Page 135 - Laboratory Manual in Physical Geology

SOME COMMON ROCK-FORMING MINERALS

Occurs in Occurs in Occurs in
Mineral Description Igneous Sedimentary Metamorphic
Rocks Rocks Rocks
Augite Very dark green to brown or dark gray, hard mineral as in situ
pyroxene (Hardness 5.5 – 6.0) with two cleavages about 90 crystals
degees apart.
Biotite mica Glossy black mineral that easily splits into thin as in situ as foliated in
transparent sheets along its excellent cleavage. crystals situ crystals
Hardness 2.5 – 3.0.
Calcite Usually colorless, yellow, white, or amber. Breaks as in situ as in situ
along three excellent cleavages (none at 90 degrees) crystals crystals
to form rhombohedrons (leaning blocks). Hardness 3.
Reacts with dilute hydrochloric acid (HCI).
Chlorite Green mica-like mineral that splits into thin glossy as microscopic as in situ
transparent sheets along its excellent cleavage. crystals that crystals
Hardness 2.0 – 2.5. Occurs in large crystals or color the rock
fine-grained masses. green
Garnet Red to black rounded crystals with no cleavage. Rarely, as in as detrital as in situ
Very hard (Hardness 7). situ crystals clasts crystals
Gypsum Colorless, white, or gray mineral. Easily scratched as in situ
(Hardness 2.0), even with a fingernail. crystals
Halite Colorless, white, yellow, gray cubes, that break into as in situ
cubic shapes because they have three excellent crystals
cleavages 90 degrees apart. Brittle. Hardness 2.5.
Hornblende Dark gray to black, hard mineral (Hardness 5.5 – as in situ as in situ
amphibole 6.0). Breaks along glossy cleavage surfaces about crystals crystals
56 and 124 degrees apart.
Kaolinite Earthy white, gray or very light brown clayey masses as in situ
that leave powder on your fingers. Very fine-grained. earthy masses
No visible crystals. Hardness 1 – 2.
Muscovite Colorless, brown, yellow, or white minerals that as in situ tiny flakes as as foliated in
mica easily splits into transparent thin sheets along its crystals clasts situ crystals
excellent cleavage. Hardness 2.0 – 2.5.
Olivine Pale to dark olive green or yellow mineral with no as in situ as in situ
cleavage. Very hard (Hardness 7). Crystals may crystals crystals
resemble sand grains. Brittle.
Plagioclase Usually white to pastel gray, but may be colorless or as in situ as detrital as in situ
feldspar black with iridescent play of colors. Exhibits fracture crystals clasts crystals
surfaces and two good cleavages. Cleavage
surfaces may have thin striations. Hardness 6.
Potassium Usually pink-orange or pale brown, may be white. as in situ as detrital as in situ
feldspar Usually has internal discontinuous streaks crystals clasts crystals
(orthoclase) (exsolution lamellae). Exhibits fracture surfaces and
two good cleavages. Hardness 6.
Quartz Usually transparent to translucent gray or milky as in situ as in situ as in situ
white, may be colorless. No cleavage. Breaks along crystals crystals and crystals
uneven fractures or curved conchoidal fractures (like commonly as
glass). Very hard (Hardness 7). detrital clasts
Serpentine Usually pale to dark green, opaque masses with no as in situ
visible crystals or cleavage. Usually scratches easily masses
(Hardness 2 – 5).
In situ (“in place”) mineral grains are present in the rock where they originally formed. Examples are mineral crystals newly formed
from cooling of lava or magma (in igneous rocks), crystals newly formed or recyrstallized in rock or hot watery solutions under
conditions of intense heat and pressure or (in metamorphic rocks), or as newly formed crystals precipitated from evaporating surface
or ground water (in sedimentary rocks).
Detrital mineral grains are not in situ. They did not form where they are now found, are not intergrown, and do not lock together to
form the rock. They were removed from the place or rock where they originally formed and were moved by wind, water, ice, organisms,
and/or gravity to a new place. Examples are pebbles and sand grains in sedimentary and metamorphic rocks.
Foliated mineral grains are flat or blade-like crystals that have been aligned and layered, like scales on a fish, during metamorphism.

FIGURE 4.3 Some common rock-forming minerals.

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