black mica

biotite

 or black mica

Biotite mica from the district of Juchitán, Oaxaca, Mex.

(credit: 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.

---

black mica [¦blak ′mī·kə]

(mineralogy)

biotite