Part 2

SHALE is a common and important sedimentary rock composed of compacted clay or mud. It is so fine grained that the minerals forming it generally cannot be identified without the aid of X-ray.

Shales are composed mainly of clay minerals but, like other sedimentary rocks, generally include other minerals. Shales containing calcium carbonate are called calcareous shales. Most shales contain silt or sand particles; if silt or sand is present in large quantity, the rock is called silty shale or sandy shale. If mica minerals are present in quantity in a shale it is called micaceous shale.

The particles of most clay minerals are thin and flat and overlap each other.

Shales have a wide range of colors but most of those in Illinois are gray. A gray, black, or blue-gray color is caused by organic matter in the shale; shades of red, brown, yellow, or green are caused by iron compounds.

Shale is widely distributed in Illinois, especially in “Coal Measures” rocks, and is used in manufacturing bricks, drain tile, building tile, and lightweight aggregate.

CLAY (11)

CLAY is an unconsolidated rock made up of a group of hydrous aluminum silicate minerals, of which chlorite, montmorillonite, kaolinite, and illite are the most abundant. These minerals are formed by the weathering or alteration of other rocks and minerals.

Clays are very fine grained and their minerals have tiny, flat crystals that can be distinguished from each other only by laboratory methods. Although clays may appear to be similar, their compositions vary greatly.

Some clays are white, but most are colored by iron compounds and organic matter. Wet clays have an earthy odor and generally are slick and plastic, but dry clays are relatively hard and are greasy to the touch.

Clays are abundant in Illinois, especially in soils, in shales, and as clay deposits. In Illinois the underclays that occur beneath coal beds are particularly well suited to the manufacture of bricks, pottery, stoneware, and drain tile.

LIMESTONE (12)

LIMESTONE is a sedimentary rock composed of particles of calcite (calcium carbonate). The crystals may range from fine to coarse. Many limestones contain other minerals, such as chert, clay, or sand, and in some places they grade into dolomite (calcium magnesium carbonate).

Many limestones are white or gray. Yellow or brown shades are caused by iron oxide impurities and dark gray to black colors by organic matter.

Limestones form in various ways. Some are deposited when calcium carbonate precipitates from solution; others are formed when the shells or skeletons of organisms such as brachiopods, clams, and corals accumulate on a sea floor. If such fossils are very abundant, the rock is called fossiliferous limestone. Limestone composed of tiny, rounded concretions is called oolite or oolitic limestone.

Limestone effervesces freely in dilute hydrochloric acid, but dolomite must be powdered before it will effervesce. In nature, limestones may be dissolved by percolating water containing weak acid (such as carbonic acid, composed of water and carbon dioxide). At many places in southern and southwestern Illinois such solution of limestones has produced caves and caverns.

Limestone outcrops are abundant in Illinois, especially along the bluffs of the Mississippi, Ohio, and Illinois Rivers.

Limestone has many uses. It is used for building stone, road surfacing, railroad ballast, in the manufacture of portland cement, and, if of high purity, for making lime and chemicals and as a flux in smelting metals. It also is used, as agricultural limestone, to add calcium to the soil.

PEAT (13)

PEAT is produced by the partial decomposition of plants that accumulate, with varying amounts of mineral matter, in old ponds, swamps, and lakes, and in abandoned channels in valley bottoms along many rivers and streams. Peat may be an early stage in the formation of coal.

Peat ranges from light to dark brown, the color, decomposition, and compaction increasing with depth in the deposit. Some of the plant remains are clearly distinguishable and appear as fibrous fragments held together by the fine peat particles. The type of peat common in Illinois has a high water content. Before it is dried it is soft and spongy to the touch; upon drying, it loses much water and becomes harder.

Peat is used as a fuel in some parts of the world, but its heating value is low compared to that of coal. It burns with a long flame and leaves a great deal of ash because of the silt and sand that were buried with the vegetation. Peat and peat moss are used chiefly as an absorbent, as stable litter, as insulating and packing material, and by gardeners to increase the water-holding capacity of soils.

Peat is found in many places in Illinois but the largest deposits are in northeastern Illinois.

