metamorphic
Banded gneiss
Foliated felsic gneiss; commonly quartz–feldspar–mica bearing
AI-generated identification · may be incorrectHardness about 6–7 Mohs; gray, white, and dark mineral bands, with a granular texture and dull to vitreous luster. Strong compositional banding and foliation are visible; cleavage is absent overall, though mica layers may split.
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Physical properties
Hardness about 6–7 Mohs; gray, white, and dark mineral bands, with a granular texture and dull to vitreous luster. Strong compositional banding and foliation are visible; cleavage is absent overall, though mica layers may split.
Formation & geological history
Formed when granite or other continental crustal rock underwent high-grade metamorphism, typically at roughly 500–800°C during mountain building. Individual specimens may be Precambrian or younger, depending on locality.
Uses & applications
Used as dimension stone, decorative facing, paving, landscaping rock, and aggregate; attractive banded pieces are collected or polished.
Geological facts
The light bands are usually quartz and feldspar, while dark bands commonly contain biotite or hornblende. Gneissic banding records intense deformation and mineral segregation deep in the crust.
Field identification & locations
Identify it by coarse interlocking grains and alternating light–dark mineral bands, distinguishing it from finer-grained slate or schist. Common in ancient continental shields and exposed mountain belts worldwide.
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