metamorphic
Banded gneiss (probable garnet–biotite gneiss)
Gneiss; chiefly quartz (SiO₂), feldspar, biotite mica, and possibly garnet
AI-generated identification · may be incorrectMedium- to coarse-grained, brown-gray, foliated and weakly banded, with a dull to slightly silky luster from mica. Typical hardness is about 6–7 Mohs; crystals are interlocked, with no strong cleavage as a whole, though mica flakes cleave readily.
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Physical properties
Medium- to coarse-grained, brown-gray, foliated and weakly banded, with a dull to slightly silky luster from mica. Typical hardness is about 6–7 Mohs; crystals are interlocked, with no strong cleavage as a whole, though mica flakes cleave readily.
Formation & geological history
Formed when granite or other felsic rock underwent high-grade regional metamorphism, producing mineral segregation and foliation; commonly Precambrian in age. The Québec City region lies within ancient Grenville/Appalachian geological terrains where gneiss and related metamorphic rocks are plausible.
Uses & applications
Used as crushed stone, aggregate, building stone, and decorative facing; attractive polished pieces are collected, though this specimen appears mainly of educational/common collecting value.
Geological facts
The planar fabric and light-dark mineral streaks distinguish gneiss from massive granite. Its prominent central fracture or vein may represent quartz-rich material or a weathered joint, but cannot be confirmed from the photograph alone.
Field identification & locations
Look for alternating mineral bands, visible mica glitter, and a hard, interlocking texture. Common regional alternatives include banded granite, migmatite, and schist; a streak test, fresh break, and magnification would refine identification.
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