Part 3

Both pyrite and marcasite are common as surface coatings, veins, and concretionary structures in coal and in dark shales associated with coal. They are referred to as “coal brasses” or “sulfur” when found as impurities in coal.

A potential use for pyrite and marcasite is in the manufacture of sulfuric acid for industrial use. Coal brasses recovered from Illinois coal have been so used.

LIMONITE (29)

LIMONITE is an iron oxide containing water and has a complex chemical composition. The limonite found in Illinois may be yellow, orange, red, brown, or black, but its streak is always yellowish brown. The mineral may have a glassy or an earthy luster. It may be too hard to be scratched by a knife. It is of medium weight.

Limonite is common and occurs as concretions and cavity fillings in sedimentary rocks, and as coatings on them, especially sandstone. It also occurs as iron rust, as scum on stagnant water, and it accumulates around rootlets in soils. Small amounts color limestone, dolomite, clay, shale, sandstone, and gravel. Some sands are firmly cemented by brown or black limonite and look much like iron ore. Clays containing a high percentage of limonite are called ocher.

In some states limonite is mined as an iron ore, and in Illinois it was so used in Hardin County in the middle 1800’s, but deposits are not large enough for profitable use now.

SPHALERITE (30)

SPHALERITE, zinc sulfide, is a major ore of zinc. It has a resinous luster and a white, yellow, or brown streak. Illinois sphalerite is generally yellow, yellowish brown, reddish brown, or brownish black. It is of medium weight, brittle, can be scratched by a piece of window glass but not by a penny. It is commonly opaque but may be translucent on thin edges.

Sphalerite is mined with galena in northwestern Illinois and in extreme southern Illinois with galena and fluorite. Small crystals occasionally are found in limestones and as crystalline masses in clay-ironstone concretions.

GALENA (31)

GALENA, lead sulfide, is the principal ore of lead. It is steel gray, heavy, opaque, and has a bright metallic luster, though the shiny surface may be dulled by a coating of lead carbonate. It has a gray or black streak, is soft enough to mark paper, and can be scratched by a penny. The cube-shaped crystals readily break into cubic, right-angled fragments. Probably the most obvious features of the mineral are its bright metallic luster on fresh surfaces, high specific gravity, and cubic cleavage.

At many places galena is argentiferous (silver-bearing), but Illinois galena is relatively unimportant for its silver content. As a source of lead, however, it is an important commercial product of the state.

Scattered pieces of galena are found at many places in Illinois. Some occur in the glacial deposits, others occur as small pockets and as crystals in limestones and geodes. In only two areas of the state are deposits of commercial value. In northwestern Illinois galena occurs in association with sphalerite; in extreme southern Illinois it occurs in association with fluorite and sphalerite.

CONCRETIONS (32)

CONCRETIONS are concentrations of inorganic sedimentary material within other sediments. Minerals that commonly form concretions are silica (in the form of opal, chert, chalcedony, and quartz), calcite, siderite, pyrite, marcasite, and limonite.

Concretions may form either as the sediment around them is forming or after the sediment around them has hardened. They may be formed when water containing dissolved minerals seeps through the sediment or rock and leaves a concentration of mineral matter in a cavity or around a central particle (nucleus) such as the remains of a plant or animal. Portions of rock may also become firmly cemented by such mineral matter.

Concretions range in size from minute particles to objects several feet in diameter. Shapes range from spheres to tubes. Many are globular or lumpy-surfaced, some are smooth. Because concretions generally are harder than the surrounding rock in which they have formed, they do not weather away as readily and may remain after the surrounding material has been eroded.

Concentrations of calcite are found in loess deposits. They may look like bizarre, knobby figurines, and the Germans called them loess kindchen (little children of the loess).

Ironstone concretions, especially common in many Illinois shales, are formed by a local concentration of the mineral siderite (iron carbonate) in the rock. The concretions found in weathered outcrops commonly are partly or entirely weathered to limonite. Some ironstone concretions grow together into odd shapes. Mazon Creek ironstone concretions of northeastern Illinois, world famous for their fossils, are sideritic. The concretions are commonly covered with limonite, the result of oxidation.

Limonite concretions, generally with a high content of clay, silt, or sand, occur in loess, shale, and sandstone.

