metamorphic
Gneiss
Gneiss (High-grade regional metamorphic rock)
Hardness: 6-7 (Mohs); Color: Alternating light (quartz/feldspar) and dark (biotite/amphibole) bands; Luster: Non-foliated to sub-vitreous; Structure: Foliated with distinct banding (gneissic foliation); Cleavage: Poorly defined outside of mineral layers; Specific Gravity: 2.6 to 2.9.
- Hardness
- 6-7 (Mohs)
- Luster
- Non-foliated to sub-vitreous
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Physical properties
Hardness: 6-7 (Mohs); Color: Alternating light (quartz/feldspar) and dark (biotite/amphibole) bands; Luster: Non-foliated to sub-vitreous; Structure: Foliated with distinct banding (gneissic foliation); Cleavage: Poorly defined outside of mineral layers; Specific Gravity: 2.6 to 2.9.
Formation & geological history
Formed through high-grade regional metamorphism of igneous (orthogneiss) or sedimentary (paragneiss) rocks at high temperatures and pressures. This occurs deep within the crust during mountain-building events (orogenies).
Uses & applications
Commonly used in construction as crushed stone for road bases and concrete, as decorative dimension stone for countertops and flooring, and for landscaping.
Geological facts
Gneiss contains some of the oldest rock formations on Earth, such as the Acasta Gneiss in Canada, which is roughly 4 billion years old. The banding is caused by the segregation of minerals under extreme pressure.
Field identification & locations
Identify in the field by looking for 'gneissic banding'—alternating stripes of different mineral colors. It is more coarse-grained than schist and lacks the tendency to split into thin sheets. Found in core areas of continents (cratons) and mountain ranges like the Appalachians or Alps.
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