metamorphic
Mica-rich metamorphic rock, likely biotite schist
Biotite schist; chiefly quartz, feldspar, and biotite mica
AI-generated identification · may be incorrectDark gray-brown, flaky and sparkly with a foliated texture; mica flakes show vitreous to pearly luster and perfect cleavage, while quartz and feldspar are harder (Mohs 6–7). Overall hardness is variable, commonly about 3–6 depending on exposed minerals; specific gravity roughly 2.7–3.0.
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Physical properties
Dark gray-brown, flaky and sparkly with a foliated texture; mica flakes show vitreous to pearly luster and perfect cleavage, while quartz and feldspar are harder (Mohs 6–7). Overall hardness is variable, commonly about 3–6 depending on exposed minerals; specific gravity roughly 2.7–3.0.
Formation & geological history
Formed when clay-rich sedimentary rock was buried and subjected to moderate-to-high heat and pressure, aligning mica into foliation. Such rocks commonly formed during ancient mountain-building events, often Precambrian to Paleozoic in age.
Uses & applications
Mostly used as a decorative or educational specimen; crushed schist may be used locally as fill or aggregate, but strong foliation limits structural construction use. Attractive pieces have modest collector value.
Geological facts
The silvery or bronze shimmer comes from reflective biotite cleavage surfaces. Foliation causes the rock to split preferentially along parallel mica-rich layers.
Field identification & locations
Identify it by visible planar layering, abundant reflective flakes, and easy splitting along foliation; a streak test and hardness test help distinguish it from slate or phyllite. Common in regional metamorphic belts worldwide, including the Appalachians, New England, Scotland, and India.
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