mineral
Biotite / Black Mica
K(Mg,Fe)3(AlSi3O10)(F,OH)2
Hardness: 2.5-3.0 Mohs; Color: Black, dark green, or dark brown; Luster: Vitreous to pearly; Crystal structure: Monoclinic; Cleavage: Perfect basal cleavage (peels into thin sheets); Specific Gravity: 2.7-3.3
- Hardness
- 2
- Color
- Black, dark green, or dark brown
- Luster
- Vitreous to pearly
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Physical properties
Hardness: 2.5-3.0 Mohs; Color: Black, dark green, or dark brown; Luster: Vitreous to pearly; Crystal structure: Monoclinic; Cleavage: Perfect basal cleavage (peels into thin sheets); Specific Gravity: 2.7-3.3
Formation & geological history
Forms in igneous rocks like granite and pegmatite, and metamorphic rocks such as schist and gneiss. It crystallizes from magma or forms through regional metamorphism.
Uses & applications
Used as a filler in paints and plastics, as a constituent in drilling muds, and as an insulator in the electrical industry. Common in classroom mineral sets.
Geological facts
Biotite was named in honor of Jean-Baptiste Biot, a French physicist who researched the optical properties of mica. It is often referred to as 'black mica' and is part of the phyllosilicate mineral group.
Field identification & locations
Identify by its dark color and the ability to flake off thin, flexible sheets with a fingernail or pocketknife. Common worldwide in granitic shields and mountain ranges.
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Epidote | Ca2(Al2,Fe3+)(SiO4)(Si2O7)O(OH)
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Mineral/Rock
Banded Gneiss (River Rock/Pebble)
Gneiss (specifically a banded variety often found as a river rock)
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Green Apatite on Albite
Fluorapatite [Ca5(PO4)3F] on Albite [Na(AlSi3O8)]
mineral
Epidote
Epidote - Ca2(Al2,Fe3+)3(SiO4)3(OH)
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