregate. This is a major use for Illinois limestones and
dolomites. Large amounts of stone aggregate also are used in bituminous
roads, popularly called “blacktop” roads.

_Agricultural Uses._—Agricultural limestone (agstone) is applied to farm
land to neutralize soil acids and otherwise benefit the soil. Both
limestone and dolomite are so used. Chickens are fed small limestone
chips to provide calcium for the formation of egg shells. Pigs and cows
get calcium from mineral supplements containing powdered pure limestone.

    [Illustration: Figure 4—An underground limestone mine.]

_Other Uses._—Illinois limestone or dolomite is used in steel making, as
building stone and marble, as road stone, as ballast for the road beds
of railroad tracks, for making refractory dolomite used in the steel
industry, and for a variety of less common uses.


                                Quarries

There are about 200 stone quarries in Illinois. Most of the larger
quarries (fig. 3) are in the Chicago, Joliet, Kankakee, and East St.
Louis areas, but one or more limestone or dolomite quarry occurs in many
counties.

If all the stone taken from Illinois quarries in 1963 were removed from
a hole 100 feet square, the hole would penetrate into the earth about 8
miles. It would take more than 350,000 railroad cars holding 100 tons
each to haul away the stone. Limestone, dolomite, and their products
added over 80 million dollars to the economy of the state in 1963,
approximately 8 dollars for each person in Illinois.

Most Illinois limestone and dolomite is quarried from open pits, but in
some places, as in the rocky bluffs along the Mississippi River, the
stone is taken from underground mines (fig. 4). There is also a dolomite
mine in Chicago. At the quarries the first step is the removal of the
earth overlying the stone. Next, in both pit and mine, the stone is
blasted to free it from the parent deposit and break it into pieces.
Mechanical shovels (fig. 3) load the stone into trucks that take it to
the crushing plant where powerful crushers further break the stone into
pieces. The pieces are sorted into various sizes by large screens. At
some of the plants, the stone is ground into powder.


                     Location of Limestone Deposits

The geologic map of Illinois prepared by the Illinois State Geological
survey shows, with reasonable exactness, what bedrock formations would
crop out at the surface if the overlying clay, sand, gravel, and earth
were removed. Thick dolomite formations would be exposed in much of the
northern fifth of the state, but would be rare elsewhere. Thick
limestone formations would occur in an almost continuous zone, varying
in width from 3 to 25 miles, along the Mississippi River from Rock
Island to southern Illinois and then eastward across the extreme
southern tip of the state. Limestone also would be seen along the
Illinois River from Havana southward.

In the central area of the state, limestones are present, but they are
rarely over 25 and often less than 15 feet thick. Consequently, most of
the larger quarries are in the northern, western, and southern parts of
Illinois. The thinner limestones, nonetheless, are of much importance
and are quarried at many places, chiefly to provide agricultural
limestone, road stone, and limestone for making cement.

The Geological Survey locates and maps limestone and dolomite deposits
and analyzes and tests samples to determine the best possible uses for
the stone. Many reports have been published about the character and
general use of the deposits in various parts of the state. Other reports
deal with the use of limestone and dolomite for specific purposes such
as cement making, building and decorative stone, rock wool, terrazzo
chips, and lime.




                      METALLIC ORES AND FLUORSPAR


                             Lead and Zinc

Lead mining was one of the earliest industries of Illinois. The early
settlers’ need for bullets for procuring food and for defense of their
lives and property made lead an important commodity, and the deposits of
lead ore in the northwestern corner of Illinois were quickly exploited.
The ore was the mineral galena (fig. 5), for which the city of Galena in
Jo Daviess County is believed to have been named.

    [Illustration: Figure 5—Galena with cubic cleavage blocks in
    foreground.]

Galena is a dark, shiny mineral that breaks readily into cubes or
combinations of cubes. It is composed of lead and sulfur (PbS). Galena
itself is not suitable for use as a metal; the lead must first be
separated from the sulfur.

The earliest method of recovering lead from galena was crude. A pile of
logs, smaller pieces of wood, and ore was built on sloping ground. Just
below it a pit was dug. When the wood was set on fire, the heat caused
the lead and sulfur to separate, and the molten lead trickled down into
the pit. The smelting process was later improved, and stone “furnaces”
were built to house the operations.

_Crevice Deposits and Residual Deposits._—Most of the lead ore mined in
the early days of the northwestern Illinois mining district came from
crevice deposits in the dolomite bedrock and from residual deposits at
or near the surface of the ground. The crevices were vertical narrow
joints or fissures. Ore was not continuously present along them but
occurred from place to place in “pods” (fig. 6 and fig. 7). Dimensions
of the pods varied, but typical ones were about 3 feet wide, 5 feet
high, and a few to a few hundred feet long. The galena, for the most
part, occurred in a mixture of clay and weathered dolomite that filled,
or partly filled, the crevices.

The residual deposits were found where the action of the weather for
many thousands of years had dissolved the dolomite from the outcropping
parts of a crevice deposit and left behind a residue of brown or red
clay containing galena.

Some of the crevice and residual deposits worked by the early miners
cropped out at the surface, but most of them were covered by earth.
Other crevices were exposed in the bluffs of the Mississippi River and
extended back into them for 1,000 feet or more.

    [Illustration: Figure 6—Diagrammatic cross section of two crevice
    deposits. A reaches ground surface and is filled with clay; B is
    only partly clay filled. Galena coats parts of the walls and occurs
    as pieces scattered through the clay. Typical crevices are about 3
    feet wide and 5 feet high.]

  SOIL
  A
    GALENA
    CLAY AND ROTTED DOLOMITE
  B
    GALENA
    CLAY AND ROTTED DOLOMITE
    DOLOMITE

When the richer deposits of ore in the crevices were worked out, some
mines were deepened into the dolomite bedrock, but usually less rather
than more galena was found.

As the amount of galena decreased, however, another mineral, which had
been present before in only small amounts, was found in increasing
quantities. This was sphalerite—a yellow, brown, or black mineral of
resinous appearance that is composed of zinc and sulfur (ZnS). It does
not look like a metallic ore.

At first the sphalerite was not used because there were no smelters in
the area that could separate the zinc from the sulfur with which it was
combined in the ore. Between 1850 and 1870, however, smelters were built
in northwestern Illinois and southern Wisconsin and the ore was shipped
there. Sphalerite is now the principal ore mineral produced in
northwestern Illinois.

The sphalerite and galena of southern Illinois, described later, are
similar to that found in northwestern Illinois.

    [Illustration: Figure 7—Crevice lead mine. Miner pries ore loose and
    pushes it into car at bottom of crevice. Shiny galena layer occurs
    above his head and another at level of his waist.]

_Mining and Milling._—One large mine is producing zinc and lead near
Galena at present. Smaller mines operate irregularly. The ore may occur
as pockets, irregularly shaped rather flat masses, vertical or inclined
veins (fig. 8), or small particles scattered through the dolomite. The
principal ore bodies that have been worked in recent years have been of
irregular shape, both horizontally and vertically, and usually have been
between 50 and 200 feet wide and from a few to as much as 100 feet high.
They lie at a depth of roughly 300 feet.

Blasting is required to loosen and break the ore. At the large mine the
ore is brought to the surface by a hoist. In some relatively shallow
mines an inclined tunnel has been driven to the ore body and the ore
brought to the surface in trucks.

    [Illustration: Figure 8—Diagrammatic cross section of a zinc ore
    deposit showing flats and pitches. The deposit occurs in limestone
    and dolomite. Much of the ore is a mixture of dolomite, limestone,
    calcite, and sphalerite. Typical dimensions of such a deposit are
    about 30 feet high and 100 to 200 feet wide.]

