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Biotite is a name used for a large group of Black Mica For Water Purification minerals that are commonly found in igneous and metamorphic rocks. These include: annite, phlogopite, siderophyllite, fluorophlogopite, fluorannite, eastonite and many others. These micas vary in chemical composition but are all sheet silicate minerals with very similar physical properties.
A generalized chemical composition for the biotite group is
K (Mg, Fe) 2-3Al1-2Si2-3O10 (OH,F)2
The name "biotite" is used in the field and in entry-level geology courses because these minerals generally can not be distinguished without optical, chemical or x-ray analysis.
Biotite is a primary mineral found in a wide range of crystalline igneous rocks such as granite, diorite, gabbro, peridotite andpegmatite. It also forms under metamorphic conditions when argillaceous rocks are exposed to heat and pressure to form schist and gneiss. Although biotite is not very resistant to weathering and transforms into clay minerals, it is sometimes found in sediments and sandstones.