COAL (14)

COAL, an organic stratified rock, is formed from accumulated plant material and partially decayed plants that were buried during the “Coal Measures” period in Illinois more than 200 million years ago.

Sediments deposited over the peat-like organic material compacted it. Chemical changes gradually took place and resulted in the loss of water and gases, leaving a higher percentage of carbon than the original material contained.

The amount of such change that has taken place determines the rank of the coal. The lowest ranks are called lignite, the intermediate group is called bituminous (soft) coal, and coals of the highest rank, with the highest carbon content, are called anthracite (hard coal). Mineral matter, such as shale, clay, or pyrite, generally is present in the coal and becomes ash when the coal is burned.

Most coal mined in the United States is bituminous coal. It is black, brittle, breaks into angular blocks, has a shiny luster, and generally shows a banded structure.

Coal mining is an important industry in Illinois, and the state contains the largest known reserves of bituminous coal in the United States. Minable coal beds underlie about two-thirds of the state. As many as 20 different coal beds have been mined in Illinois, the most important being the Herrin (No. 6) and the Springfield (No. 5) or Harrisburg (No. 5). The coal in most mining areas averages 5 to 7 feet thick and in places attains a thickness of 15 feet.

In underground mines the coal is approached by vertical or inclined shafts. In open cut, or strip, mines all of the overlying material (overburden) is removed, leaving the coal exposed. Coal as much as 100 feet deep is now being strip mined in Illinois.

Illinois coal is now used mainly for generating electric power, for industrial purposes, and for heating. In industry, coal is used extensively for power, heating, burning cement, firing clay products such as brick, tile, pottery, porcelain, and china, and making coke.

Certain Illinois coals when mixed with coal from the eastern part of the United States, produce metallurgical coke for making steel. Gases, oils, and tars derived in processing coal for coke have been used for making many chemical products, including dyes, perfumes, explosives, medicines, insecticides, plastics, and road tar.

PETROLEUM (15)

PETROLEUM (crude oil) is classed as a mineral resource although it is a liquid hydrocarbon and not technically a mineral or rock. It is, however, found in the pores and fractures of rocks. The color of crude oil ranges from yellow through green and brown to black.

Arch (anticline) Gas Porous rock Nonporous rock Oil saturated rock Water Break (fault) Nonporous rock Oil saturated porous rock Water saturated porous rock Trap formed by change in character of rock Water Nonporous rock Porous rock

Petroleum had its origin in the plants and animals buried in ancient sediments. The organic matter changed slowly into the complex mixture of hydrogen and carbon compounds that is petroleum.

Because gas is lighter than oil, and oil in turn lighter than water, gas and oil move upward in a porous rock containing all three. Gas moves to the highest position with oil next below and water in the lowest part of the rock. Oil pools exist where geologic barriers have stopped the movement of gas and oil.

Arches (upward folds or anticlines), breaks (faults), and lateral changes from porous to nonporous rock are geologic features that serve to localize oil pools within the reservoir rock.

Petroleum is obtained by drilling wells into the reservoir rock. Gas that is free or dissolved in the petroleum expands as pressure is released when the well is drilled and drives the oil to the well. Water in the reservoir rock also acts as a driving force. When this natural pressure is no longer effective, other methods (secondary recovery) are used to recover the oil remaining in the reservoir.

Porous sandstones and limestones are the oil-bearing rocks. Illinois also has deposits of oil shale from which petroleum may be produced in the future.

Thousands of products are derived from petroleum, including gasoline, kerosene, naphtha, lubricating oils and waxes, medicinal oils, salves, heavy fuels, road oils, tar, and asphalt.

GLACIATED PEBBLES (16)

GLACIATED PEBBLES are small stones whose shapes have been altered by the grinding action of a glacier. Such pebbles commonly have at least one flattened side that shows scratches (called striae). The striae were produced when the pebbles were pushed over bedrock or ground against other pieces of rock.

Glaciers tore fragments from the bedrock over which they moved and the fragments accumulated in, on, and under the mass of ice. The rock fragments were transported, some of them far from their source, and were deposited as the glacier moved along or when the ice melted.