  ORE
  PITCHES
  FLATS

Because the ore consists of galena and sphalerite attached to and
scattered through dolomite, it must be milled to free and separate the
metals from the rock. Crushing, grinding, and other operations are
involved. The dolomite is discarded and the galena and sphalerite
concentrates are shipped away to be smelted. No smelters have operated
in the Galena area for some time.

_Aids to Prospecting._—Finding deposits of ore 300 feet underground is
not easy. Inspection of the surface usually tells little. To find and
outline a commercial ore deposit many holes often must be drilled to
explore the unexposed rock strata. Because this is a costly process,
every possible means is employed to drill the holes where ore is most
likely to be found. This is where geologists are useful—geologists of
the mining companies and of the Illinois Geological Survey. Three
examples of how their investigations help to find ore are given here.

It was noted early in the development of the northwestern Illinois
mining district that zinc ore deposits were most common along small
downfolds in the bedrock, called synclines, that were a few hundred feet
wide and a mile or so long. The synclines were associated with much
larger synclines that extended for several miles. A map prepared by the
Illinois Geological Survey shows the possible location and extent of
many of these downfolds and has had much practical use in the selection
of the most promising areas for test drilling to find ore.

The Survey also collects the records of borings made by companies and
individuals in their search for ore. The records are on permanent file
at the Survey offices and are valuable in several ways. Some indicate
where no ore was found and where it is, therefore, useless to drill
further; others show only traces of ore but suggest that more drilling
in the vicinity might discover a deposit large enough to be mined
profitably. Still other records are of borings that encountered rich ore
in which mines have been developed.

The third aid to prospecting is the study of ore bodies and their
minerals to determine how the deposits were formed. The ore bodies have
been and are being studied in the mines. Ore specimens are carefully
examined in the Survey laboratories. If geologists can learn how the
known deposits were formed, it may be possible to direct exploration
into promising new areas.


                               Fluorspar

In the southeastern tip of Illinois lie deposits of a mineral that
contains the chemical element fluorine. This element is used in making
the propellant that activates aerosol sprays, a plastic that resists
chemicals and oil and is strong enough to be used for bearings,
compounds that are said to help to prevent tooth decay, and many other
useful chemicals.

The mineral is fluorite (fig. 9), commonly called fluorspar. It is
composed of calcium fluoride (CaF₂), a compound of calcium and fluorine,
and is a glassy mineral that is generally white or gray but may be
purple, rose, yellow, blue, or green. In rare instances it is colorless.

Fluorspar mining in Hardin and Pope Counties began with lead mining.
Galena was first discovered there in 1839 in a well being dug at the
town of Rosiclare. Mining of galena began in the early 1840’s, and
somewhat later ore was being smelted by three furnaces, all of which
have long since disappeared.

The veins that were worked for galena also contained fluorspar, but as
there was little or no use for fluorspar in the 1840’s it was considered
waste. In time, uses developed, however, and about 1870 it was mined and
shipped in commercial quantities. Since then the tonnage and value of
the fluorspar produced from the Rosiclare area have increased until
fluorspar is the major product.

    [Illustration: Figure 9—Group of cubic fluorite crystals.]

The fluorspar mining district north of the town of Cave in Rock in
eastern Hardin County also was an early producer of galena. In that area
the fluorspar-galena deposits are elongate and approximately flat. The
first miners followed the ore bodies from outcrops by tunneling into the
hillsides. In the late 1930’s and early 1940’s, many holes were drilled
into the bedrock in search of new deposits. Ore was found that contained
not only galena and fluorspar but also important amounts of sphalerite.

_Vein Deposits._—In the Rosiclare district, the fluorspar and its
accompanying minerals occur as steeply inclined veins a few inches to 25
feet or more wide (fig. 10), usually in limestone strata. The veins are
not uniformly thick but widen or narrow from place to place both
vertically and horizontally. They occur along faults—planes along which
the rocks of the earth’s crust have broken and slipped. A fault may be a
single plane of slippage but more often is a zone of broken and
displaced rocks. In most of the faults that contain fluorspar, the
slippage is vertical, or nearly so. Along one of the faults in the
Rosiclare district, the rocks on one side of the fault have moved
downward as much as 650 feet in relation to the rocks on the other side.
Some faults are more than 10 miles long, and the depth to which they
extend into the earth is unknown. Fluorspar has been mined from one of
them at depths of 800 feet. Not all faults, nor all parts of any one
fault, contain fluorspar.

    [Illustration: Figure 10—Diagrammatic cross section of fluorspar
    vein along a fault. The strata on the left side of the fault have
    moved downward with reference to those on the right side.]

  SOIL
  FAULT
    FLUORSPAR VEIN ALONG FAULT
    down
      SANDSTONE A
      LIMESTONE B
      SHALE C
      SANDSTONE D
      LIMESTONE E
    up
      LIMESTONE B
      SHALE C
      SANDSTONE D
      LIMESTONE E

_Bedding Deposits._—In contrast to the vein deposits of the Rosiclare
district, the bedding deposits of the Cave in Rock area are flat, or
nearly flat, commonly 5 to 15 feet thick, and from a few to 200 feet
wide (fig. 11). They may be as much as 2000 feet long, widening or
narrowing and thickening or thinning throughout their extent. They are
called bedding deposits because they lie along the beds or layers of the
limestone in which they occur. Most of the ore bodies are associated
with a fracture or a small fault.

    [Illustration: Figure 11—Diagrammatic cross section of bedding
    deposit of fluorspar, lead, and zinc ore.]

  SOIL
  SANDSTONE AND SHALE
  ORE
  LIMESTONE
  FRACTURE OR SMALL FAULT

_Grades and Uses of Fluorspar._—There are three principal grades of
fluorspar—metallurgical, acid, and ceramic. The metallurgical spar is
used as a flux in making steel and in metal foundries. Acid spar is used
to make hydrofluoric acid, which plays a part in the preparation of
uranium isotopes and in the production of a synthetic mineral essential
in refining aluminum. The acid is also used in the production of
high-octane gasoline and is the basis for a variety of important
chemical compounds, among them refrigerants and insecticides. Ceramic
grade fluorspar is used in making enamels, glazes, and certain kinds of
glass.

_Mining and Milling._—Fluorspar and its associated ores are mined in
different ways in the Rosiclare and Cave in Rock districts because the
types of deposits differ. However, in both areas most of the larger
mines are entered by vertical shafts. In the mines, explosives are
placed in holes drilled by machines and are then detonated to shoot down
the ore (fig. 12). Mine cars, trucks, or conveyor belts carry the ore to
the bottom of the mine shaft where hoists raise it to the mills at the
surface. In the mills a variety of ore-classifying machines separate the
galena, sphalerite, and fluorspar from the waste mineral materials
(chiefly limestone and calcite) with which they occur. Some fluorspar
after the separation process is almost flour-fine. To increase its use,
much of this spar is mixed with a binder and made into pellets or
briquets one-half to one inch in size.

    [Illustration: Figure 12—Machine loading fluorspar ore in mine near
    Cave in Rock.]

_Geological and Chemical Studies._—Because much of the fluorspar
produced in southern Illinois has come from veins along faults,
geologists have mapped the faults of the area by investigating the
distribution and nature of the various bedrock outcrops. The work was
complicated by the mantle of earth and vegetation that covers the
bedrock at many places. However, a geologic map was made that shows
where the various rock formations—sandstone, limestone, and shale—lie
beneath the surface and where the numerous faults crisscross the
district.

