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Biotite is a common phyllosilicate mineral within the mica group, with the approximate chemical formula K(Mg, Fe)<sub>3</sub>AlSi<sub>3</sub>O<sub>10</sub>(F, OH)<sub>2</sub>. More generally, it refers to the dark mica series, primarily a solid-solution series between the iron-endmember annite, and the magnesium-endmember phlogopite; more aluminous endmembers include siderophyllite.