Biotite

A K-rich subgroup of the trioctahedral mica group of the Mica Group. The CNMMN Subcommittee on Nomenclature of the Micas (1998, 1999) has recommended that the name biotite be used for any minerals in the “series” between the joins Annite-Phlogopite and Siderophyllite-Eastonite, and is therefore no longer to be regarded as a species name, more strictly a subgroup. Other closely related species such as Fluorophlogopite and Fluorotetraferriphlogopite are usually included in the subgroup. The name is most commonly used generically (in the field or petrologically) for any unanalysed or incompletely analysed dark-coloured, Fe-rich micas, including Annite, Fluorannite, Tetra-ferri-annite and Siderophyllite. Commonly alters to vermiculite or chlorite, and these may be intimately interlayered. Rock-forming biotite may be confused with stilpnomelane.