The first geologic map of the fluorspar district was made in 1920 by the
Illinois and U. S. Geological Surveys. New maps on a much larger scale
have been made recently by the Illinois Survey to meet the needs of the
modern fluorspar industry.

The Illinois Survey also has studied the ores and ore deposits of
southern Illinois to determine how they were formed. The records of many
borings and pits sunk to find ore have been collected and filed at the
Survey to guide future prospecting.

Survey chemists are finding new uses for Illinois fluorspar. Their
research has produced new organic fluorine compounds that are being
tested for use in agriculture, medicine, and industry. They also have
worked out easier and cheaper methods of making certain fluorine
compounds. Survey chemical engineers have helped to obtain needed
information about the physical properties of the pellets made from
fluorspar powder.


                    Origin of Illinois Ore Deposits

The ore deposits of northwestern and southern Illinois were formed so
many millions of years ago that it is possible to propose only theories
of their origin. Most geologists think that the minerals, dissolved in
warm or hot water, came from deep in the earth. Perhaps the
mineral-bearing water came from, or was associated with, rocks that were
or had been molten (igneous rocks), but it may have had some other
source. Why the ores occur where we now find them is not fully known.
The cooling of the solutions and the lessening of pressure as the
solutions rose toward the surface may have had a part in ore deposition.
Faults and the nature of the rocks encountered by the depositing
solutions also appear to have had an influence.


                       Illinois as a Mining State

Although Illinois is not usually thought of as a mining state,
northwestern and southern Illinois together produced in 1963 nearly
$5,000,000 worth of zinc, about $600,000 worth of lead, and $6,500,000
worth of fluorspar. The annual total value is about $12,000,000.

The southern Illinois fluorspar district has another distinction—for
many years its mines have been the major domestic source of the nation’s
fluorspar.




                              SILICA SAND


About 450,000,000 years ago, a shallow ocean covered Illinois. Its waves
and currents carried clean white sand and deposited it as curving
beaches, sand bars, and dunes. This sand differed from many sands in
that it was composed almost exclusively of grains of the mineral quartz
instead of being a mixture of quartz and other minerals.

Quartz is composed of silica (SiO₂), and sands such as the ancient
Illinois sand that are composed of quartz are known as silica sands.
Quartz is very hard and will scratch glass and some steel. Perfect
quartz crystals, which are rare, are longer than they are thick and end
in pyramids. Probably not many grains of the ancient sand came from
perfect crystals; they more likely resulted from the decaying and
breaking down of rocks such as granite, which are mixtures of quartz
grains and other mineral particles.

The quartz grains probably did not come directly from their source to
Illinois. Instead, it is likely they first were deposited elsewhere and
formed into sandstone. That sandstone was subsequently broken down by
weathering agents and the grains transported to the ancient Illinois sea
by streams.

As a result of the erosive action of the agents that transported them,
many of the originally angular grains, particularly the coarser ones,
were rounded and their surfaces dulled like that of frosted glass (fig.
13). Consequently, they appear white, although they actually are
colorless.

Since the ancient sea deposited its silica sand, other seas have covered
Illinois at various times and each has left deposits of sand, mud, or
limy materials. The silica sand thus was buried by hundreds of feet of
other sediments and became sandstone. This sandstone is called the St.
Peter Sandstone. It is named from the St. Peter River, now the Minnesota
River, in Minnesota where the sandstone was first described and named by
geologists. The overlying deposits also were consolidated into rock.

St. Peter Sandstone is exposed at the surface at many places in northern
Illinois and in one small area in the western part of the state. The
sandstone exposed in northern Illinois generally varies from 125 to 300
feet thick. The fact that it crops out at the surface indicates that the
materials that formerly covered it have been removed.

The uncovering was not a single, simple event but rather a series of
events that took place at various times during the many years since the
St. Peter sand was deposited. Among these was the up-bowing of the rocks
of central northern Illinois into a broad arch. Streams then began to
cut across the arched rock, slowly but persistently stripping away the
top layers until the core of the arch was laid bare. Among the rocks
thus exposed was the St. Peter Sandstone, which may be seen in northern
Illinois in the valleys and tributaries of the Rock River near Dixon and
Oregon and along the Illinois and Fox Rivers and some of their
tributaries near Ottawa, Wedron, Millington, and Troy Grove. The St.
Peter Sandstone at Starved Rock and Matthiessen state parks near LaSalle
and along the highway between Dixon and Oregon is eroded into scenic
bluffs and canyons.

    [Illustration: Figure 13—Enlarged photograph of St. Peter sand
    showing the rounded and frosted character of the grains.]


                          Silica Sand Industry

The Illinois silica sand industry is based on the St. Peter Sandstone.
It centers around Ottawa, Wedron, Troy Grove, and Utica in LaSalle
County and in Oregon in Ogle County. Two principal grades of silica sand
are produced—washed and crude. The value of the silica sand produced in
Illinois in 1963 was about $9,000,000.

_Washed Sand._—Although the St. Peter Sandstone is composed almost
entirely of quartz grains, a small amount of clay is present. For some
uses it is not necessary to remove the clay, for others its elimination
is important and is achieved by washing the sand.

In the mining of silica sand that is to be washed, the sandstone is
first blasted loose from the parent deposit to break it into sand or
pieces of various sizes. Some of the larger pieces may require a second
blasting to disintegrate them.

At some pits the material is loaded mechanically and transported to the
washing plant. At others a powerful stream of water is directed against
the broken sandstone (fig. 14) and the resulting mixture of sand and
water flows to a collecting basin from which it is pumped through large
pipes to the processing mill.

In both types of operation the sand is thoroughly washed at the plants.
After it is washed, the sand is further processed to suit the needs of
its users. Much of it is screened into different size grades.

_Uses of Washed Silica Sand._—The washed silica sand produced in
Illinois has many uses, some of which are briefly mentioned below. The
suitability of the sand for some purposes depends in part on its having
been screened to specified sizes.

The high purity of Illinois washed silica sand makes it suitable for
making glass, which is more than half silica sand. Each year over a
million tons is used for this purpose. The purity of the sand also is of
importance for chemical and metallurgical uses such as the manufacture
of sodium silicate and silicon carbide and in alloying.

The hardness of the sand makes it useful for grinding large sheets of
plate glass to prepare them for polishing and also makes it an effective
abrasive agent for sandblasting. Metal castings in foundries and the
exteriors of buildings are cleaned by this process. Illinois produces
thousands of tons of sand yearly for such abrasive purposes.

Because the coarser grains of the washed silica sand are rounded,
strong, and available in uniform sizes, oil operators use thousands of
tons of it annually in the hydraulic fracturing of oil-bearing strata.
The sand is mixed with oil, other petroleum products, or water and is
forced by powerful pumps into sandstone or limestone formations that
contain oil. The great force thus exerted opens fractures in the rock
strata and pushes the liquid and sand into them. When the pressure is
relieved, the sand grains serve as props to hold the fractures open. The
oil can then flow more easily into the wells and oil production is thus
increased.

The washed sand, because it is clean and does not dissolve in water, is
used to filter impurities from drinking water. Its whiteness makes it a
desirable constituent in plaster, mortar, and precast building panels.

    [Illustration: Figure 14—Hydraulic mining of silica sand near
    Ottawa.]

Because it is round grained and withstands high temperatures without
melting, large tonnages of the washed silica sand are used to make molds
into which molten metal is poured to make various kinds of castings.

A special type of coarse silica sand from Illinois that is carefully
prepared so that it is always of the same grain size is used throughout
the world as a standard in laboratories that test cement and other
commercial products.

    [Illustration: Figure 15—Loading crude silica sand.]

