metamorphic
Biotite Schist
Biotite-bearing Schist (K(Mg,Fe)3AlSi3O10(F,OH)2)
Hardness: 2.5-3 (on mica flakes), 5-6 (overall); Color: Dark brown to black with silvery/greenish streaks; Luster: Pearly to sub-metallic; Crystal structure: Monoclinic (foliated); Cleavage: Perfect basal.
- Hardness
- 2
- Color
- Dark brown to black with silvery/greenish streaks
- Luster
- Pearly to sub-metallic
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Physical properties
Hardness: 2.5-3 (on mica flakes), 5-6 (overall); Color: Dark brown to black with silvery/greenish streaks; Luster: Pearly to sub-metallic; Crystal structure: Monoclinic (foliated); Cleavage: Perfect basal.
Formation & geological history
Formed through regional metamorphism of clay-rich sedimentary rocks (pelites) or certain igneous rocks under intermediate temperatures and pressures. The presence of large mica flakes indicates significant recrystallization.
Uses & applications
Mainly used as a decorative stone, in construction as fill, or as a source of mica for electrical insulation and heat resistance in industrial applications.
Geological facts
The term 'Schist' comes from the Greek word 'schistos', meaning 'split', referring to the ease with which the rock can be cleaved into thin plates. Biotite is often called 'black mica'.
Field identification & locations
Identify by its distinct flaky layering (schistosity) and the presence of shiny, dark mica crystals. It is common in mountain belts like the Appalachians or the Alps. Collectors look for garnet inclusions within these specimens.
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