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