Some silica sand is ground to a fine, white powder. The powder, called
ground quartz, ground silica, silica flour, or potter’s flint, has many
uses. It is an ingredient in paints, potters use it in making pottery
and china, it goes into scouring powders, into molds used for precision
types of metal castings, and into enamels.

_Crude Silica Sand._—The crude silica sand produced from the St. Peter
Sandstone generally is yellow or yellowish white and is not washed
before it is used. It probably originally was white, but iron oxide,
similar to the rust that forms on iron, now coats many of the sand
grains and colors the small amount of clay in the sand. Thousands of
tons of crude silica sand are mined annually (fig. 15). Because it is
highly heat resistant, foundries buy much of it to make the molds used
for castings, especially steel castings, and for automobile engine
blocks, train wheels, and a variety of other metal products. Crude
silica sand also is used around industrial furnaces to seal cracks and
openings to prevent the loss of heat, in certain ceramic products, and
for adjusting the silica content of the raw materials used for making
portland cement.


                   Studies of the St. Peter Sandstone

The Illinois Geological Survey has made field studies and prepared maps
showing where the St. Peter Sandstone is exposed in northern and western
Illinois. Many samples have been screened and examined under a
microscope to determine how the sand of different deposits, or different
parts of the same deposit, varies in grain size and mineral composition.
The possibility of using Illinois silica sand for making silica brick
also has been investigated.




                            GRAVEL AND SAND


Some 225,000 years ago, most of what is now Illinois was buried under
the ice of the Illinoian glacier. Two earlier glaciers had covered large
parts of Illinois, and another, known as the Wisconsinan glacier, came
into the state later, about 50,000 to 70,000 years ago (fig. 16).

The relatively small glaciers in the United States today, such as those
in the northern Rocky Mountains, are concentrated in valleys and are
called valley glaciers. The glaciers that covered Illinois were parts of
huge ice sheets that extended over much of the North American continent
and are called continental glaciers. They spread over most of Canada,
then pushed southward to bury New England and a great area in the
north-central part of the United States north of the Ohio and Missouri
Rivers.


                 Formation of Gravel and Sand Deposits

As the glacial ice edged slowly southward from Canada, it froze fast to
and picked up soil and loose pieces of rock, with enormous force tore
away huge chunks of bedrock, and mixed and ground these materials
together (fig. 17).

Into Illinois the glacier carried rock materials from Canada, Wisconsin,
Minnesota, and Michigan; other rock fragments were picked up in Illinois
as the ice front advanced. When the glacier melted, it left behind its
load of rock flour and rock fragments, much of it as a gray clay
containing pebbles, cobbles, and boulders. Geologists call such deposits
glacial till.

    [Illustration: Figure 16—Extent of the exposed deposits of the
    Wisconsinan, Illinoian, and Kansan glaciers in Illinois, and the
    unglaciated areas of the state.]

  UNGLACIATED
  WISCONSINAN GLACIER
    Freeport
    Fulton
    Peoria
    Decatur
    Charleston
  ILLINOIAN GLACIER
    Kewanee
    Waterloo
    Carbondale
    Harrisburg
  KANSAN GLACIER
  UNGLACIATED
    Hardin

The ice in the continental glaciers usually crept forward, sometimes
slowly, sometimes more rapidly. Whether the front of a glacier moved
forward or back depended on the balance between the rate of forward
motion of the ice and the rate of melting. When the ice advanced faster
than it melted, the front of the glacier moved forward. When the glacial
ice melted faster than it moved forward, the front of the glacier
receded. When the rates of melting and advance were about equal, the
front of the glacier stood still or moved back and forth in a narrow
zone.

    [Illustration: Figure 17—Striated boulder. Scratches and flattened
    surfaces were caused by abrasion by other rocks while boulder was
    embedded in glacial ice.]

When such a more or less stationary front existed, an enormous amount of
clay, silt, sand, pebbles, and boulders was deposited in a belt only a
few miles wide along the front of the glacier, creating a line of hills
and ridges that extended for many miles. Such belts, called end
moraines, can be seen today in many parts of Illinois.

The building of end moraines often was accompanied by the release of
great quantities of water (meltwater) from the melting ice. The water,
laden with rock debris, flowed from the front of the glacier in many
streams.

As the meltwater flowed away from the glacier it sorted its load,
although the sorting was rarely perfect. The heavy boulders and pebbles
usually were dropped first, then the sand, next the silt, and finally
the clay. In general, the farther the deposits were from the glacier the
finer they were. The major streams frequently carried pebbles 50 to 100
miles from the glacier and it was many more miles before all the sand
was dropped. They carried some of the fine silt and clay as far as the
Gulf of Mexico.

Sometimes the floods of glacial water were greater and flowed faster
than usual and so were able to carry coarse rock materials farther. As a
result, gravel was laid down on top of earlier sand deposits. Later
there may have been further sand deposition.

The debris-laden meltwater that flowed into valleys often deposited in
them a considerable filling of sand and gravel. Some valleys were filled
to a depth of as much as 100 feet. Such deposits are called valley fills
or valley trains. Modern streams have cut their courses into many of
these fills and even worn away large parts of them. Remnants of valley
train deposits are now large terraces or benches along streams, many of
them well above the present stream channels.

Where many small streams flowed from the glacier, they deposited sand
and gravel as a large apron in front of the glacier. Such deposits are
called outwash plains and many of them extend for miles.

Two other types of sand and gravel deposits made by glacial meltwaters
also are significant. One was formed where water issued from the front
of a glacier or poured into holes or crevasses in the ice. The sand and
gravel in the water formed a deposit that now appears as a rounded hill
associated with a terminal moraine and is called a kame. The second type
of deposit was laid down in beds of streams flowing under, through, or
on the glaciers and was left as a more or less continuous ridge of sand
and gravel when the ice melted. Such a deposit is called an esker. Some
eskers in Illinois are about a quarter of a mile wide and several miles
long. Typical are the Kaneville Esker northwest of Aurora, the Adeline
Esker south of Freeport, and the Exeter Esker west of Jacksonville.

The deposits of both the Illinoian and Wisconsinan glaciers are widely
distributed throughout the state. Melting of the Illinoian glacier
caused comparatively little flooding; consequently, extensive gravel
deposits were formed in only a few places. The ice of the Wisconsinan
glacier, however, melted rapidly and produced great floods laden with
sand and gravel. Thus, most major gravel deposits in Illinois are
related to the Wisconsinan glacier.

Wind sweeping across the sand and gravel deposits blew the sand into
hills or sand dunes near such places as Havana, Prophetstown, Kankakee,
and Watseka. Even today the wind shifts sand of long-forgotten glacial
floods.


                      Studies of Glacial Deposits

The foregoing discussion of glaciers and their deposits is greatly
simplified. For some time geologists of the Illinois Geological Survey
have been mapping the moraines, valley trains, outwash plains, and other
glacial deposits of the state. Because the Illinoian and Wisconsinan
glaciers advanced and retreated several times, they built many moraines.
The Survey has made a map (fig. 18) that shows the complexity of the
moraines left by the Wisconsinan glacier. They are roughly concentric,
indicating that the general shape of the glacier front remained about
the same.


            Principal Commercial Sources of Sand and Gravel

The sand and gravel industry is widely distributed throughout Illinois.
The principal commercial sources of sand and gravel are valley trains
and outwash plains. The Fox, Rock, Illinois, Mississippi, and Wabash
Rivers and many smaller streams have terraces in their valleys that are
parts of valley trains. In these deposits are some of the largest sand-
and gravel-producing operations in the state.


