biotite
biotite
A monoclinic mineral, K2Mg6(Si6Al2O20)(OH,F)2; mica group, with Fe2+replacing Mg and Fe3+replacing Al; in masses with perfect basal cleavage; dark brown, dark green, black; a common rock-forming mineral in crystalline rocks, either as an original crystal in igneous rocks or as a metamorphic product in gneisses and schists; a detrital constituent of sedimentary rocks.