Biotite
Potassium
iron magnesium aluminum silicate hydroxide fluoride
K(FE, Mg)3AlSi3O10(F, OH)2
Biotite
is a common rock forming mineral, being present in at least some
percentage in most igneous and both regional and contact metamorphic
rocks. The typical black to brown color of biotite is characteristic
although it is difficult to distinguish brown biotite from dark brown
phlogopite.
The two are actually end members in a series that is dependent on the
percentage of iron. Phlogopite is iron poor and biotite is iron rich.
The darker color and density increase with an increase in the iron
content. Biotite tends to form in a wider range of conditions than
phlogopite which is limited mostly to ultramafic rocks and magnesium
rich marbles and pegmatites.
Try scratching the black grains with a nail or knife. Biotite will
flake off easily. Biotite is differentiated from amphibole by shape of
the crystals (hexagonal for biotite and elongated or needle-like for
amphibole) and by hardness (biotite is soft, amphibole is hard).
It is differentiated from pyroxene by hardness, color (biotite is
black and pyroxene dark green) and occurrence (biotite is found in
light-colored igneous rocks like granites, diorites and rhyolites
while pyroxene occurs in dark-colored rocks like gabbro and basalt).
Expect to find biotite as a common accessory in granite, and as
phenocrysts in some rhyolites.
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