metamorphic
Gneiss (High-Grade Metamorphic Rock)
Gneiss (primarily composed of Quartz, Feldspar, and Mica/Amphiboles)
Hardness: 6-7 on Mohs scale; Color: Gray, tan, white, and black banding; Luster: Vitreous to dull; Crystal Structure: Foliated (banded texture); Cleavage: Distinct foliation planes, though not as easily split as schist.
- Hardness
- 6-7 on Mohs scale
- Color
- Gray, tan, white, and black banding
- Luster
- Vitreous to dull
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Physical properties
Hardness: 6-7 on Mohs scale; Color: Gray, tan, white, and black banding; Luster: Vitreous to dull; Crystal Structure: Foliated (banded texture); Cleavage: Distinct foliation planes, though not as easily split as schist.
Formation & geological history
Formed through high-grade regional metamorphism of pre-existing igneous or sedimentary rocks (protoliths like granite or shale) subjected to extreme pressure and temperatures exceeding 600°C. Typically formed during mountain-building events (orogenies).
Uses & applications
Used primarily in the construction industry as crushed stone for road aggregates and as decorative architectural stone, countertops, and flooring when polished (often sold as 'Granite' in the trade).
Geological facts
Gneiss is some of the oldest rock on Earth, with the Acasta Gneiss in Canada dated to approximately 4 billion years old. The term comes from the Middle High German verb 'gneist' (to spark), describing the way the rock glints under light.
Field identification & locations
Identify in the field by looking for 'gneissose banding'—distinct light and dark alternating layers (segregation of minerals). Common globally in the 'roots' of ancient mountain ranges or continental shields. To collect, look for specimens with Sharp, contrasting bands and visible garnets or large feldspar phenocrysts.
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