Guide to Rocks and Minerals of Illinois

KEYS FOR IDENTIFICATION OF COMMON ILLINOIS ROCKS AND MINERALS

Reading edition

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 S—streak H—hardness Cl—cleavage F—fracture Remarks Name and composition

A. Hardness not more than 2.5

C—lead gray S—black H—2.5 Cl—cubic; perfect in 3 directions F—subconchoidal or even Very heavy; occurs as crystals, grains, or masses; easily identified by color and cleavage Galena (31) PbS

C—copper red S—metallic, shiny H—2.5 Cl—none F—jagged Very heavy; apt to have green coating; distorted or wirelike forms; malleable Native copper Cu

B. Hardness greater than 2.5 but not greater than 6.5

C—yellow-brown to black S—yellow-brown H—5.5 (may be as low as 1) Cl—none F—uneven In earthy masses; coloring material in many sandstones, conglomerates, and soils; often mixed with and difficult to distinguish from goethite and other iron minerals Limonite (29) FeO(OH)·H₂O

C—brassy yellow S—greenish black H—6 Cl—poor F—conchoidal to uneven As compact masses, grains, cubes, and in 8- and 12-sided crystals; commonly associated with coal, and with lead-zinc ores of northwestern Illinois Pyrite (28) FeS₂

C—pale brassy yellow to silver white S—greenish gray H—6 Cl—poor F—uneven As fibrous, radiating, tabular, and cocks-comb crystals or compact masses; usually lighter colored than pyrite, but difficult to distinguish from pyrite; associated with coal, and with lead-zinc ores of northwestern Illinois Marcasite FeS₂ (28)

II. NONMETALLIC LUSTER, STREAK WHITE

A. Hardness not greater than 2 (can be scratched by fingernail)

C—usually white but may be almost any color H—2 Cl—perfect in one direction, less perfect in two others Commonly found in Illinois as twinned or needle-shaped crystals in weathered shales containing pyrite and calcium carbonate Gypsum (26) CaSO₄·2H₂O

C—white or a shade of green H—2 As needle-shaped crystals or powdery coating on pyrite or marcasite; has an astringent taste Melanterite FeSO₄·7H₂O

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

C—colorless, silver white, gray, brown H—2-2.5 Cl—perfect in one direction In scales or “books”; in splits into thin sheets; common in sandstones, shales, and in igneous and metamorphic rocks Muscovite (white mica) (23) (OH)₂KAl₂ AlSi₃O₈

C—brown or black H—2.5-3 Cl—perfect in one direction As scales or “books”; splits into thin sheets; common in igneous and metamorphic rocks but not in sedimentary rocks such as sandstone or shale Biotite (23) (black mica) (OH)₂K(Mg,Fe)₃ AlSi₃O₈

C—colorless, white, gray, and various tints H—3 Cl—perfect in three directions, not at right angles (rhombohedral) Common mineral; effervesces vigorously in cold acid; occurs in many crystal forms and as fibrous, banded, and compact masses; chief mineral in limestones Calcite (24) CaCO₃

C—white, gray, red, or almost any color H—3 Cl—perfect in one direction, less perfect in two other directions Very heavy; commonly in tabular crystals united in diverging groups, as laminated or granular masses; associated with fluorite in southern Illinois Barite BaSO₄

C. Hardness greater than 3 but not greater than 5

(Cannot be scratched by penny; can be scratched by knife)

C—white, gray, light yellow H—3.5 Cl—in one direction F—uneven Relatively heavy; effervesces in acid; associated with fluorite and barite in southern Illinois but is not abundant Witherite BaCO₃

C—white, pink, gray, or light brown H—3.5 Cl—perfect in three directions, not at right angles (rhombohedral) In grains, rhombohedral crystals and cleavable or granular masses; effervesces slowly in cold acid when powdered, more vigorously in warm acid; principal mineral in rock called dolomite Dolomite CaMg(CO₃)₂

C—colorless, white, gray, grayish black H—3.5 In fibrous or compact masses or may be in orthorhombic crystals as a coating on galena; very heavy; effervesces in acid; formed by alteration of galena Cerussite PbCO₃

C—brown to gray S—usually white but may tend toward brown when weathered H—3.5 Cl—in three directions not at right angles (rhombohedral) slightly curved surfaces In fibrous or botryoidal masses or rhombohedral crystals; effervesces in hot acid Siderite FeCO₃

C—yellow, yellow-brown to almost black S—light yellow to brown H—3.5 Cl—parallel to dodecahedral faces; in six directions In crystals, in fibrous or layered masses; associated with galena in northwestern Illinois, with fluorite and galena in southern Illinois Sphalerite ZnS (30)

C—colorless, white, yellow, purple, green, blue H—4 Cl—perfect, parallel to octahedral faces; in four directions In cubes and cleavable masses; many colors; mined in Hardin and Pope counties Fluorite (25) (Fluorspar) CaF₂

C—white, tinted yellow, blue, or green H—5 As crystalline incrustations or in earthy or compact masses; associated with fluorite-sphalerite ores in southern Illinois, with galena and sphalerite in northwestern Illinois Smithsonite ZnCO₃

D. Hardness greater than 5 but not greater than 7

C—white, green, brown, black H—5-6 Cl—in two directions intersecting at about 60° and 120° In long, slender 6-sided crystals; cleavage angle important in differentiating from pyroxenes; common in metamorphic and some igneous rocks Amphibole Group (Mg,Fe,Ca)₇(Si₈O₂₂)(OH)₂ (may also contain Na or Al)

C—gray, dark green, black, dark brown, bronze H—5-6 Cl—in two directions intersecting at about 90° Crystals short, stout, and 8-sided; cleavage angle important in differentiating from amphiboles; common in igneous and some metamorphic rocks Pyroxene Group (Mg,Ca,Fe)₂(Si₂O₆)

C—white, gray, pink, light blue, green H—6 Cl—in two directions nearly at right angles As crystals, cleavable masses and grains; common in igneous and metamorphic rocks, also in stream gravel and sand; many varieties Feldspar Group (22) K, Na, Ca, Ba (Al, Si)₄O₈

C—white when pure; may be colored by impurities H—7 Cl—none F—conchoidal Finely crystalline variety of quartz; botryoidal or concretionary masses; lining in geodes Chalcedony SiO₂

C—colorless, white, or almost any color H—7 F—conchoidal Most abundant mineral; occurs in 6-sided crystals capped by pyramids, in grains or masses; principal mineral in sandstone, also abundant in igneous and metamorphic rocks; is a variety of silica Quartz (21) SiO₂

C—red H—7 F—conchoidal A variety of quartz usually colored red by hematite inclusions; common in glacial and river sand and gravel found along Lake Michigan shores and in the Mississippi River Jasper SiO₂

C—many; arranged in bands H—7 F—conchoidal Cloudy banded variety of silica; widely used as semi-precious stones. Onyx and silicified wood are forms of agate; found in glacial gravels and upper Mesozoic sediments in southern Illinois Agate SiO₂

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

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