metamorphic
Banded gneiss
Gneiss, commonly quartz–feldspar–mica gneiss
AI-generated identification · may be incorrectTypically medium-to-coarse grained, with alternating light quartz/feldspar and dark biotite or amphibole bands; Mohs hardness about 6–7, vitreous to dull luster, and weak foliation rather than distinct cleavage.
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Physical properties
Typically medium-to-coarse grained, with alternating light quartz/feldspar and dark biotite or amphibole bands; Mohs hardness about 6–7, vitreous to dull luster, and weak foliation rather than distinct cleavage.
Formation & geological history
Formed by high-grade regional metamorphism of granite or sedimentary protoliths, commonly during Precambrian to Paleozoic mountain building; mineral segregation produces the prominent banding.
Uses & applications
Used as dimension stone, building and landscaping stone, riprap, monuments, and aggregate; attractive polished pieces have modest decorative and collector value.
Geological facts
Gneissic banding reflects intense heat, pressure, and partial recrystallization, not ordinary sedimentary layering; folds within the bands can record ancient tectonic deformation.
Field identification & locations
Identify it by coarse crystals and continuous light-dark compositional bands, unlike slate’s fine cleavage or sandstone’s granular layers. Common in continental shields and exposed mountain cores, including Scandinavia, Canada, the northeastern United States, and the Alps.
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