Biotite–K(Mg,Fe)3AlSi3O10(OH)2–is a dark, iron-rich mica that has a monoclinic crystal system. The octahedral layer is entirely occupied by Mg2+ and Fe2+ and the mineral is therefore trioctahedral, since all octahedra are occupied. The source of the lattice charge is the presence of Al3+ in place of Si4+ in 1/4 of the tetrahedral sites. The interlayer potassium is neatly nested among 12 oxygens and 2 hydroxyl groups, half from each layer.

Distribution

Biotite occurs worldwide in granites, gabbros, schists, and gneisses.

Brigatti, M. F., and P. Davoli. 1990. Crystal-structure refinement of 1M plutonic biotites sample M62 from a rock transitional between granitic and monzonitic in the Valle del Cervo pluton. American Mineralogist. 75:305-313