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biotite
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biotite granite Bowen's reaction series
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Basalt sample returned by Apollo 15, from near a long sinous lunar valley called Hadley Rille. Measured at 3.3 years old.
(Bed) Rocks and (Flint) Stones
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Also called:
black mica
Related Topics:
mica
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biotite
biotite

biotite

mineral
Also known as: black mica
Written and fact-checked by
Britannica Editors
Encyclopaedia Britannica's editors oversee subject areas in which they have extensive knowledge, whether from years of experience gained by working on that content or via study for an advanced degree....
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biotite, a silicate mineral in the common mica group. It is abundant in metamorphic rocks (both regional and contact), in pegmatites, and also in granites and other intrusive igneous rocks. For chemical formula and detailed physical properties, see mica (table).

Biotite is regarded as a mixture composed of variable proportions of four basic aluminosilicates of potassium, iron, magnesium, or aluminum: annite, K2Fe6(Si6Al2O20)(OH)4; siderophyllite, K2Fe5Al(Si5Al3O20)(OH)4; phlogopite, K2Mg6(Si6Al2O20)(OH)4; and eastonite, K2Mg5A1(Si5Al3O20)(OH)4. Biotite is arbitrarily designated as having a magnesium-to-iron ratio less than 2:1.

This article was most recently revised and updated by Amy Tikkanen.