mineral
Biotite (Black Mica)
Biotite (K(Mg,Fe++)3AlSi3O10(OH,F)2)
AI-generated identification · may be incorrectHardness: 2.5-3.0; Color: Black, dark green, or dark brown; Luster: Pearly to vitreous; Crystal Structure: Monoclinic; Cleavage: Perfect basal (peels into thin sheets); SG: 2.7-3.3.
- Hardness
- 2
- Color
- Black, dark green, or dark brown
- Luster
- Pearly to vitreous
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Physical properties
Hardness: 2.5-3.0; Color: Black, dark green, or dark brown; Luster: Pearly to vitreous; Crystal Structure: Monoclinic; Cleavage: Perfect basal (peels into thin sheets); SG: 2.7-3.3.
Formation & geological history
Forms primarily in igneous rocks such as granite and pegmatites, and metamorphic rocks like schist and gneiss. It is a common rock-forming mineral crystallizing from cooling magma.
Uses & applications
Used as an insulating material in the electronics industry (though muscovite is preferred), as a filler in paints and rubber, and in specialized optical equipment.
Geological facts
Unlike muscovite, which is clear or silver, biotite is dark due to high iron and magnesium content. It is often nicknamed 'iron mica' and can weather into 'fool's gold' under certain conditions.
Field identification & locations
Identified by its dark color and the ability to peel off paper-thin, flexible sheets from the specimen. Found globally in granitic terrains.
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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