—biotitic /buy'euh tit"ik/, adj./buy"euh tuyt'/, n.a very common mineral of the mica group, occurring in black, dark-brown, or dark-green sheets and flakes: an important constituent of igneous and metamorphic rocks.[1860-65; named after J. B. Biot (1774-1862), French mineralogist and mathematician; see -ITE1]
* * *or black micaIt 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.
* * *▪ mineralalso called black mica,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.
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