metamorphic
Biotite Schist
Biotite-rich Schist (containing K(Mg,Fe)3AlSi3O10(OH,F)2)
Hardness: 2.5-3 (on the Mohs scale for biotite); Color: Dark brown, black, or dark green; Luster: Pearly to submetallic; Crystal Structure: Monoclinic, typically appearing as flaky or platy layers; Cleavage: Perfect basal cleavage.
- Hardness
- 2
- Color
- Dark brown, black, or dark green
- Luster
- Pearly to submetallic
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Physical properties
Hardness: 2.5-3 (on the Mohs scale for biotite); Color: Dark brown, black, or dark green; Luster: Pearly to submetallic; Crystal Structure: Monoclinic, typically appearing as flaky or platy layers; Cleavage: Perfect basal cleavage.
Formation & geological history
Formed through the regional metamorphism of shale or mudstone under intermediate to high temperatures and pressures. This sample displays 'schistosity,' where micas align perpendicular to the direction of pressure.
Uses & applications
Primarily used as an ornamental stone in landscaping or interior design. Occasionally used in electrical insulators or ground up for use in paints and drilling muds.
Geological facts
Biotite is often called 'black mica.' Because it contains iron and magnesium, it is heavier and darker than its cousin, muscovite. Large crystals are sometimes called 'books' because the cleavage flakes look like pages.
Field identification & locations
Identify in the field by its distinct layered (foliated) texture and the way the dark mica flakes sparkle in sunlight. It is commonly found in mountain belts such as the Appalachians, Alps, or Himalayas.
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