metamorphic
Biotite Schist
Biotite-rich Schist (K(Mg,Fe)3AlSi3O10(F,OH)2)
Hardness: 2.5–3 on Mohs scale; Color: Black, dark brown, or dark green; Luster: Vitreous to pearly; Crystal structure: Monoclinic; Cleavage: Perfect basal cleavage (flakes easily); Texture: Schistose (foliated with visible grains).
- Hardness
- 2
- Color
- Black, dark brown, or dark green
- Luster
- Vitreous to pearly
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Physical properties
Hardness: 2.5–3 on Mohs scale; Color: Black, dark brown, or dark green; Luster: Vitreous to pearly; Crystal structure: Monoclinic; Cleavage: Perfect basal cleavage (flakes easily); Texture: Schistose (foliated with visible grains).
Formation & geological history
Formed through the regional metamorphism of shale or mudstone under high pressure and temperature conditions. It typically develops in the roots of mountain ranges where crustal plates collide.
Uses & applications
Used primarily as a decorative stone in landscaping, for making heat-resistant materials (due to mica content), or as a specimen for geological education and collecting.
Geological facts
Biotite is often called 'black mica.' Because it contains iron and magnesium, it is heavier and darker than its cousin, muscovite. Large grains in schist indicate high levels of metamorphic grade.
Field identification & locations
Identify in the field by its distinct 'shimmer' or glittery appearance and its tendency to flake off in thin, brittle sheets when scratched with a fingernail or knife. Common in the Appalachian Mountains and the Alps.
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