metamorphic
Biotite Schist
Biotite Gneiss/Schist (approximate formula K(Mg,Fe)3AlSi3O10(OH)2)
Hardness: 2.5-3 (on the Mohs scale for biotite); Color: Dark brown to black with sparkling silvery-gray highlights; Luster: Pearly to submetallic; Structure: Foliated or flaky/platy; Cleavage: Perfect basal.
- Hardness
- 2
- Color
- Dark brown to black with sparkling silvery-gray highlights
- Luster
- Pearly to submetallic
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Physical properties
Hardness: 2.5-3 (on the Mohs scale for biotite); Color: Dark brown to black with sparkling silvery-gray highlights; Luster: Pearly to submetallic; Structure: Foliated or flaky/platy; Cleavage: Perfect basal.
Formation & geological history
Formed from the regional metamorphism of clay-rich sedimentary rocks or igneous rocks at intermediate to high temperatures and pressures, causing the minerals to align and grow.
Uses & applications
Primarily used as an indicator mineral for metamorphic grade; occasionally used in the production of drilling muds or as a filler in paints and plastics, but mostly of geological interest.
Geological facts
Biotite is often called black mica. It is a key constituent in many granitic rocks and is named after the French physicist Jean-Baptiste Biot who researched the optical properties of micas.
Field identification & locations
Identify by its dark color, flaky texture, and the way it sparkles in the sun. It can be easily peeled into thin, flexible sheets with a thumbnail if the mica flakes are large enough.
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