metamorphic
Biotite Schist
Biotite Schist (predominantly K(Mg,Fe)3AlSi3O10(OH,F)2)
Hardness: 2.5-3; Color: Dark brown, black, or dark green; Luster: Pearly to vitreous (metallic-like sheen); Structure: Foliated with visible platy mica crystals; Cleavage: Perfect basal cleavage in individual grains.
- Hardness
- 2
- Color
- Dark brown, black, or dark green
- Luster
- Pearly to vitreous (metallic-like sheen)
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Physical properties
Hardness: 2.5-3; Color: Dark brown, black, or dark green; Luster: Pearly to vitreous (metallic-like sheen); Structure: Foliated with visible platy mica crystals; Cleavage: Perfect basal cleavage in individual grains.
Formation & geological history
Formed through regional metamorphism of shale, mudstone, or igneous precursors at intermediate pressures and temperatures. Common in mountain belts and Precambrian shields.
Uses & applications
Used as an indicator of metamorphic grade in geology, as a filler in paints and plastics, and occasionally in construction as decorative stone or gravel.
Geological facts
Biotite is often called 'black mica.' Schist is distinctive because it contains more than 50% platy and elongated minerals that are visible to the naked eye.
Field identification & locations
Identify by its 'schistose' texture, which creates a glittery or sparkly appearance due to light reflecting off parallel mica flakes. Found in metamorphic terrains like the Appalachian or Himalayan mountains.
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