Concretions of chert and other forms of silica are common in limestones. In many places, because of their greater resistance to weathering, lenses and nodules of chert protrude from the beds.

Pyrite or marcasite occur as concretions or concretion-like masses in some coal beds and in the black shales, sometimes popularly called “slates,” above coal beds. Some other Pennsylvanian clays and shales also contain concretions or coarsely crystalline aggregates of these minerals.

GEODES (33)

GEODES are roughly spherical bodies that may be filled with layers of minerals, lined with crystals, or both. The outer layer of geodes found in Illinois as a rule is composed of chalcedony, a form of finely crystalline silica.

Geodes differ from concretions in that they form inward from the outer shell, whereas concretions develop outward from a center. Even if geodes have been completely filled by mineral matter, their inward-projecting crystals prove that they formed within a cavity.

In a partly filled cavity, crystals generally are well formed because they grew without being crowded. Some of the best mineral specimens known in Illinois are found as crystal linings in geodes.

Quartz is the most common mineral deposited in geodes, but calcite, aragonite, dolomite, siderite, pyrite, galena, fluorite, and sphalerite also are found.

Geodes ranging in size from less than one inch to a foot or more in diameter can be gathered from streams where they have accumulated as residual boulders after the rock in which they were enclosed has been eroded.

Hollow geodes are the most desirable because they have better crystals. They can be distinguished from solid ones by their comparative lightness of weight.

Geodes are commonly associated with limestone and dolomite, at some places with shale. In Illinois they can be found most easily in the Warsaw Formation in the area of Nauvoo, Hamilton, and Warsaw, but they also occur in other areas and other formations.

ANIMAL FOSSILS (34)

Prehistoric animals lived in water, on land, and in the air, and left both direct and indirect evidence of their existence, evidence we now call fossils.

Millions of ancient animals died without leaving a trace, but some, especially those that had hard parts such as shells, bones, or teeth, may be found preserved in rocks much as they were when buried beneath sediment on the floor of an ancient sea. Sometimes only imprints of the outside or fillings of the inside of the shells remain, the original material having been completely dissolved. Footprints of land or amphibious animals, burrows made by clams, or holes made by worms also are fossils.

The animals whose remains are fossilized lived and died while the sediments that contained them were being deposited, and they provide clues to the types of life and climate then existing. Fossils of animals characteristic of a certain time are an index to the age of formations where they occur. For example, if a certain trilobite (an ancient relative of the crayfish and lobster) is known to have lived only during a definite time, then all rocks in which it is found are the same age.

Fossils of animals that lived in the sea are exposed in rocks in many parts of Illinois, especially in quarries, river bluffs, and road cuts.

The oldest fossils found in Illinois are shells of marine animals—snails, corals, crinoids, brachiopods, trilobites, pelecypods (clams), cephalopods, bryozoa, arthropods, and others. The youngest fossils are teeth and bones of prehistoric bison, giant beavers, deer, mammoths and mastodons of the “Ice Age,” and snails found in glacial loess.

PLANT FOSSILS (35)

PLANT FOSSILS are the remains of prehistoric plants. Woody structures of plants aid preservation just as hard parts of animals do. Leaves and plants without much woody material generally were well preserved only if they were buried quickly in fine, soft sediment.

The most famous Illinois plant fossils are those from the Mazon Creek area in Grundy and Will Counties of northeastern Illinois. The plant material acted as a nucleus around which iron minerals accumulated to form concretions. Many good fossils—of trunks, branches, leaves, and seeds—are found in coals and in shale directly overlying coals. Descendants of “Coal Measures” plants, such as ferns, mosses, and rushes, are still living today, but they no longer thrive as they did in the warm, moist climate of the Pennsylvanian forests.

Some plants of Pennsylvanian age are petrified, and occasionally such trees or stumps are found. Petrified trees are found also in the upper Mesozoic deposits of southern Illinois. Fossils of “Ice Age” plants closely related to forms living at the present time are occasionally found in peat bogs or scattered throughout glacial deposits.

KEYS FOR IDENTIFICATION OF COMMON ILLINOIS ROCKS AND MINERALS

Two keys, one for minerals and one for rocks, briefly present clues that may aid the collector in identifying rocks and minerals found in Illinois. In outline form, the keys are a guide to some of the easily observable properties that various rocks and minerals display.

