Biotite
ID: 130
Biotite
Chemical Formula
K(Mg,Fe2+)3AlSi3O10(OH,F)2
Hardness (Mohs)
2.5 - 3
Crystal System
Monoclinic
Streak
Gray to white
Luster
Vitreous to Pearly
Diaphaneity
Transparent to translucent to opaque
Color
Dark brown, greenish-brown, blackish-brown, black; also yellow, white.
Crystal Habit / Mode of Occurrence
Lamellar (thin sheets), micaceous (platy texture with flexible plates), pseudo-hexagonal crystals. Often forms "books" of stacked sheets.
Fun Facts
Biotite is commonly referred to as "black mica" due to its typically dark color, and it's easily identified by its perfect cleavage into flexible sheets.