mineral
Biotite (Black Mica)
Biotite - K(Mg,Fe)3(AlSi3O10)(F,OH)2
Hardness: 2.5-3.0 (Mohs); Color: Dark brown to greenish black; Luster: Vitreous to pearly; Crystal structure: Monoclinic; Cleavage: Perfect basal (peels into thin sheets); Specific Gravity: 2.7-3.3
- Hardness
- 2
- Color
- Dark brown to greenish black
- Luster
- Vitreous to pearly
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Physical properties
Hardness: 2.5-3.0 (Mohs); Color: Dark brown to greenish black; Luster: Vitreous to pearly; Crystal structure: Monoclinic; Cleavage: Perfect basal (peels into thin sheets); Specific Gravity: 2.7-3.3
Formation & geological history
Formed in igneous and metamorphic environments, specifically in granite, diorite, and schists. It is a common rock-forming mineral found in the Earth's crust.
Uses & applications
Used as an insulating material in electrical equipment, as a filler in paints and plastics, and as an additive in oil-well drilling muds. It is also significant for geological dating (Potassium-Argon dating).
Geological facts
Biotite is known as 'black mica' and is part of the mica group. Its distinctive perfect cleavage allows it to be split into extremely thin, flexible, and elastic sheets.
Field identification & locations
Identified in the field by its dark color and the ability to flake off thin, flexible sheets with a fingernail or pocketknife. Common in mountainous regions with exposed granite.
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Sandstone (with potential mineral staining/concretions)
Arenite (SiO2 based)
sedimentary
Sandstone with Mineral Veining or Crust
Sedimentary Sandstone (mostly SiO2 with Fe2O3 tinting)
sedimentary
Schist
Schist
Metamorphic
Gypsum (variety Selenite or Alabaster)
Calcium sulfate dihydrate (CaSO4·2H2O)
Mineral
Epidote
Epidote | Ca2(Al2,Fe3+)(SiO4)(Si2O7)O(OH)
metamorphic
Nephrite Jade
Nephrite
Mineral/Rock