mineral
Biotite Mica
K(Mg, Fe)3(AlSi3O10)(F, OH)2
Hardness: 2.5–3.0. Color: Dark brown, greenish-black, or 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, greenish-black, or black
- Luster
- Vitreous to pearly
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Physical properties
Hardness: 2.5–3.0. Color: Dark brown, greenish-black, or black. Luster: Vitreous to pearly. Crystal structure: Monoclinic. Cleavage: Perfect basal (peels into thin sheets). Specific gravity: 2.7–3.3.
Formation & geological history
Commonly forms in metamorphic and igneous rocks, particularly schists and granites. Formed over various geological periods in environments with high heat and pressure.
Uses & applications
Used as a filler in paints, rubber, and plastics. Historically used as a thermal insulator in electrical equipment. Primarily a collector and educational specimen.
Geological facts
Biotite is known as 'black mica'. Its name honors Jean-Baptiste Biot, a French physicist who researched the optical properties of micas. It is a vital mineral for potassium-argon dating of rocks.
Field identification & locations
Identify by its dark color and the way it flakes into flexible, elastic 'books' or sheets. Commonly found in mountainous regions under metamorphic conditions.
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