metamorphic
Banded gneiss (possibly migmatitic gneiss)
Quartz–feldspar–mica gneiss; composition commonly includes quartz, feldspar, biotite/hornblende, and accessory dark minerals
AI-generated identification · may be incorrectTypically medium–coarse grained with strongly alternating light and dark mineral bands; vitreous quartz, feldspathic patches, and dark mica/amphibole. Hardness is about 6–7 Mohs, with weak-to-moderate foliation and irregular fracture; the specimen appears naturally weathered and iron-stained.
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Physical properties
Typically medium–coarse grained with strongly alternating light and dark mineral bands; vitreous quartz, feldspathic patches, and dark mica/amphibole. Hardness is about 6–7 Mohs, with weak-to-moderate foliation and irregular fracture; the specimen appears naturally weathered and iron-stained.
Formation & geological history
Formed when preexisting igneous or sedimentary rocks underwent high-temperature, high-pressure metamorphism, causing minerals to recrystallize and segregate into bands. Such rocks commonly formed deep in Precambrian to Paleozoic continental crust, though exact age requires locality data.
Uses & applications
Used as crushed stone, aggregate, building stone, and decorative dimension stone; attractive polished pieces are collected but are generally not gemstone-grade.
Geological facts
The banding records metamorphic deformation and mineral segregation, often preserving multiple episodes of crustal heating and compression. Light layers are usually quartz/feldspar-rich, while dark layers contain mica or amphibole.
Field identification & locations
Identify it by persistent alternating light and dark bands, interlocking grains, and a hard, nonporous texture; distinguish it from striped sedimentary rock by its crystalline fabric and disrupted or folded banding. Common in ancient continental shields and regional metamorphic belts.
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