metamorphic
Biotite–quartz–feldspar gneiss
Gneiss, composed chiefly of quartz (SiO₂), feldspar, and dark mica, likely biotite
AI-generated identification · may be incorrectTypically 6–7 Mohs; gray to black with white crystalline bands and rusty brown weathering. Vitreous quartz/feldspar luster, platy mica sparkle, no strong cleavage as a whole, and specific gravity about 2.6–2.8; visible compositional banding is characteristic.
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Physical properties
Typically 6–7 Mohs; gray to black with white crystalline bands and rusty brown weathering. Vitreous quartz/feldspar luster, platy mica sparkle, no strong cleavage as a whole, and specific gravity about 2.6–2.8; visible compositional banding is characteristic.
Formation & geological history
Formed by high-grade metamorphism of granite or other felsic protoliths, causing recrystallization and mineral segregation into light and dark layers. Such rocks commonly formed during Precambrian to Paleozoic mountain-building events, though exact age requires laboratory analysis.
Uses & applications
Used as building stone, architectural slabs, landscaping rock, and aggregate; attractive pieces may be collected but are rarely suitable as gemstones.
Geological facts
The specimen shows coarse interlocking grains and probable gneissic banding, with dark biotite-rich zones and pale quartz–feldspar veins. It may be confused with coarse granite or migmatite if banding is weak.
Field identification & locations
Identify it by alternating light and dark mineral-rich bands, abundant mica sparkle, and hard quartz grains; a glass scratch test and inspection of fresh surfaces help distinguish it from softer schist. Common in ancient continental shields and metamorphic mountain belts worldwide.
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