A dark-brown or dark-green to black mica,
K(Mg,Fe)3(Al,Fe)Si3O10(OH)2, found in igneous and metamorphic rocks. A
ferro-magnesian mineral. Black, shiny, basal cleavage (one cleavage
resulting in thin sheets). In this specimen we see face up the basal
cleavage face, very flat, very smooth. Up on end view the crystal will
look like a stack of paper with all the sheets laying together.
In felsic and intermediate igneous rocks
biotite may be confused with amphibole but biotite has flat, slick,
shiny faces not broken by the numerous cleavages typical of amphibole.
Biotites are found commonly in igneous and
metamorphic rocks. They are the common ferromagnesian phase in most
granitic rocks, and are also found in some siliceous and intermediate
volcanic rocks. In basaltic rocks biotite sometimes occurs in the
crystalline groundmass, and is a common late interstitial phase in
gabbroic rocks. It has been recognized in samples of the Earth's mantle
found as inclusions in volcanic rocks. Biotites are not stable at the
surface of the Earth, as they decompose by both hydrolysis and
oxidation when exposed to the Earth's atmosphere. They alter to
vermiculite, chlorite, and iron oxides, and thus are uncommon in
sedimentary rocks.
Biotites are important constituents of metamorphic
rocks such as schist and gneiss, and the first appearance of biotite is
an important marker in metamorphism. Biotite persists to very high
grades of metamorphism, where it reacts with quartz to form granulites
made up of potassium feldspar and orthopyroxene, garnet, or cordierite,
in addition to quartz and plagioclase. Prismatic or tabular (short
prismatic) crystals. Crystals may appear rhombohedral and are called
pseudorhombohedral. Normally biotite occurs as foliated masses or as
scaly aggregates.
Under conditions of ultrametamorphism, biotite
reacts with quartz, plagioclase, and alkali feldspar to form siliceous
melts. Biotite is also a common gangue mineral in ore deposits. The
mineral has been used as an indicator of H2O, HF, O2, and S2 activities
in both rock- and ore-forming processes.
Material Notes:
One cleavage direction. Weathering end product(s):
Loss of Fe, Mg. Occurrence: Igneous, sediments, metamorphic. Rusty
discoloration by loss of Fe. It is common in a wide range of igneous
and metamorphic rocks and may be an important detrital mineral in
sediments. Biotite is a widespread mineral found in igneous rocks,
especially granite and syenite. It is also found in large sheets in
pegmatites and in felsic lavas as phenocrysts. Biotite mica is dark
green to black in color because it contains iron and magnesium.
