Biotite

Biotite is a member of the mica group of silicates (sheet silicates), like chlorite and muscovite. It occurs in more geological environments than any of the other micas. It is a common rock forming mineral, being present in at least some percentage in many igneous rocks (e.g. granite and rhyolite), and metamorphic rocks (e.g. schist, gneiss).

Chemical composition - K(Fe, Mg) ₃AlSi ₃O ₁₀(F, OH) ₂
Hardness - 2.5-3
Specific gravity - 2.9-3.4
Transparency - Transparent to translucent
Colour - Brown to black
Streak - Very pale brown
Lustre - Vitreous to pearly
Cleavage/fracture - Perfect in one direction producing thin sheets or flakes / uneven
Crystal habit/mode of occurrence - Tabular (sheets or flakes) / granular