Soft rocks like limestone and dolomite are easily scratched, but soft rocks cannot make grooves in hard igneous and metamorphic rocks. Therefore soft rocks have more striae.

Glaciated pebbles can be found in deposits in many parts of the state, especially in northeastern and east-central Illinois. However, many deposits near the surface have been weathered and striae have been destroyed. An especially good place to look for striated pebbles is in quarries and strip mines where glacial drift overburden has been removed. Pebbles found in such deposits show good striae for they are but little weathered.

SILICA SAND (17)

SILICA SAND is the commercial name for sand composed almost entirely of grains of quartz. Sand of this kind is mined in Illinois from the St. Peter Sandstone in LaSalle and Ogle Counties. The coarser grains of the sand are characteristically rounded and frosted. The frosting causes the sand to look white.

Illinois silica sand has many uses. It is used just as it is mined for molding sand in which metal castings are made, for lining industrial furnaces, and for many other purposes. Some of the sand is washed to remove the small amount of impurities present. The washed sand is used for such purposes as making glass, for grinding plate glass smooth, for sand blasting, for molding metal, as fracturing sand to increase the production of oil wells. Some silica sand is ground to a fine powder and used as a fine abrasive, as a filler in paint, and as an ingredient in pottery, glazes, and enamel.

A specially prepared St. Peter sand, known throughout the world as Standard Ottawa Testing Sand, is used to test the strength of cements and as a laboratory standard in physical tests of other sands.

MOLDING SAND (18)

MOLDING SAND is a mixture of sand and clay or other bonding material and is used to make molds in which metal is cast into various useful shapes. There are two kinds of molding sands, synthetic and natural bonded. The first is an artificial mixture of silica sand and clay; the second is a naturally occurring mixture of sand and a bonding material.

For casting, molding sand is first moistened with water and shaped into a mold of the metal part to be reproduced. The mold is then allowed to dry and the molten metal poured into it. The bonding material in the sand must be strong enough to keep the mold in shape during these operations.

Natural bonded molding sand is produced in Fayette, Bond, Bureau, Carroll, Kankakee, and Rock Island Counties, Illinois.

TRIPOLI (19)

TRIPOLI, called amorphous silica in southern Illinois, is a white or light brown, powdery substance that rubs off on the hands like chalk. It consists mostly of very small particles of quartz that result from the weathering of calcareous chert or highly siliceous limestone.

It is finely ground and used as “white rouge” for polishing optical lenses, as a filler in paints, in making ceramic products, as a component of buffing compounds, and as a fine abrasive.

Tripoli occurs in Alexander and Union Counties and is milled at Elco and Tamms in Alexander County.

FULLER’S EARTH (20)

FULLER’S EARTH is clay or silty clay material that contains very fine silica. It is soft, nonplastic, opaque, has a greasy feel when wet, and does not readily break up in water. Its color varies from blue-gray to yellow or buff.

Fuller’s earth is valuable for its unique property of absorbing and decolorizing substances. The material was first used to “full” or remove grease from woolen cloth, hence its name. It also has been used to filter and bleach mineral and vegetable oils by absorbing dark organic matter.

In Pulaski County in extreme southern Illinois the Porter’s Creek Formation contains deposits of clay that were at one time the source of fuller’s earth and still afford clay whose absorbent properties make it useful as litter and as sweeping and cleaning compound.

QUARTZ (21)

QUARTZ is the most common of all minerals, making up about 12 percent of the earth’s crust. There are two main types of quartz—crystalline quartz and dense, crypto-crystalline (submicroscopic) quartz. Many dense varieties occur in Illinois, probably the most common are chert or flint.

Crystals of quartz are typically six-sided, elongated, have sharply pointed pyramid-like ends, and are apt to grow together forming twins. Good crystals are rare in Illinois, and the crystal structure is not apparent in the commonly occurring grains and masses.

Quartz is brittle and hard. It may be colorless or tinted, transparent or translucent, but more commonly it is white and nearly opaque. Transparent quartz looks much like ordinary glass, but it scratches glass easily. It has a glassy to brilliant luster and breaks irregularly or with a good conchoidal fracture.

