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.
Highlighting FeaturesMagnesium (Mg) atoms Aluminum (Al) atoms Silicon (Si) atoms Oxygen (O) atoms Iron (Fe) atoms Potassium (K) atoms Potassium (K) and nearest neighbors Single potassium (K) and nearest neighbors (12 O and 2 OH-) All atoms
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