metamorphic
Gneiss (specifically Amphibolite Gneiss)
Gneiss
Hardness: 6-7 (Mohs scale); Color: Black, gray, and white banded appearance; Luster: Dull to vitreous; Crystal Structure: Foliated (gneissose banding); Cleavage: Poorly defined along bands; Specific Gravity: 2.6-2.9
- Hardness
- 6-7 (Mohs scale)
- Color
- Black, gray, and white banded appearance
- Luster
- Dull to vitreous
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Physical properties
Hardness: 6-7 (Mohs scale); Color: Black, gray, and white banded appearance; Luster: Dull to vitreous; Crystal Structure: Foliated (gneissose banding); Cleavage: Poorly defined along bands; Specific Gravity: 2.6-2.9
Formation & geological history
Formed through high-grade regional metamorphism of igneous or sedimentary rocks (protolith). High temperature and pressure cause minerals to segregate into bands. Often associated with Precambrian or Paleozoic mountain-building events.
Uses & applications
Used in construction as crushed stone for road bases or as dimension stone for counter tops, floor tiles, and building veneers. Valuable to collectors for demonstrating metamorphism.
Geological facts
Gneiss is among the oldest known rocks on Earth, with the Acasta Gneiss in Canada dating back up to 4 billion years. The distinct banding is called 'gneissose banding.'
Field identification & locations
Identify in the field by its distinct alternating dark (mafic) and light (felsic) bands. It is coarse-grained and does not split easily along planes like schist. Common in mountain cores and shield areas like the Canadian Shield or Appalachians.
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