The rocks and minerals in the school set of “Typical Rocks and Minerals of Illinois” are included, plus other relatively common ones you might find in Illinois. Because of the great diversity of rocks and minerals in this state, the keys are not conclusive. It is therefore suggested you consult other more complete keys (such as that in Dana’s _Manual of Mineralogy_) when identifying rocks and minerals that are either from other states or are difficult to identify.

The minerals (p. 30-35) are arranged in two groups: 1) those with a metallic luster, and 2) those with a nonmetallic luster. Each group is arranged according to increasing hardness. Other characteristics such as color, streak, cleavage, fracture, and composition are listed.

The rocks (p. 36-39) are arranged according to their reaction to dilute hydrochloric acid applied to a scratched surface. (The acid reacts more readily to powdered material produced by scratching the rock.) After the reaction to acid has been determined, the texture and components of the rock should be noted. Because rocks grade into one another, clear distinctions are not always possible.

MINERAL IDENTIFICATION KEY

I. METALLIC LUSTER, STREAK COLORED C—color H—hardness Remarks Name and S—streak Cl—cleavage composition F—fracture A. Hardness not more than 2.5

C—lead gray H—2.5 Very heavy; occurs as Galena (31) S—black Cl—cubic; crystals, grains, or PbS perfect in 3 masses; easily directions identified by color and F—subconchoidal cleavage or even C—copper red H—2.5 Very heavy; apt to have Native copper S—metallic, Cl—none green coating; distorted Cu shiny F—jagged or wirelike forms; malleable

B. Hardness greater than 2.5 but not greater than 6.5

C—yellow-brown H—5.5 (may be as In earthy masses; Limonite (29) to black low as 1) coloring material in FeO(OH)·H₂O S—yellow-brown Cl—none many sandstones, F—uneven conglomerates, and soils; often mixed with and difficult to distinguish from goethite and other iron minerals C—brassy yellow H—6 As compact masses, Pyrite (28) S—greenish black Cl—poor grains, cubes, and in 8- FeS₂ F—conchoidal to and 12-sided crystals; uneven commonly associated with coal, and with lead-zinc ores of northwestern Illinois C—pale brassy H—6 As fibrous, radiating, Marcasite yellow to Cl—poor tabular, and cocks-comb FeS₂ (28) silver white F—uneven crystals or compact S—greenish gray masses; usually lighter colored than pyrite, but difficult to distinguish from pyrite; associated with coal, and with lead-zinc ores of northwestern Illinois

II. NONMETALLIC LUSTER, STREAK WHITE A. Hardness not greater than 2 (can be scratched by fingernail)

C—usually white H—2 Commonly found in Gypsum (26) but may be Cl—perfect in Illinois as twinned or CaSO₄·2H₂O almost any color one direction, needle-shaped crystals less perfect in in weathered shales two others containing pyrite and calcium carbonate C—white or a H—2 As needle-shaped Melanterite shade of green crystals or powdery FeSO₄·7H₂O coating on pyrite or marcasite; has an astringent taste

B. Hardness greater than 2 but not greater than 3 (Can be scratched by a penny)

C—colorless, H—2-2.5 In scales or “books”; in Muscovite silver white, Cl—perfect in splits into thin sheets; (white mica) gray, brown one direction common in sandstones, (23) shales, and in igneous (OH)₂KAl₂ and metamorphic rocks AlSi₃O₈ C—brown or black H—2.5-3 As scales or “books”; Biotite (23) Cl—perfect in splits into thin sheets; (black mica) one direction common in igneous and (OH)₂K(Mg,Fe)₃ metamorphic rocks but AlSi₃O₈ not in sedimentary rocks such as sandstone or shale C—colorless, H—3 Common mineral; Calcite (24) white, gray, Cl—perfect in effervesces vigorously CaCO₃ and various three in cold acid; occurs in tints directions, not many crystal forms and at right angles as fibrous, banded, and (rhombohedral) compact masses; chief mineral in limestones C—white, gray, H—3 Very heavy; commonly in Barite red, or almost Cl—perfect in tabular crystals united BaSO₄ any color one direction, in diverging groups, as less perfect in laminated or granular two other masses; associated with directions fluorite in southern Illinois

