Biotite

K₂(Mg,Fe²⁺)_6-4(Fe³⁺,Al,Ti)_0-2[Si_6-5Al_2-3O₂₀](OH,F)₄

Biotite is a sheet silicate in the mica group. Micas break into thin sheets; biotite can be distinguished from other micas by its black or dark brown color. Biotites usually occur in both plutonic igneous (e.g., granite) and metamorphic rocks, but can also be found as shiny, black flakes in beach sands.