metamorphic
Banded gneiss
Gneiss, likely biotite–quartz–feldspar gneiss
AI-generated identification · may be incorrectMedium- to coarse-grained, strongly compositional-banded rock with pale quartz/feldspar layers and dark biotite or amphibole bands; feldspar and quartz have Mohs hardness 6–7, with no obvious cleavage at specimen scale. Typically nonmetallic, with a granular crystalline texture and specific gravity around 2.6–2.8.
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Physical properties
Medium- to coarse-grained, strongly compositional-banded rock with pale quartz/feldspar layers and dark biotite or amphibole bands; feldspar and quartz have Mohs hardness 6–7, with no obvious cleavage at specimen scale. Typically nonmetallic, with a granular crystalline texture and specific gravity around 2.6–2.8.
Formation & geological history
Formed when preexisting igneous or sedimentary rock underwent intense high-grade metamorphism, causing mineral segregation and foliation, commonly during Precambrian to Paleozoic mountain-building events. The image does not permit precise age or mineral identification.
Uses & applications
Used as dimension stone, decorative landscaping rock, aggregate, and occasionally countertops or architectural facing; attractive pieces are collected as examples of metamorphic banding.
Geological facts
Its alternating light and dark bands reflect metamorphic recrystallization rather than ordinary sedimentary layering. Gneiss commonly forms in deep continental crust alongside schist, granite, and migmatite.
Field identification & locations
Identify it by coarse interlocking grains and persistent light–dark banding that can split along foliation; distinguish it from granite by stronger banding and from schist by less platy mica. Common in ancient continental shields and exposed mountain belts worldwide.
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