metamorphic
Biotite-rich gneiss (possibly mica schist)
Quartz–feldspar–biotite gneiss; mica-rich metamorphic rock
AI-generated identification · may be incorrectTypically gray to black, coarse-grained, with sparkling flaky biotite mica and lighter quartz/feldspar bands; uneven foliation and irregular fracture. Hardness is about 2.5–3 for biotite and 6–7 for quartz/feldspar; nonmetallic luster and no clear cleavage as a whole, though mica flakes cleave readily.
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Physical properties
Typically gray to black, coarse-grained, with sparkling flaky biotite mica and lighter quartz/feldspar bands; uneven foliation and irregular fracture. Hardness is about 2.5–3 for biotite and 6–7 for quartz/feldspar; nonmetallic luster and no clear cleavage as a whole, though mica flakes cleave readily.
Formation & geological history
Formed when preexisting igneous or sedimentary rock underwent intense heat and pressure, producing mineral segregation and foliation during regional metamorphism, commonly in Precambrian to Paleozoic terranes.
Uses & applications
Used locally as dimension stone, landscaping rock, aggregate, and building facing; attractive polished pieces may be sold as decorative specimens, but mica-rich material has limited durability outdoors.
Geological facts
The dark reflective flakes are likely biotite, while pale gray translucent grains are quartz or feldspar. Strong foliation and mica cleavage distinguish it from ordinary granite.
Field identification & locations
Identify it by its layered or streaked texture, abundant shiny mica, and hard light-colored grains; common in metamorphic mountain belts worldwide, including the Appalachians, Canadian Shield, and European Alps.
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