mineral
Biotite (Black Mica)
Biotite (K(Mg,Fe)3AlSi3O10(OH)2)
AI-generated identification · may be incorrectHardness: 2.5–3.0 on Mohs scale; Color: Black, dark brown, or dark green; Luster: Vitreous to pearly; Crystal structure: Monoclinic; Cleavage: Perfect basal cleavage (splits into thin, flexible sheets); Specific gravity: 2.7–3.3
- Hardness
- 2
- Color
- Black, dark brown, or dark green
- Luster
- Vitreous to pearly
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Physical properties
Hardness: 2.5–3.0 on Mohs scale; Color: Black, dark brown, or dark green; Luster: Vitreous to pearly; Crystal structure: Monoclinic; Cleavage: Perfect basal cleavage (splits into thin, flexible sheets); Specific gravity: 2.7–3.3
Formation & geological history
Formed in a wide range of geological environments, including igneous rocks (like granite and diorite) and metamorphic rocks (like schist and gneiss). It crystallizes from magma or forms through metamorphism of clay-rich sedimentary rocks.
Uses & applications
Used as an insulating material in electrical equipment, as a filler in paints and rubber, and occasionally in the production of drilling mud for the oil industry. It is also highly valued by mineral collectors for its unique platelet structure.
Geological facts
Biotite is often referred to as 'black mica' and is a member of the mica group. It is named after the French physicist Jean-Baptiste Biot, who researched the optical properties of micas in the early 19th century.
Field identification & locations
Identify in the field by its dark color and the way it peels into thin, elastic sheets with a fingernail or knife. It is commonly found in mountainous regions or areas with exposed granite and metamorphic bedrock. Collectors should look for its characteristic 'books' or stacks of plates.
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Other mineral specimens
Gneiss or Schist
Gneiss or Mica Schist with Garnet
Metamorphic
Sandstone (with potential mineral staining/concretions)
Arenite (SiO2 based)
sedimentary
Tumbled granite
Granite, a coarse-grained felsic intrusive igneous rock
igneous
Epidote
Epidote | Ca2(Al2,Fe3+)(SiO4)(Si2O7)O(OH)
metamorphic
Pegmatite with Tourmaline and Quartz
Igneous Pegmatite
igneous
Granite
Granite (Phaneritic intrusive igneous rock)
igneous