metamorphic
Banded Gneiss
Gneiss (High-grade regional metamorphic rock)
Hardness: 6-7 on Mohs scale; Color: Dark grey with light grey or white bands; Luster: Dull to sub-vitreous; Texture: Gneissose (alternating layers of different minerals); Cleavage: Poor, but splits along bands; Specific Gravity: 2.6-2.9.
- Hardness
- 6-7 on Mohs scale
- Color
- Dark grey with light grey or white bands
- Luster
- Dull to sub-vitreous
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Physical properties
Hardness: 6-7 on Mohs scale; Color: Dark grey with light grey or white bands; Luster: Dull to sub-vitreous; Texture: Gneissose (alternating layers of different minerals); Cleavage: Poor, but splits along bands; Specific Gravity: 2.6-2.9.
Formation & geological history
Formed through high-pressure, high-temperature regional metamorphism of igneous or sedimentary protoliths. The banding, or 'gneissose foliation', results from the separation of felsic (light) and mafic (dark) minerals during recrystallization.
Uses & applications
Used primarily as decorative stone, crushed stone for road construction, and occasionally for high-end countertops or landscaping features.
Geological facts
Gneiss contains some of the oldest rocks on Earth, such as the Acasta Gneiss in Canada, which is over 4 billion years old. The banding implies the rock was buried deep enough to be soft but not deep enough to melt into magma.
Field identification & locations
Identify by distinct 'zebra' banding of light and dark minerals. Commonly found in shield regions of continents (like the Canadian Shield) and in the roots of mountain ranges. To identify in the field, look for visible grain alignment and mineral segregation.
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