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Biotite

Biotite is a common group of phyllosilicate minerals within the mica group, with the approximate chemical formula K(Mg, Fe)₃AlSi₃O₁₀(F, OH)₂. Wikipedia
Crystal system: Monoclinic
Birefringence: δ = 0.03–0.07
Category: Phyllosilicate
Cleavage: Perfect on the
Color: Dark brown, greenish-brown, blackish-brown, yellow
Crystal habit: Massive to platy
Fracture: Micaceous