Chapter 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

No effervescence or very slight effervescence
I Coarse-grained (p. 37)
II Fine-grained (p. 38)
III Organic (p. 39)
Slight effervescence
gray, light gray, white, or brown: Dolomite
Vigorous effervescence
Composed of pebbles that effervesce
Rounded pebbles: Limestone conglomerate
Angular pebbles: Limestone breccia
Composed of crystals of calcite, fossil shells, or oolites:
Limestone
Composed of banded layers of crystalline calcite; commonly found in
caves, forming stalactites and stalagmites: Travertine
Large amount of insoluble residue left on acid-treated surface
Individual grains seen with unaided eye: Calcareous sandstone
Individual grains not seen with unaided eye: Calcareous shale
Composed of porous or cellular mass of calcite; commonly found near
springs and waterfalls: Tufa