Some varieties of quartz that are used for semiprecious gems are chalcedony, agate, onyx, and jasper. Chalcedony is waxy, smooth, generally translucent, white to gray, blue, brown, or black. Agate is a form of chalcedony that has a mottled or variegated banded appearance and may be yellow, green, red, brown, blue, gray, or black. Onyx is agate with parallel bands that as a rule are brown and white or black and white. Jasper, an impure opaque quartz, generally is red.

Quartz occurs as rock crystal (colorless, transparent), milky quartz (white, nearly opaque), and smoky quartz (smoky yellow to gray or brown) in geodes from the Warsaw and Keokuk Limestones of the Nauvoo-Hamilton-Warsaw area and as vein and cavity fillings associated locally with fluorite, sphalerite, and galena in extreme southern Illinois. It also occurs as vug (cavity) fillings in limestones and sandstones.

FELDSPAR (22)

FELDSPAR is the name applied to a group of minerals that are the second most common of all the earth’s minerals. All feldspars are composed of aluminum, silicon, and oxygen, combined with varying amounts of one or more metals, particularly potassium, sodium, calcium, and lithium.

The minerals are hard, have a smooth glassy or pearly luster, and cleave along two planes nearly at right angles to each other. Feldspars are fairly light weight. The streak is white, but the color of the mineral is highly variable, although potassium and sodium-bearing feldspar are commonly white or pink and most plagioclase feldspar is gray.

Feldspars are essential parts of the crystalline igneous rocks. Their decomposition products are present in most soils. In Illinois relatively small feldspar crystals can be found associated with quartz and other minerals in granite and gneiss boulders and pebbles in glacial drift.

MICA (23)

MICA is the name of a family of complex aluminum silicate minerals that can be split easily into paper-thin, flexible sheets. If broken across the grain at right angles to the flat, smooth surface they fracture raggedly. In a single mica crystal the sheets range from more or less transparent to translucent and are arranged one on top of another like a deck of cards.

Micas are tough and somewhat elastic, soft enough to be split and scratched by a fingernail, and are light weight. They have a nonmetallic, glassy or pearly luster, although yellow mica may appear to be metallic. Color and streak depend upon the chemical composition of the mineral. Muscovite, or white mica, contains potassium and makes a colorless or white streak. Biotite, or black mica, contains iron and magnesium and is commonly dark green or black, although it may be shades of yellow or brown; its streak is uncolored.

Mica is abundant as tiny, shimmering flakes in Illinois sands, sandstones, and shales (which are then said to be micaceous). It also is common in many varieties of igneous and metamorphic rocks. White or yellow flakes may show a brilliant luster and may be mistaken for silver, platinum, or gold, but those minerals are heavy and malleable whereas mica is not.

CALCITE (24)

CALCITE, a common rock-forming mineral, consists of calcium carbonate. The mineral is white or colorless, but impurities may tint it shades of yellow or gray. Transparent calcite is more rare than the tinted varieties.

Transparent calcite possesses the property of double refraction; an image appears double when viewed through a calcite cleavage block.

Calcite has a glassy luster, its streak is white or colorless. The mineral is of medium hardness and can be scratched by a penny or a piece of window glass but not by the fingernail. It is fairly light weight and effervesces freely in cold dilute hydrochloric acid.

Calcite has a variety of crystal forms but in Illinois flattened block-shaped crystals and elongate crystals with tapering points (“dogtooth spar”) are the most common. When broken, calcite cleaves into six-sided blocks called rhombs.

Crystals of calcite are found in Illinois as linings in geodes in certain limestones and shales, especially in the Nauvoo-Hamilton-Warsaw area, and as crystalline masses in limestone and dolomite. Small amounts of clear crystalline calcite are associated with various ores in northwestern and extreme southern Illinois.

Calcite is the principal mineral in limestones and occurs as a component of many concretions.

FLUORITE (25)

FLUORITE, or fluorspar, is made up of the elements calcium and fluorine. The mineral is easily identified by its perfect cleavage, color, and hardness.

