metamorphic
Biotite mica schist
Mica schist dominated by biotite (K(Mg,Fe)₃AlSi₃O₁₀(F,OH)₂), with quartz and feldspar
AI-generated identification · may be incorrectDark gray to black, strongly foliated, with abundant reflective mica flakes and platy cleavage; biotite hardness is about 2.5–3 Mohs, with pearly to vitreous luster. The photographed material appears crushed or screened, so individual crystal fabrics are fragmented.
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Physical properties
Dark gray to black, strongly foliated, with abundant reflective mica flakes and platy cleavage; biotite hardness is about 2.5–3 Mohs, with pearly to vitreous luster. The photographed material appears crushed or screened, so individual crystal fabrics are fragmented.
Formation & geological history
Formed when shale or other clay-rich sediment is subjected to regional metamorphism, typically at greenschist to amphibolite facies. Common ages range from Precambrian to Paleozoic, depending on the deposit.
Uses & applications
Used locally as decorative aggregate, landscaping stone, fill, and occasionally building or facing stone; fresh, attractive pieces are collected but are generally not jewelry grade.
Geological facts
Its glitter comes from light reflecting off thousands of mica cleavage surfaces. Biotite-rich schist can weather into flaky, dark soil and may release iron-colored staining.
Field identification & locations
Look for aligned, easily separable mica sheets, pronounced foliation, and a sparkly black-gray appearance; it differs from slate by coarser visible mica and from gneiss by lacking strong alternating light-dark banding. Common in regional metamorphic belts worldwide, including the Appalachians and Alps.
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