                              Composition

An examination of glacial gravel deposits in Illinois reveals pebbles
and larger pieces of many kinds of rock. Some are gray, others white,
pink, brown, or black. They commonly include limestone, dolomite,
granite, and many rocks with less common names such as quartzite,
schist, and basalt. The limestone and dolomite were picked up by the
glaciers from outcrops in northern Illinois, Wisconsin, and Michigan.
Some of the granite pebbles resemble outcrops in Wisconsin; others look
like granite that crops out in Canada. The quartzite probably came from
Wisconsin, and black shale fragments found in some gravel deposits came
from the floor of Lake Michigan or from western Michigan. Occasionally
pieces of metallic copper are found that probably had their source in
the Lake Superior copper-bearing area.

In addition to the sand associated with gravel deposits, extensive
deposits of sand alone are found at many places in Illinois. Most of the
sand grains are pieces of minerals that were constituents of rocks until
weathering, the grinding action of the glaciers, and other erosive
agencies broke the rocks into sand. The principal mineral in glacial
sand is quartz, but many others occur in lesser amounts, including
calcite, dolomite, feldspar, pyroxene, tourmaline, garnet, magnetite,
and hornblende. Most of these are foreign to Illinois, although the
calcite and dolomite may be native.

    [Illustration: Figure 18—Moraines left by Wisconsinan glacier.]

  Modified from George E. Ekblaw, 1960
  _ILLINOIS STATE GEOLOGICAL SURVEY_

    [Illustration: Figure 19—Gravel dredge, with plant for processing
    the gravel in background.]


                        Uses of Sand and Gravel

In 1963 more than 27 million tons of sand and gravel, directly or
indirectly of glacial origin, was sold by Illinois producers for almost
25 million dollars. The sand alone would have filled a child’s sand box,
with an area of 8 square miles, to a depth of 1 foot. The gravel would
have covered an even larger area.

The gravel was used in making concrete for roads and buildings, for
surfacing roads, for ballast for railroad tracks, and for other
purposes. The sand found its way into plaster, mortar, concrete, and a
variety of other products and uses. Some of it was produced for use as
molding sand.


                     Production of Sand and Gravel

The production of sand and gravel from its deposits may be a relatively
simple operation or one of considerable complexity. Gravel for surfacing
a road may be dug from a conveniently located pit and loaded
mechanically into trucks that haul it to the road. A large sand- and
gravel-producing operation, however, may include not only mechanical
equipment to load and transport the material but also a processing plant
where it can be washed if necessary and screened to various sizes.

In a “dry pit” operation, mechanical cranes or shovels pick up the
gravel and sand and load it into trucks, railroad cars, or conveyor
belts to be transferred to the processing plant. There clay and dirt may
be washed out and the sand and gravel is sized by screens. Conveyor
belts carry sand and gravel to the various processing operations and to
storage bins or piles on the ground.

A “wet pit” operation produces sand and gravel from an artificial pond
or lake. In some operations the sand and gravel is mined by a dredge
that floats on the water (fig. 19). In some pits, a stream of water is
directed from the dredge against the bank of gravel to wash the gravel
into the lake. A large metal pipe at the front of the dredge slants down
into the water and sucks up the sand and gravel from the underwater part
of the deposit. The gravel, sand, and water is then pumped through a
pipe at the rear of the dredge to the processing plant on the shore.

In some types of wet pit operations a large scoop or bucket operated
from a crane on the shore, or by cables fastened to the shore, is used
to dig sand and gravel from beneath the water.

Dredges are used to pump up sand and gravel from the beds of Illinois
rivers, especially the Mississippi, Ohio, and Wabash.

The sand and gravel resources of many parts of the state have been
mapped and studied by the Illinois Geological Survey, and work of this
kind is continuing.




    SILICA (TRIPOLI) AND OTHER MINERAL MATERIALS OF EXTREME SOUTHERN
                                ILLINOIS


The hills of extreme southern Illinois contain several mineral materials
that are entirely or largely restricted, in important quantities, to
that part of the state. Some of them, such as silica and novaculite,
come directly from bedrock deposits of great age; others, such as some
of the sands and gravels, are of more recent origin.


                            Silica (Tripoli)

A mineral material unique in Illinois is the amorphous silica, or
tripoli, mined in the hills of southern Illinois about 20 miles north of
Cairo.

Most of the silica mines are found at the heads of valleys or in
tree-covered hill slopes along the less traveled roads. An arched
opening (fig. 20) with a road leading into it may be all that is visible
of the mine from the outside. Inside are many rooms, some 30 feet high,
separated by rounded pillars about 20 feet thick that have been left to
support the arched mine roof.

Most of the silica in the mines is white, and this is the part that is
mined. A silica deposit is made up of layers—some of white, powdery rock
material, others that look chalky but are firm or hard.

    [Illustration: Figure 20—Entrance to a silica mine.]

In the mines the silica is blasted loose from the natural deposit and
loaded into trucks that take it to the processing mills at Elco and
Tamms, where it is crushed and then transferred to huge grinding mills
that pulverize it to a very fine powder. Air currents of different
velocities separate the powder into various grades of fineness, and the
finished silica goes into large paper bags for shipment.

_Origin of Silica._—Some of the history of the formation of silica
deposits is not fully known, but it seems probable that their original
character was quite different from what it is today. Investigations by
Survey geologists suggest that the parent rock formation was limestone
and chert. The limestone was composed of the mineral calcite, but
scattered through it were myriads of small particles of quartz.
Interlayered with it were bands and beds of chert that contained varying
amounts of calcite.

As the rocks above them were worn away by the ceaseless erosion of
streams and rivers, these original deposits were uncovered. Rain and
snow-water then worked down into the limestone and chert deposits
through cracks and crevices and dissolved the calcite in the rock. The
quartz remained because it is much less soluble than the calcite. After
many thousands of years all the calcite had been removed, leaving behind
a “skeleton” of quartz—the silica deposits of today.

_Uses of Silica._-The silica mined and milled in southern Illinois has
many uses. A superfine grade, known as white rouge, is used to polish
optical lenses. Other grades are used in scouring compounds, metal
polishes, paints, electrical resistors, high-temperature pipe coverings,
fiberglass manufacture, plastics, silicone rubber, wood filler, caulking
compounds, ceramic products, floor tile, billiard cue chalk, as foundry
parting or facing, concrete admixture, in the manufacture of buffing
compounds that are used to polish metal objects, and for other purposes.
Industry uses many thousands of tons of silica each year.


                         Chert and Chert Gravel

Deposits of chert, chert gravel, and ganister also are among the variety
of mineral materials found in extreme southern Illinois. Chert consists
principally of minutely crystalline particles of quartz. Some chert is
popularly called flint. In southern Illinois chert occurs in two
principal kinds of deposits, those composed of solid ledges and those
consisting of gravel. The term novaculite is used in southern Illinois
for those solid deposits that are white, comparatively thick, and free
of other interlayered materials. No novaculite is mined at present, but
it is said to have been sold in past years for making sodium silicate
and silica brick.

The chert gravels of southern Illinois are of three kinds—novaculite
gravel, Elco gravel, and “Lafayette” Gravel. The novaculite and Elco
gravels consist of fragments of chert plus lesser amounts of fine silica
particles and clay. The chert fragments of the novaculite gravel are
angular, but the Elco gravel includes both angular and rounded
fragments. These gravels are white, yellow, brown, or reddish brown, the
novaculite gravel usually being the more highly colored. Deposits in
Union and Alexander Counties have been used for road surfacing and other
purposes. Deposits of chert gravel also occur in Hardin and Saline
Counties, and some stream valleys in southern and western Illinois also
contain such gravel.

