Guide to Rocks and Minerals of Illinois
Mineral Identification Key
Name and Composition
Metalic luster, streak colored, with a hardness of ≥
Cubic; perfect in 3 directions
Subconchoidal or even
Very heavy; occurs as crystals, grains, or masses; easily identified by
color and cleavage.
Very heavy; apt to have green coating; distored or wirelike forms;
Native copper Cu
Metalic luster, streak colored, with a hardness of >2 but
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
Pale brassy yellow to silver-white
As fibrous, radiating, tabular, and cockscomb 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.
Nonmetalic luster, streak colored, with a hardness of >2 but
Yellow-brown to black
(may be as low as 1)
In earthy masses; coloring material in many sandstones, conglomerates,
and soils; often mixed with and difficult to distinguish from goethite
and other iron minerals.
Nonmetalic luster, streak white, with a hardness of
but may be
almost any color
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.
As needle-shaped crystals or powdery coating on pyrite or marcasite.
Nonmetalic luster, streak white, with a hardness >2 but
≤3 (can be scratched by a penny)
Colorless, silver-white, gray, brown
Perfect in one direction
As scales or "books"; splits into thin sheets; common in sandstones,
shales, and igneous and metamorphic rocsk, such as schist.
Muscovite (white mica) KAI2
Brown or black
Perfect in one direction
As scales or "books"; splits into thin sheets; common in igneous and
metamorphic rocks such as granite or gneiss, but not in sedimentary
Colorless, white, gray, and various tints
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
White, gray, red or almost any color
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
Nonmetalic luster, streak white, with a hardness >3 but
(cannot be scratched by a penny; can be scratched by a knife)
White, gray, light yellow
In one direction
Relatively heavy; effervesces in acid; associated with fluorite and
barite in southern Illinois, but is not abundant.
White, pink, gray, or light brown
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 minerals in rock called dolomite; crystals found
Colorless, white, gray, grayish black
In fibrous or compact masses or may be in orthorhombic crystals as a
coating on galena; very heavy; formed by alteration of galena.
Brown to gray
Usually white, but may tend toward brown when weathered
In three directions; not at right angles (rhombohedral) slightly
In fibrous or botryoidal masses or rhombohedral crystals; effervesces
in hot acid.
Yellow, yellow-brown to almost balck
Light yellow to broan
Parallel to dodecahedral faces; in six directions
In crystals, in fibrous or layered masses; associated with galena in
northwestern Illinois and with fluorite and galena in southern Illinois.
Colorless, white, yellow, purple, green, blue
Perfect, parallel to octahedral faces; in four directions.
In cubes and cleavable masses; many colors; mined in Hardin and Pope
White, tinted yellow, blue, or green
As crystaline incrustations or in earthy or compact masses; associated
with fluorite-sphalerite ores in southern Illinois and with galena and
sphalerite in northwestern Illinois.
Nonmetalic luster, streak white, with a hardness of >5 but
White, green, brown, black
In two directions intersecting at about 60° and 120°
In long, slender six-sided crystals; cleavage angle important in
differentiating from pyroxenes; common in metamorphic and some igneous
Amphibole Group (Mg, Fe, Ca)7
May also contain Na or Al
Gray, dark green, black, dark brown, bronze
In two directions intersecting at about 90°
Crystals short, stout, and eight-sided; cleavage angle important in
differentiating from amphiboles; common in igneous and some
(Mg, Ca, Fe)2
White, gray, pink, light blue, green
In two directions; nearly at right angles
As crystals, cleavable masses, and grains; common in igneous and
metaborphic rocks, also found in river sands and gravels; many
(K, Na, Ca)
White when pure; may be colored by impurities
Finely crystaline variety of quartz; botryoidal or concretionary
masses; lining in geodes; forms agates.
Colorless, white, or almost any color
Most abundant mineral; occurs in six-sided crystals capped by pyramids
in geodes. In grains or masses; principal mineral in sandstone, also
abundant in igneous and metamorphic rocks.
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.
Many; arranged in bands
Cloudy banded variety of silica; widely used as semi-precious stones.
Silicified wood is a form of agate; found in glacial gravels and upper
Mesozoic sediments in southern Illinois.
Nonmetalic luster, streak white, with a hardness of >7
Red, brown, yellow, green, black, white
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.
(Ca, Mn, Fe, Mg)3
(Al, Cr)2 (SiO4)3
Rocks and Minerals of Illinois
Table of Contents
The printed version of Guide to Rocks and Minerals in
Illinois can be purchased from the Shop ISGS