metamorphic
Biotite Schist
Biotite Schist (K(Mg,Fe)3AlSi3O10(OH,F)2)
Hardness: 2.5-3 (on mica flakes); Color: Black, dark brown, or dark green with bronze highlights; Luster: Pearly to submetallic; Crystal structure: Monoclinic (foliated); Cleavage: Perfect basal cleavage; Specific Gravity: 2.8-3.4
- Hardness
- 2
- Color
- Black, dark brown, or dark green with bronze highlights
- Luster
- Pearly to submetallic
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Physical properties
Hardness: 2.5-3 (on mica flakes); Color: Black, dark brown, or dark green with bronze highlights; Luster: Pearly to submetallic; Crystal structure: Monoclinic (foliated); Cleavage: Perfect basal cleavage; Specific Gravity: 2.8-3.4
Formation & geological history
Formed through the regional metamorphism of clay-rich sedimentary rocks (shales) or igneous rocks under medium-grade pressure and temperature conditions (Barrovian metamorphism).
Uses & applications
Used primarily as a geological indicator of metamorphic grade; sometimes used in ground form as a filler in paints or as a lubricant. Mica flakes can be used as decorative 'glitter' in construction materials.
Geological facts
Biotite belongs to the mica group and is often called 'black mica'. In schist, the mica flakes align perpendicularly to the direction of pressure, giving the rock its characteristic 'schistosity' or shimmer.
Field identification & locations
Identify in the field by its distinct platy or flaky texture and shiny, metallic-looking surfaces that reflect light. It will crumble into thin, flexible sheets if poked with a needle. Commonly found in mountain belts and Precambrian shield areas.
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