“Lafayette” Gravel consists principally of brown chert pebbles. Most of
the pebbles are rounded and have a smooth, semi-polished surface. The
sand and clay occurring with the gravel are brown or dark red. In some
places there are deposits of coarse, red quartz sand. The gravel is most
abundant in the four southernmost counties of the state, and deposits
may be as much as 65 feet thick. It is used principally as a
road-surfacing material.


                                Ganister

Ganister occurs in the hills of Alexander and Union Counties and is a
loosely consolidated, granular material consisting of irregular
particles up to about an inch in diameter or of masses of material
readily disintegrated into such particles. The particles are composed
mainly of fine crystalline quartz. Ganister is white, cream, light
yellow, or red and occurs in deposits up to 25 feet or more thick.
Relatively small tonnages of the light colored ganister are now used,
principally in making refractories, but ganister is said to have been
more widely used in that field in the past. It is produced from
underground mines. Some red ganister, or ganister like material, mined
from open pits has been used for road surfacing.


                 Studies of Southern Illinois Materials

Survey geologists have mapped the chert- and silica-bearing formations
of Alexander and Union Counties and also many of the various kinds of
gravel deposits. Samples have been tested to determine their chemical
composition and the size of the particles composing them. Laboratory
studies by ceramists indicate that novaculite and novaculite gravel,
when suitably processed, can be used for making silica brick, which
withstands great heat. Canister and the gravels of southern Illinois
offer a variety of raw materials awaiting increased industrial use.


                   Sands of Extreme Southern Illinois

In southern Illinois deposits of sand laid down in an arm of the ocean
that once extended northward into Illinois from the Gulf of Mexico are
found in Alexander, Union, Pulaski, Pope, and Massac Counties. The
deposits are commonly a light color—white, cream, yellow, or gray.

The grains of the sand are almost all quartz and generally are angular.
Some of the sands are of almost powder-like fineness, others are fine or
medium grained. Many of the sands contain flakes of white mica, a
glistening, silvery-looking mineral often mistaken for silver or
platinum. Unlike these metals, however, mica is comparatively light in
weight and is not metallic. Also present in some sands are small flakes
of the mineral graphite.

The southern Illinois sands have not been widely used, but some of them
have been employed in making concrete. They also may have possibilities
for molding and core sand.

As a result of work by Survey geologists, the location and properties of
many of the southern Illinois sand deposits are known.




                             CLAY AND SHALE


Man has used clay in various ways for many hundreds of years. From it he
made, and still makes, bricks to build his dwellings, pottery utensils
of many kinds, and other useful products.

Everyone knows what clay is, yet it is a substance difficult to define.
All clays are earth materials, most of them plastic or sticky when wet
but firm when dry. If heated sufficiently (fired) they become hard.

Clays are composed of various minerals. Of these, the so-called clay
minerals—complex substances composed mostly of alumina, silica, and
water—generally are the most important. They impart the property of
plasticity and also cause clays to become hard when fired.

Most clays are what geologists call unindurated (unhardened) rocks. Clay
that has been indurated and occurs in layered deposits is commonly
called shale. The layers may be from a fraction of an inch to several
inches thick. Most Illinois shales are not plastic when dug from freshly
exposed deposits, but they become plastic when crushed and kneaded with
water. The clays and shales of Illinois are the basis of a huge and
important industry.


                         Early Uses in Illinois

Clays and shales are useful because they can be made plastic by adding
water, formed into desired shapes, and fired to a rock like hardness. As
a result, various kinds of bricks, drain tile, pottery, and other useful
products are made from them. In its early years, Illinois had many
widely distributed potteries that used clay from nearby deposits to make
a variety of jugs, crocks, and bowls that served in place of many
present-day glass or metal articles.

Drain tile has been of major importance in the development of the state.
Early settlers found many low lying, swampy areas and tracts of land
that drained poorly after heavy rains. Ditches were dug to carry away
the water from some areas, but others were drained by means of drain
tile—pieces of fired clay pipe several inches in diameter and about a
foot long that were laid end to end in trenches below plough depth and
then covered with earth. Water seeped into the tile, which discharged it
into ditches. Tile factories, built throughout Illinois near clay or
shale deposits, did an active business. Gradually, however, as more and
more farm land was drained the demand slackened and many tile factories
went out of business. Although there are fewer factories, much drain
tile is still manufactured in Illinois.

Many of the early tile plants also made bricks to be used for making
foundations, buildings, sidewalks, and other structures. The bricks were
made by hand-operated equipment. Some of the old hand-molded bricks may
still be seen in older buildings. Now the brick-making process is highly
mechanized and even though there are fewer plants they produce more
bricks.


                        Clay and Shale Deposits

Illinois shales are a part of the bedrock—that is, they are associated
with indurated rocks such as sandstone and limestone. Most clays are
surficial rocks occurring in deposits near the surface, where they lie
above the bedrock. Exceptions are certain clays found in extreme
southern Illinois and the underclays, also called fireclays, that occur
beneath coal seams and are part of the bedrock.

The surficial clays are of two principal kinds—till and loess. Till is a
deposit left by glaciers. It is a gray, blue-gray, or brown clay
containing varying amounts of sand, pebbles, cobbles, and even boulders.
Till is found at many places in the state and is used for brick making,
especially in the Chicago area.

Loess is a wind-deposited silty clay or clayey silt and is found in many
parts of Illinois. It is thickest on or near the bluffs of the
Mississippi, Illinois, and Ohio Rivers. It generally is brown and stands
in steep faces in roadcuts and other excavations. It once was widely
used for making brick and tile.

Of major importance in making clay products in Illinois are the bedrock
shales and the clays associated with the coal-bearing rocks that
underlie much of the state. The shales, and the clays to a lesser
extent, are dug at many places for making structural clay products such
as bricks, structural tile, and drain tile. They also are used to make
lightweight aggregate for concrete. The underclays of some of the older
coal seams are used to make buff-colored brick, stoneware, and a highly
heat-resistant brick (firebrick) that is used in industrial furnaces or
in other operations involving high temperatures. Some fireclay, ground
as fine as flour, is added to molding sand to make it coherent enough to
form into molds for metal casting. Sewer pipe and flue lining also are
made from underclays.

Clays unlike those found elsewhere in the state occur in extreme
southern Illinois. One of these has the property of removing color from
oils and was so used at one time by petroleum refineries. Another,
kaolin, was extensively used during World War I for making crucibles.


                             Clay Minerals

The uses of clay and shale are determined to a large degree by the
properties of their clay minerals and to a lesser degree by the
impurities present. A clay or shale containing the clay mineral illite,
and other similar but less important clay minerals, commonly becomes red
when fired and gets hard at a relatively low temperature. It therefore
is used to make red bricks, drain tile, building tile, and other
structural clay products.

Another clay mineral, kaolinite, generally burns to a light color and is
difficult to fuse. Therefore, clays composed wholly or mainly of
kaolinite can be used for making buff or light-colored bricks and for
the manufacture of highly heat-resistant (refractory) bricks.

The clay mineral in the southern Illinois clay that was used to
decolorize oil is montmorillonite. This clay is now used in sweeping
compounds, as an oil absorbent, as animal litter, and for other
purposes.


                       Studies of Clay and Shale

In view of the significant relationship between the clay minerals and
the utilization of the clays and shales in which they occur, the
Illinois Geological Survey has investigated extensively the clay
minerals in the clay and shale deposits of Illinois. Many samples were
studied by means of powerful microscopes, X-ray, and chemical analysis.
Most of the surface clays and shales proved to be composed principally
of illite or related minerals. The kaolin clay of extreme southern
Illinois contains the mineral kaolinite. The older underclays also
contain kaolinite, but many of them also contain smaller amounts of
illite.

