mineral
Biotite (Black Mica)
K(Mg,Fe)3AlSi3O10(F,OH)2
AI-generated identification · may be incorrectHardness: 2.5-3 on Mohs scale; Color: dark brown, greenish-black, or jet black; Luster: vitreous to submetallic, pearly on cleavage faces; Cleavage: perfect basal (micaceous), easily splits into thin, flexible, elastic sheets; Crystal system: monoclinic.
- Hardness
- 2
- Color
- dark brown, greenish-black, or jet black
- Luster
- vitreous to submetallic, pearly on cleavage faces
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Physical properties
Hardness: 2.5-3 on Mohs scale; Color: dark brown, greenish-black, or jet black; Luster: vitreous to submetallic, pearly on cleavage faces; Cleavage: perfect basal (micaceous), easily splits into thin, flexible, elastic sheets; Crystal system: monoclinic.
Formation & geological history
Common rock-forming silicate mineral found in intrusive felsic igneous rocks (granite, pegmatites) and metamorphic rocks (schist, gneiss) under medium-to-high temperature conditions.
Uses & applications
Used primarily for radiometric potassium-argon dating of rocks, as a non-conducting electrical insulator, filler in paints, joint compound, and drywall, and as a popular educational specimen for geology students.
Geological facts
Named in honor of French physicist and mineralogist Jean-Baptiste Biot, who investigated the optical properties of mica. Because it forms thin sheets that bend without breaking, ancient cultures occasionally used mica sheets as heat-resistant windows.
Field identification & locations
Easily identified by its dark black-to-brown color, sheet-like structure, and ability to easily peel into thin, flexible flakes with a fingernail or knife edge. Found globally in granitic pegmatites and mica schists.
