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