C. Hardness greater than 3 but not greater than 5 (Cannot be scratched by penny; can be scratched by knife)

C—white, gray, H—3.5 Relatively heavy; Witherite light yellow Cl—in one effervesces in acid; BaCO₃ direction associated with fluorite F—uneven and barite in southern Illinois but is not abundant C—white, pink, H—3.5 In grains, rhombohedral Dolomite gray, or light Cl—perfect in crystals and cleavable CaMg(CO₃)₂ brown three or granular masses; directions, not effervesces slowly in at right angles cold acid when powdered, (rhombohedral) more vigorously in warm acid; principal mineral in rock called dolomite C—colorless, H—3.5 In fibrous or compact Cerussite white, gray, masses or may be in PbCO₃ grayish black orthorhombic crystals as a coating on galena; very heavy; effervesces in acid; formed by alteration of galena C—brown to gray H—3.5 In fibrous or botryoidal Siderite S—usually white Cl—in three masses or rhombohedral FeCO₃ but may tend directions not crystals; effervesces in toward brown at right angles hot acid when weathered (rhombohedral) slightly curved surfaces C—yellow, H—3.5 In crystals, in fibrous Sphalerite yellow-brown to Cl—parallel to or layered masses; ZnS (30) almost black dodecahedral associated with galena S—light yellow faces; in six in northwestern to brown directions Illinois, with fluorite and galena in southern Illinois C—colorless, H—4 In cubes and cleavable Fluorite (25) white, yellow, Cl—perfect, masses; many colors; (Fluorspar) purple, green, parallel to mined in Hardin and Pope CaF₂ blue octahedral counties faces; in four directions C—white, tinted H—5 As crystalline Smithsonite yellow, blue, incrustations or in ZnCO₃ or green earthy or compact masses; associated with fluorite-sphalerite ores in southern Illinois, with galena and sphalerite in northwestern Illinois

D. Hardness greater than 5 but not greater than 7

C—white, green, H—5-6 In long, slender 6-sided Amphibole Group brown, black Cl—in two crystals; cleavage angle (Mg,Fe,Ca)₇ directions important in (Si₈O₂₂)(OH)₂ intersecting at differentiating from (may also about 60° and pyroxenes; common in contain Na or 120° metamorphic and some Al) igneous rocks C—gray, dark H—5-6 Crystals short, stout, Pyroxene Group green, black, Cl—in two and 8-sided; cleavage (Mg,Ca,Fe)₂ dark brown, directions angle important in (Si₂O₆) bronze intersecting at differentiating from about 90° amphiboles; common in igneous and some metamorphic rocks C—white, gray, H—6 As crystals, cleavable Feldspar Group pink, light Cl—in two masses and grains; (22) blue, green directions common in igneous and K, Na, Ca, Ba nearly at right metamorphic rocks, also (Al, Si)₄O₈ angles in stream gravel and sand; many varieties C—white when H—7 Finely crystalline Chalcedony pure; may be Cl—none variety of quartz; SiO₂ colored by F—conchoidal botryoidal or impurities concretionary masses; lining in geodes C—colorless, H—7 Most abundant mineral; Quartz (21) white, or F—conchoidal occurs in 6-sided SiO₂ almost any color crystals capped by pyramids, in grains or masses; principal mineral in sandstone, also abundant in igneous and metamorphic rocks; is a variety of silica C—red H—7 A variety of quartz Jasper F—conchoidal usually colored red by SiO₂ hematite inclusions; common in glacial and river sand and gravel found along Lake Michigan shores and in the Mississippi River C—many; H—7 Cloudy banded variety of Agate arranged in F—conchoidal silica; widely used as SiO₂ bands semi-precious stones. Onyx and silicified wood are forms of agate; found in glacial gravels and upper Mesozoic sediments in southern Illinois

E. Hardness greater than 7 (cannot be scratched by quartz)

C—red, brown, H—7.5 Irregular grains or Garnet Group yellow, green, Cl—poor masses; sometimes as (Ca,Mn,Fe,Mg)₃ black, white F—even 12-, 24-, and 36-sided (Al,Cr)₂(SiO₄)₃ crystals; abundant in glacial sands and Lake Michigan beach sands; common in metamorphic rocks

ROCK IDENTIFICATION KEY

SAMPLE Scratch with a knife and apply dilute acid (HCl) If rock does not scratch, go directly to I, II, or III