metamorphic
Biotite schist (mica schist), possibly garnet-bearing
Mica schist; chiefly quartz (SiO₂), feldspar, and biotite mica, with possible garnet
AI-generated identification · may be incorrectMedium-grained, sparkly black-and-silvery mica with pale quartz/feldspar; foliated and weakly schistose, with mica’s perfect basal cleavage. Hardness is about 2.5–3.5 for biotite and 6–7 for quartz; luster is pearly to vitreous, and specific gravity is roughly 2.7–3.0.
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Physical properties
Medium-grained, sparkly black-and-silvery mica with pale quartz/feldspar; foliated and weakly schistose, with mica’s perfect basal cleavage. Hardness is about 2.5–3.5 for biotite and 6–7 for quartz; luster is pearly to vitreous, and specific gravity is roughly 2.7–3.0.
Formation & geological history
Formed when clay-rich sedimentary rock was buried and subjected to regional metamorphism, typically during mountain building; ages vary widely from Precambrian to younger orogenic belts.
Uses & applications
Used locally as decorative stone, aggregate, landscaping material, and occasionally building facing; attractive specimens are collected, but it is not usually a jewelry stone.
Geological facts
Its glitter comes from aligned mica flakes reflecting light. Foliation can cause the rock to split into thin, irregular sheets.
Field identification & locations
Common in metamorphic mountain belts, including the Appalachians, Alps, and Himalaya. Identify it by visible mica, layered foliation, and surfaces that sparkle or peel; a garnet-bearing variety shows rounded red-brown crystals.
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