metamorphic
Gneiss
Gneiss
Hardness: 6-7 on Mohs scale; Color: Gray, black, pink, or white alternating bands; Luster: Variable (dull to vitreous/sub-metallic due to mica); Structure: Foliated with distinct compositional banding (gneissose banding); Cleavage: Fractures irregularly, though individual mica grains show perfect cleavage.
- Hardness
- 6-7 on Mohs scale
- Color
- Gray, black, pink, or white alternating bands
- Luster
- Variable (dull to vitreous/sub-metallic due to mica)
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Physical properties
Hardness: 6-7 on Mohs scale; Color: Gray, black, pink, or white alternating bands; Luster: Variable (dull to vitreous/sub-metallic due to mica); Structure: Foliated with distinct compositional banding (gneissose banding); Cleavage: Fractures irregularly, though individual mica grains show perfect cleavage.
Formation & geological history
Formed through high-grade regional metamorphism of pre-existing igneous rocks (like granite) or sedimentary rocks (like shale or sandstone) subjected to intense heat and pressure deep within the Earth's crust.
Uses & applications
Primarily used as a crushed stone in road construction and as a decorative building stone. Highly durable and often sold as 'granite' in the countertop industry due to its hardness and polishability.
Geological facts
Gneiss is some of the oldest rock found on Earth, with some specimens like the Acasta Gneiss dating back over 4 billion years. The distinct banding occurs because minerals separate into different layers during recrystallization.
Field identification & locations
Identify in the field by looking for 'zebra-striping' or coarse alternating light and dark bands. It does not split along thin planes as easily as schist. Commonly found in the cores of mountain ranges and ancient continental shields.
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