metamorphic
Biotite-bearing metamorphic rock, possibly schist or gneiss
Mica schist/gneiss; chiefly quartz (SiO₂), feldspar, and biotite mica
AI-generated identification · may be incorrectDark gray, coarse-grained, foliated specimen with sparkly black mica and lighter quartz/feldspar bands. Hardness is approximately 2.5–3 for biotite and 6–7 for quartz; mica has perfect basal cleavage and a vitreous to pearly luster.
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Physical properties
Dark gray, coarse-grained, foliated specimen with sparkly black mica and lighter quartz/feldspar bands. Hardness is approximately 2.5–3 for biotite and 6–7 for quartz; mica has perfect basal cleavage and a vitreous to pearly luster.
Formation & geological history
Formed when shale or granitic material was subjected to intense heat and pressure during mountain building, typically hundreds of millions to billions of years ago. The visible layering reflects mineral alignment during metamorphism.
Uses & applications
Used locally as decorative stone, aggregate, and building stone; attractive pieces may be collected, though this small weathered specimen has little commercial value.
Geological facts
Biotite flakes can peel into elastic sheets and commonly produce the dark, glittering bands characteristic of schist and gneiss.
Field identification & locations
Look for planar foliation, alternating light and dark layers, and black reflective mica flakes; a knife scratches mica but not quartz. Common in metamorphic belts worldwide, including the Appalachians, Canadian Shield, and Scandinavia.
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