metamorphic
Banded gneiss
Gneiss, commonly quartz–feldspar–biotite gneiss
AI-generated identification · may be incorrectTypically medium-to-coarse grained, with alternating light quartz/feldspar bands and dark biotite or amphibole layers; hardness about 6–7, vitreous to pearly luster, and weak cleavage controlled by foliation. The specimen shows strongly contorted compositional banding and brown-black weathering.
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Physical properties
Typically medium-to-coarse grained, with alternating light quartz/feldspar bands and dark biotite or amphibole layers; hardness about 6–7, vitreous to pearly luster, and weak cleavage controlled by foliation. The specimen shows strongly contorted compositional banding and brown-black weathering.
Formation & geological history
Formed when pre-existing igneous or sedimentary rock underwent high-grade metamorphism, commonly at roughly 600–800°C during continental collision; many gneisses are Precambrian, though exact age requires laboratory analysis.
Uses & applications
Used as crushed aggregate, dimension stone, landscaping rock, and decorative slabs; attractive polished pieces are collected but are generally low-value.
Geological facts
Its alternating bands record mineral segregation and intense deformation deep in Earth’s crust. It may resemble schist, but gneiss has thicker, more compositionally distinct bands and less obvious platy mica alignment.
Field identification & locations
Identify it by coarse grains and light-dark mineral banding, often folded or streaked; common in ancient continental shields and mountain belts worldwide, including Scandinavia, Canada, the northeastern United States, and India.
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