metamorphic
Gneiss
Gneiss (High-grade Regional Metamorphic Rock)
Hardness: 6-7 on Mohs scale; Color: Altnernating dark and light bands (salt and pepper appearance); Luster: Dull to vitreous; Crystal Structure: Coarse-grained, crystalline; Cleavage: Poor (fractures along grain boundaries rather than clean planes); Specific Gravity: 2.6–2.9.
- Hardness
- 6-7 on Mohs scale
- Luster
- Dull to vitreous
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Physical properties
Hardness: 6-7 on Mohs scale; Color: Altnernating dark and light bands (salt and pepper appearance); Luster: Dull to vitreous; Crystal Structure: Coarse-grained, crystalline; Cleavage: Poor (fractures along grain boundaries rather than clean planes); Specific Gravity: 2.6–2.9.
Formation & geological history
Formed by high-pressure, high-temperature regional metamorphism of pre-existing igneous rocks (like granite) or sedimentary rocks (like shale). This process typically occurs in the roots of mountain belts during tectonic collisions over millions of years.
Uses & applications
Commonly used in construction as crushed stone for road bases or as 'granite' in the countertop and flooring industry due to its durability and aesthetic banding. Also used for ornamental garden stones and monuments.
Geological facts
Gneiss contains some of the oldest known rocks on Earth, such as the Acasta Gneiss in Canada, which is approximately 4 billion years old. The term comes from an Old High German word meaning 'to spark' or 'bright' because of its mineral glisten.
Field identification & locations
Identify in the field by looking for 'gneissic banding'—alternating layers of light-colored minerals (quartz, feldspar) and dark-colored minerals (biotite, amphibole). Unlike schist, it does not split easily into thin sheets. Common in the Appalachian Mountains, Scandinavia, and Canada.
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