metamorphic
Gneiss (Granitic Gneiss)
Gneiss
Hardness: 6-7 on Mohs scale; Color: Pinkish-orange (orthoclase), white (quartz), and black (biotite/hornblende); Luster: Vitreous to dull; Crystal structure: Coarse-grained foliated; Cleavage: Poorly defined overall but mica flakes show perfect basal cleavage; Specific gravity: 2.6-2.9.
- Hardness
- 6-7 on Mohs scale
- Luster
- Vitreous to dull
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Physical properties
Hardness: 6-7 on Mohs scale; Color: Pinkish-orange (orthoclase), white (quartz), and black (biotite/hornblende); Luster: Vitreous to dull; Crystal structure: Coarse-grained foliated; Cleavage: Poorly defined overall but mica flakes show perfect basal cleavage; Specific gravity: 2.6-2.9.
Formation & geological history
Formed by high-grade regional metamorphism of a protolith (likely granite or sedimentary rock) under intense heat and pressure, causing minerals to segregate into bands. Often Precambrian in age.
Uses & applications
Used as crushed stone for road construction, as dimension stone in architecture (countertops, flooring, facing), and as decorative landscaping rock.
Geological facts
Gneiss contains some of the oldest known rocks on Earth, such as the Acasta Gneiss in Canada, which is approximately 4.03 billion years old.
Field identification & locations
Identify by 'gneissose banding' (alternating light and dark layers). Unlike schist, it does not split easily along planes. Found in the cores of mountain ranges and ancient continental shields.
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