metamorphic
Gneiss (with Quartzite layers)
Gneissic Metamorphic Rock
Hardness: 6-7 on Mohs scale; Color: Grey, white, and tan bands; Luster: Dull to vitreous; Structure: Foliated with distinct mineral banding (gneissose), medium to coarse grained; Cleavage: Generally poor, breaks along foliations.
- Hardness
- 6-7 on Mohs scale
- Color
- Grey, white, and tan bands
- Luster
- Dull to vitreous
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Physical properties
Hardness: 6-7 on Mohs scale; Color: Grey, white, and tan bands; Luster: Dull to vitreous; Structure: Foliated with distinct mineral banding (gneissose), medium to coarse grained; Cleavage: Generally poor, breaks along foliations.
Formation & geological history
Formed through high-grade regional metamorphism of pre-existing igneous or sedimentary rocks. High temperatures and pressures cause chemical recrystallization and mineral alignment into bands. Often dating to the Precambrian or Paleozoic eras.
Uses & applications
Used primarily as crushed stone for road construction and aggregate. Higher quality slabs are used as dimension stone for flooring, countertops, and grave markers.
Geological facts
Gneiss contains some of the oldest rocks on Earth, such as the Acasta Gneiss in Canada, which is approximately 4 billion years old. The banding is often caused by the migration of minerals during metamorphism.
Field identification & locations
Identify in the field by looking for 'salt and pepper' appearance and distinct compositional banding. It is commonly found in mountain belts and Precambrian shield areas like the Appalachians or the Canadian Shield.
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