The Survey also has tested many clays to determine their burning
properties and color when fired, and hence their potential uses. The
bonding capabilities of other clays have been measured to find out
whether they can be used as a bonding material for molding sand. The
bloating properties of Illinois clays and shales from many deposits have
been studied to determine which are suitable for making lightweight
aggregate for the manufacture of concrete.

The object of these studies has been to discover the location,
character, and possible uses of the state’s clay and shale resources.
Special studies are continuing in several parts of the state. Illinois
is well endowed with clays and shales that can be used for a variety of
purposes and has resources to fill future as well as present needs.


                          How Bricks Are Made

Conversion of Illinois clays and shales into useful products is an
interesting process and is exemplified by the making of building bricks.
Mechanical shovels dig the clay or shale and load it into trucks or
small railroad cars that take it to the brick plant. There, machines
grind the raw material and mix water with it until it has the
consistency of stiff mud.

Next, a machine, which operates somewhat like a meat grinder, extrudes a
brick-sized column of clay. As the column moves forward, it is
automatically cut into bricks by wires. The bricks are then dried in
large heated rooms.

    [Illustration: Figure 21—Beehive brick kiln.]

From the driers, the bricks go to huge ovens (kilns) and are heated
until they are hard and have attained the desired color. This is known
as firing or burning the bricks. Temperatures employed are rarely lower
than 1800° F.

Three kinds of kilns are used in Illinois for burning bricks—beehive,
tunnel, and scove. A beehive kiln (fig. 21) has a round base and a
dome-shaped top and somewhat resembles an oversized beehive. Unfired
bricks are stacked in the kiln and the doors are sealed with burned
bricks and clay. Fires are started in hearths or fire boxes in the wall
of the kiln and the heat is circulated into and through the kiln. It
usually takes several days to fire the bricks adequately and let the
kiln cool so that the bricks can be removed.

Tunnel kilns, made from heat-resistant bricks, are actually tunnels big
enough for a man to stand in. The unburned bricks are loaded on flat
steel cars on top of a layer of refractory blocks that protect the steel
from the heat. The cars enter the kiln and heating begins. As they move
through the kiln, they carry the bricks through a firing area, then
through a cooling zone, and finally out into the air.

In some brickyards in the Chicago area, dried unburned bricks are
carefully stacked by machines into piles about 17 feet high, 35 feet
wide, and 115 feet long, which are known as scoves or scove kilns. A
layer of burned bricks that is plastered with clay covers the sides of
the scove. A jet of flame is directed through small tunnels at the base
of the scove, and the heat fires the bricks.

During 1963, more than 325,000,000 bricks were produced by Illinois
brick plants. In the same year, the value of all the clay and clay
products produced in Illinois was nearly $54,000,000. Besides brick and
drain tile, the products of the clay and shale industry of Illinois
include refractory brick, building block and tile, fire-proofing, sewer
pipe, flue liners, stoneware, lightweight bloated burned clay aggregate
for concrete, and a variety of unburned clays for special purposes,
including bonding clay, refractory fireclays, absorbent for use on
garage floors, and litter for animal cages.




                                  PEAT


After the retreat of the last of the great ice sheets from Illinois,
numerous ponds and lakes were left in northern Illinois, especially in
the eastern section. Some of them were soon drained by natural
processes, but others remained. In the shallow water along their shores
grew various plants, chiefly reeds and sedges and, locally, a variety of
moss. As the plants died, their partially decomposed remains were
preserved beneath the water. Ultimately, the ponds and lakes were
overgrown and more or less completely filled by the plants and their
remains, giving rise to peat (fig. 22) bogs.

Some peat bogs have been drained and are now used as farm land. Others
remain and a few of them are the source of peat or humus for
horticultural purposes. Producing operations are located in northeastern
Illinois and in Whiteside County in northwestern Illinois.




                        OTHER MINERAL RESOURCES


In the future, new uses will be made of the Illinois industrial minerals
already discussed. In addition, other mineral resources of the state
that are not now being used may be the bases of new mineral industries.
Some of these minerals are at present too costly to mine because the
deposits are deeply buried or are not sufficiently rich to be worked at
a profit. Others are not convenient to markets, and still others have no
present commercial use. In years to come, however, changes may occur
that will make it practical to mine, process, and use some of these
resources. Furthermore, some other mineral deposits that are now being
utilized in a limited way may have greater future use. The Illinois
Geological Survey continues to study the location, character, and
composition of many such mineral materials and is alert for the
development of new uses. Some of the materials are discussed briefly
below.

    [Illustration: Figure 22—Peat from Kane County showing its fibrous
    nature and remains of plants.]


                          Gypsum and Anhydrite

Gypsum is a mineral that consists of calcium sulfate plus two molecules
of water (CaSO₄·2H₂O). By suitably heating it, the amount of water can
be reduced, and a product called calcined gypsum (plaster of paris)
results. This material changes back to gypsum if mixed with an
appropriate quantity of water. The ability of calcined gypsum to “set”
when water is added makes it important in the manufacture of a variety
of plasters and related products, especially building materials. Gypsum
also is used in cement making and in agriculture.

Anhydrite (CaSO₄) is like gypsum except that it contains no water and
hence cannot be made into plaster of paris. Its uses are limited in the
United States.

Wells that were drilled for oil, water, or coal have encountered gypsum
and/or anhydrite in some parts of south-central Illinois, but the gypsum
and anhydrite are not known to crop out at the surface. A study of
diamond drill cores and well cuttings on file at the Survey showed that
the shallowest gypsum and anhydrite reported occurred at a depth of 470
feet in Madison County. The greatest continuous thickness of gypsum
found was 2 feet; but in one well, over 6 feet of strata was penetrated
that averaged almost 75 percent gypsum. It is possible that thicker
deposits of gypsum might be found if drilling were done especially in
search of it.


                         Feldspar-Bearing Sands

Feldspar is the name applied to a group of minerals that are mainly
silicates of potassium, sodium, and calcium. Various kinds of feldspar
are used industrially in making glass, enamels, pottery, and other
products. All the feldspar now used in Illinois is shipped into the
state. The discovery by the Illinois Survey that some Illinois sands
contain considerable feldspar led Survey geologists and chemists to find
where deposits highest in feldspar occur, what kinds of feldspar they
contain, and whether it could be separated from the sand in which it
occurs. Beach sands, river sands, dune sands, and sands from other kinds
of deposits were studied.

It was found that many sands contain more than 15 percent feldspar and
some as much as 25 percent. Means of separating the feldspar from the
sand are believed to exist, but problems relating to the purity of the
separated spar remain to be solved.


                                 Brines

No salt is now produced in Illinois, but at one time the state was a
major salt producer. Salt works were in operation near Equality, Central
City, Murphysboro, St. John, Danville, and possibly other places. The
salt was obtained by evaporating salt water (brine) that came from
natural springs or from wells. The Equality area was a particularly
important producer of salt in the 1800’s. Discovery elsewhere in the
Middle West of deposits of rock salt and brines that contained more salt
than those of Illinois is said to have been responsible for the
discontinuance of salt making in the state.

No salt beds crop out in Illinois, nor are any known to have been
encountered in the many wells that have been drilled for coal, oil, or
water. However, most oil well drilling encounters brines containing
various amounts and kinds of salts, including the common table salt,
sodium chloride.

For reasons relating to the production of oil, Survey geologists and
chemists have collected and analyzed many samples of Illinois oil field
brines, and data are therefore available on their salt content. No
commercial use is being made of the brines as sources of salt.