It occurs in cubic crystals that may be twinned but is more often found as irregular masses. It can be split into diamond-shaped, eight-sided forms (octahedrons). Fluorite is commonly gray, white, or colorless, but it may be green, blue, purple, pink, or yellow. The streak is colorless and the luster glassy. It can be scratched by a knife or a piece of window glass, is fairly light weight, and is transparent to translucent.

Extensive deposits of fluorite, one of Illinois’ important commercial minerals, occur in Hardin and Pope Counties in extreme southern Illinois, where it is associated with galena, sphalerite, calcite, barite, and other less abundant minerals.

Fluorite is used to make hydrofluoric acid, to form a fluid slag in the production of iron and steel, in the manufacture of aluminum, to make many chemical products, and in the ceramic industry, in which it is used to make colored glass, enamels, and glazes.

GYPSUM (26)

GYPSUM, hydrous calcium sulfate, is a colorless, transparent to translucent mineral when pure, but it often is stained yellow by impurities. It has a white streak, is soft enough to be scratched by a fingernail, and is light weight.

Gypsum occurs in several forms. Selenite is a coarsely crystalline, transparent variety, composed of flat, nearly diamond-shaped crystals that can be split easily into thin sheets, have a glassy luster, and often grow together to form “fishtail twins.” Crystals of selenite occur in shales of the “Coal Measures” of southern, north-central, and western Illinois, and can be picked up at the surface.

Satin spar has crystals like silky threads closely packed together, splits parallel to the fibers, and is found as fillings in rock cracks and as thin layers in shales. Massive gypsum is granular.

Gypsum deposits occur deep underground in Illinois but thus far have not been mined.

CHERT (27)

CHERT, one of the main forms of silicon dioxide, is cryptocrystalline (submicroscopic) quartz. Most of the chert in Illinois is white or gray, but impurities stain many deposits yellow, brown, or even pink. Chert is so hard that it can scratch glass and ordinary steel. It is fairly light weight, dense, opaque, and brittle; the luster is dull.

Flint, a variety of chert, is generally dark colored, more dense, may have a glassy luster and be transparent in thin flakes. Both chert and flint have a smooth, curved (conchoidal) fracture, but flint tends to break with thinner, sharper edges. Indians used flint and chert to make arrow points and spearheads.

Chert occurs as rounded masses (nodules and concretions) or as irregular layers in limestones and dolomites in northern, western, southwestern, and southern Illinois. Because chert is hard and more resistant to weathering than limestone or dolomite, it often remains after the rest of the rock has weathered away.

Chert also is abundant in many glacial deposits because it is hard and resists solution. Streams that flow through cherty bedrock or glacial deposits carry pebbles along and concentrate them as gravel in stream channels. Cherty stream gravels are especially abundant in western and southern Illinois.

Brown chert gravels in the southern part of the state are used for road gravel. Other deposits in extreme southern Illinois, consisting of angular fragments of chert and a small amount of clay (known locally as novaculite gravel) also are used for road surfacing.

White and gray chert occur as massive bedrock deposits several hundred feet thick in Union and Alexander Counties.

PYRITE AND MARCASITE (28)

PYRITE and MARCASITE are iron disulfide compounds. They look much alike but have different crystal forms. Both are brittle, hard, brassy yellow with metallic luster, and opaque. The best distinguishing feature is crystal shape. The pyrite crystals are cubes, but the marcasite crystals are blade- or needle-shaped.

Pyrite and marcasite have been mistaken for gold because they are yellow and metallic and hence are sometimes referred to as “fool’s gold”. They, however, are harder than gold, tarnish, and leave a dark streak, whereas gold is soft, very heavy, does not tarnish, and leaves a yellow streak. Gold is malleable, but pyrite or marcasite are reduced to powder if they are pounded and give off a noticeable odor of sulfur dioxide gas if they are heated.

Pyrite and marcasite are found in many deposits in Illinois. They occur as grains or larger masses in some clays, shales, and limestones. They also occur with the lead and zinc ores of northwestern Illinois and, in small amounts, with the fluorite and associated minerals in the extreme southern part of the state.