Biotite mica from the district of Juchitán, Oaxaca, Mex.—Courtesy of the Field Museum of Natural History, Chicago; photograph, John H. Gerard-EB Inc.

Silicate mineral in the common mica group. It is abundant in metamorphic rocks, in pegmatites, and in granites and other igneous rocks. Biotite is a layer silicate structure in which aluminum and silicon occur in infinitely extending Si-Al-O sheets that alternate with potassium-rich and magnesium- (and iron-) containing sheets.

Variants of BIOTITE

biotite or black mica

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