                               Oil Shale

Illinois has a large oil-producing industry that obtains oil from wells.
The state also contains beds of shale that yield oil when the shale is
heated.

In order to estimate the present and future importance of the oil shale
resources, the Survey collected and tested more than 100 shale samples
from 41 Illinois counties. A few samples contained more than 25 gallons
of oil per ton of shale, but most contained less than 15 gallons per
ton. A study of the crude oil distilled from selected shale samples
showed it to be somewhat different from the oil that comes from wells.
It could, nevertheless, be made to yield gasoline, fuel oil, and other
products if suitably processed.

The shale strata generally the richest in oil are found above coal
seams, are black, and are sometimes called slate by coal miners. They
are rarely more than 3 feet thick, but they extend over large areas.


                               Sandstone

Sandstone has a long history of use in Illinois. Pioneers built
foundations for their houses and barns and curbs around their wells from
it. Slabs of sandstone were once a popular material for sidewalks, some
of which are still in use. Churches and other sizable buildings have
been constructed from it, and at one time an Illinois sandstone was used
to make grindstones. Except for the St. Peter Sandstone, which was
discussed under “Silica Sand,” the use of sandstone has decreased,
although comparatively small quantities are still used as building
stone.

Most Illinois sandstones may be thought of as a mass of sand whose
grains are more or less firmly cemented together by clay, iron oxide,
and quartz, either singly or in combination, or, less commonly, by
calcite. The grains are particles of various minerals, but most of them
are quartz.

Sandstones are especially common in the hill country of extreme southern
Illinois. The Survey’s investigations in this area revealed that if they
are suitably processed some of the sandstones may have possibilities for
commercial use. Sandstones in other parts of the state also have been
studied, with similar conclusions.


                                 Barite

Barite (barium sulfate, BaSO₄) is a deceptive mineral—it is much heavier
than it looks. Barite found in Illinois is generally white or light
colored, and, although some of it looks rather like white limestone, it
is more than half again as heavy as limestone. Barite’s unusual weight
is responsible for one of its major uses—as a constituent of drilling
muds for the oil industry. These muds are a mixture of clay, water, and
a weighting material such as barite. They are used in various ways in
the drilling of oil wells by rotary drills. Barite also is an important
raw material for the manufacture of chemicals.

Barite is found in Hardin and Pope Counties, the site of the fluorspar
industry. According to studies made by Survey geologists, the barite
occurs both as veins and beds associated with fluorspar, but its
distribution is irregular and the deposits are of limited size. A barite
mine is said to have been worked years ago, and more recently
comparatively small tonnages have been taken from open pits. Future
exploration in southern Illinois may reveal deposits of barite that will
be profitable to mine.


                               Greensand

In some parts of Illinois occur sands or sandstone that contain numerous
grains of the green mineral glauconite. If the sands are not discolored
by iron compounds or other substances, they too have a greenish color
and, therefore, are called greensands. Glauconite varies in composition
but contains potassium, magnesium, iron, aluminum, silicon, and water.
Greensand is said to be used in relatively small amounts as a soil
conditioner and as a water-softening agent.

Greensand is known to occur in the general vicinity of Olmsted in
southern Illinois. Near Oregon in northern Illinois an old quarry
exposed 10 feet of greenish brown sandstone that contains glauconite.
Samples from southern Illinois and from the sandstone at Oregon
contained more than 6 percent potassium oxide.


                                  Marl

In some of the lakes and ponds left by the glaciers lived numerous small
mollusks with calcium carbonate shells. As the animals died, their
shells formed a deposit on the bottom of the lakes and ponds. Certain
plants, especially algae, may have added a mudlike precipitate of
calcium carbonate to the deposits, and varying amounts of clay washed
from the shores mixed with both these materials. The resultant deposit
is called marl. Some marl deposits have peat mixed with them, and peat
also overlies some marl deposits.

Only comparatively small amounts of marl are known to have been dug in
Illinois. One deposit containing many shells and shell fragments, some
of it associated with peat, was worked in southeastern Livingston County
as a source of agricultural liming material. Other deposits have been
reported at other places in northeastern Illinois. The available
information indicates that the marl deposits are likely to be
principally of local importance.


                          Tufa and Travertine

The tufa (fig. 23) and travertine occurring as surficial deposits in
Illinois were formed by springs. The deposits usually occur at or near
the outcrop of a layer of porous water-bearing earth material, such as
gravel, sandstone, or fissured limestone, that is underlain by a
nonporous clay or shale formation. Water moving down through the porous
layer cannot sink through the clay or shale and so is forced to move
laterally. Where valleys have cut into the layers of gravel or rock, the
water emerges as springs.

If the material through which the water has passed is limestone, or
gravel containing limestone as most Illinois gravels do, some of the
limestone is likely to have dissolved in the water. When the water
issues as a spring, conditions may be such as to cause precipitation of
the dissolved limestone as tufa or, more rarely, travertine.

Tufa is generally highly porous and more or less impure, whereas
travertine is harder and less porous.

The middle or lower slopes of bluffs are common sites for
spring-deposited tufa or travertine. Deposits have been seen in various
parts of Illinois, but are not known to have been worked, except in
Calhoun County where small quantities of tufa were produced for use as
agricultural limestone. It is thought that the tufa and travertine
deposits of Illinois are relatively small, but they may be of local
importance.

    [Illustration: Figure 23—Calcareous tufa from Pike County. In
    addition to the large visible pores there are numerous tiny ones.]


                          Pyrite and Marcasite

The mineral pyrite consists of iron and sulfur as the compound iron
sulfide (FeS₂) and, because of its shiny, brassy yellow color, is
sometimes called fool’s gold. Marcasite has the same composition as
pyrite, but its crystals have a different shape and it is often lighter
colored. Both minerals occur in various parts of the state; they are
particularly prevalent in some coal seams, and when so occurring are in
some cases called coal brasses or “sulfur.”

Pyrite and marcasite are used commercially as raw materials for making
sulfuric acid, although sulfur itself is more extensively used for that
purpose. At one time coal brasses recovered during coal-cleaning
operations at a northwestern Illinois coal mine were sold for acid
making. A large quantity of coal brasses probably could be recovered
from such operations at Illinois coal mines.


                                Uranium

Uranium has been sought in Illinois by many people in recent years. The
Geological Survey also carried out a wide search for uranium and
particular attention was paid to certain clays and other rocks in Hardin
County and certain black shales in other parts of the state.

About 200 samples from Hardin County and 175 samples of the shales were
tested by the Survey. No deposits were found that are known to be of the
required richness and quantity.


                                Iron Ore

About the middle of the 19th century two furnaces in Hardin County in
extreme southern Illinois for a time produced iron from local limonite
ores. The ore is said to have occurred as pellets, chunks, and masses
scattered through soil and clay and apparently was of irregular
distribution. Little can be seen of the deposits today. Their extent is
not known, but they are believed to be of limited size.


The Illinois State Geological Survey carries on a continuous program of
research on the industrial minerals and metals of Illinois and their
uses.

In addition to the investigations mentioned in this booklet, many others
have been made or are in progress. Such studies are necessarily a
continuing activity if the full potentialities of Illinois mineral
resources are to be realized, for industry continually demands new raw
materials and changes its requirements for those now used.

The Survey has issued numerous reports that deal with the resources
discussed here and mimeographed lists of these are available upon
request.


If reproduction is made of the material herein, acknowledgment of the
Illinois State Geological Survey is requested.




                          Transcriber’s Notes


—Silently corrected a few typos.

—Retained publication information from the printed edition: this eBook
  is public-domain in the country of publication.

—In the text versions only, text in italics is delimited by
  _underscores_.