Biotite
K(Mg,Fe)₃(AlSi₃O₁₀)(OH)₂ · Monoclinic · also called black mica
Biotite is the black mica in granite, and it accounts for most of the dark specks in a light-coloured igneous rock. It behaves exactly like muscovite but is dark and iron-bearing.
Properties
- Mohs hardness
- 2.5–3
- Streak
- white, colorless
- Lustre
- vitreous, pearly
- Colour
- Dark brown to black, with a bright shine on cleavage faces.
- Cleavage
- Perfect basal cleavage into elastic sheets, as in muscovite.
- Fracture
- Not applicable
- Specific gravity
- 2.7–3.3 (water = 1)
- Magnetism
- Not magnetic
- Acid test
- No reaction
- UV
- Not fluorescent
Tests that confirm biotite
- Elastic sheets that spring back when bent.
- Very thin flakes are translucent brown when held to light, never fully opaque.
- Peels with a fingernail.
What biotite gets confused with
Each of these is settled by one test. Run that test first.
Colour alone: biotite is dark, muscovite is silvery.
Chlorite is green and its flakes are flexible but not elastic — they stay bent.
Where it is found
Granite, schist, and gneiss; extremely widespread.
Is it worth anything?
None.