metamorphic
Banded Gneiss
Gneiss (High-grade metamorphic rock typically composed of Quartz, Feldspar, and Biotite/Amphiboles)
Hardness: 6-7 on Mohs scale; Color: Gray, black, and white bands; Luster: Vitreous to dull; Crystal structure: Foliated (gneissose banding); Cleavage: Poor, splits irregularly along bands; Specific gravity: 2.6-2.9
- Hardness
- 6-7 on Mohs scale
- Color
- Gray, black, and white bands
- Luster
- Vitreous to dull
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Physical properties
Hardness: 6-7 on Mohs scale; Color: Gray, black, and white bands; Luster: Vitreous to dull; Crystal structure: Foliated (gneissose banding); Cleavage: Poor, splits irregularly along bands; Specific gravity: 2.6-2.9
Formation & geological history
Formed through high-grade regional metamorphism of pre-existing igneous or sedimentary rocks under extreme pressure and temperatures (usually deeper than 10-15km in the Earth's crust). Age varies by location, often associated with ancient Precambrian shields.
Uses & applications
Used primarily in construction as crushed stone for road bases and concrete aggregate. High-quality specimens with attractive banding are used as decorative building stone, countertops, and facing stone.
Geological facts
Gneiss contains some of the oldest known rocks on Earth, such as the Acasta Gneiss in Canada which is roughly 4 billion years old. The distinct 'gneissose' banding occurs because different minerals segregate into layers during intense tectonic compression.
Field identification & locations
Identify in the field by looking for distinct alternating light (felsic) and dark (mafic) mineral bands that are coarser than schist. It is commonly found in mountainous regions or stable continental interiors (cratons) like the Appalachians or the Canadian Shield.
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