Biotite
Mineral K(Mg,Fe)3(AlSi3O10)(OH,F)2
Also called Black mica
Take care.
- Fragile: Handle it gently and pack it well; it breaks or crumbles easily.
About it
Dark brown to black mica in flexible sheets, a common grain in granite, gneiss, and schist; a series name covering the annite and phlogopite compositions. Big books from Bancroft, Ontario.
- Type
- Mineral
- Group
- Silicate (phyllosilicate), mica group
- Formula
- K(Mg,Fe)3(AlSi3O10)(OH,F)2
- Hardness (Mohs)
- 2.5–3
- Specific gravity
- 2.7–3.3
- Streak
- white to grey
- Lustre
- vitreous (glassy), pearly, submetallic
- Crystal system
- Monoclinic
- Colours
- black, dark brown, dark green
- Transparency
- transparent, translucent, opaque
- Cleavage and fracture
- perfect in one direction, flexible elastic sheets
Typical values: real specimens vary. For more, see mindat.org.