metamorphic
Gneiss
Gneiss
Hardness: 6-7 (Mohs scale); Color: Banded grey, white, and black with visible mica; Luster: Dull to sub-vitreous; Crystal structure: Gneissic banding (foliated); Cleavage: None (fractures irregularly); Specific gravity: 2.6 - 2.9
- Hardness
- 6-7 (Mohs scale)
- Color
- Banded grey, white, and black with visible mica
- Luster
- Dull to sub-vitreous
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Physical properties
Hardness: 6-7 (Mohs scale); Color: Banded grey, white, and black with visible mica; Luster: Dull to sub-vitreous; Crystal structure: Gneissic banding (foliated); Cleavage: None (fractures irregularly); Specific gravity: 2.6 - 2.9
Formation & geological history
Formed through high-grade regional metamorphism of igneous (granite) or sedimentary (shale/mudstone) protoliths under intense heat and pressure deep within the Earth's crust, often during mountain-building events (orogenesis).
Uses & applications
Used primarily as decorative stone in architecture (flooring, countertops), as crushed stone for road construction and railroad ballast, and for landscape design.
Geological facts
Gneiss contains some of the oldest rocks on Earth, such as the Acasta Gneiss in Canada, which is dated at approximately 4.03 billion years old. The distinct bands are caused by the separation of light (quartz/feldspar) and dark (biotite/amphibole) minerals.
Field identification & locations
Identify by looking for 'gneissic banding,' which appears as distinct alternating layers of light and dark minerals that are often contorted or folded. Common in the core of major mountain ranges like the Appalachians, Alps, and Himalayas.
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