metamorphic
Biotite-rich mica schist (tentative)
Mica schist, dominated by biotite; approximate formula K(Mg,Fe)₃AlSi₃O₁₀(F,OH)₂
AI-generated identification · may be incorrectDark gray-brown, strongly micaceous, with shiny platy cleavage and uneven crystalline texture. Biotite has Mohs hardness 2.5–3, vitreous to pearly luster, perfect basal cleavage, and specific gravity about 2.7–3.1; the specimen may also contain quartz and feldspar.
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Physical properties
Dark gray-brown, strongly micaceous, with shiny platy cleavage and uneven crystalline texture. Biotite has Mohs hardness 2.5–3, vitreous to pearly luster, perfect basal cleavage, and specific gravity about 2.7–3.1; the specimen may also contain quartz and feldspar.
Formation & geological history
Formed when shale or another clay-rich sediment was buried and metamorphosed under elevated temperature and pressure, commonly during mountain building. Its exact geological age cannot be determined from the photograph and depends on its source locality.
Uses & applications
Used mainly as a teaching, decorative, and collecting specimen; schist can be used locally as facing stone or aggregate, while mica has industrial electrical and heat-insulating applications.
Geological facts
The abundant mica flakes reflect light and split into thin elastic sheets. Similar dark material could be mica-rich gneiss, phyllite, or a sulfide-bearing rock, so laboratory testing would improve certainty.
Field identification & locations
Look for glittering aligned flakes, foliation, and easy splitting along mica layers; a streak, hardness test, and close inspection for metallic sulfides help distinguish it. Common in metamorphic belts worldwide, including the Appalachians, Alps, and Precambrian shields.
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