metamorphic
Banded Gneiss
Gneiss (High-grade regional metamorphic rock)
Hardness: 6-7 (Mohs scale); Color: Gray or dark blue matrix with white or cream-colored bands; Luster: Dull to vitreous; Crystal structure: Crystalline with visible foliation; Cleavage: Generally poor, though mica layers may flake; Specific gravity: 2.6-2.9
- Hardness
- 6-7 (Mohs scale)
- Color
- Gray or dark blue matrix with white or cream-colored bands
- Luster
- Dull to vitreous
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Physical properties
Hardness: 6-7 (Mohs scale); Color: Gray or dark blue matrix with white or cream-colored bands; Luster: Dull to vitreous; Crystal structure: Crystalline with visible foliation; Cleavage: Generally poor, though mica layers may flake; Specific gravity: 2.6-2.9
Formation & geological history
Formed through high-grade regional metamorphism of pre-existing igneous or sedimentary rocks (protoliths) under extreme temperature and pressure. The distinct 'gneissoid banding' occurs as minerals segregate into light and dark layers over millions of years.
Uses & applications
Used primarily in construction as crushed stone or road base, occasionally as dimension stone for counter tops (often sold as 'Granite'), and in landscape design.
Geological facts
Gneiss is some of the oldest rock on Earth; the Acasta Gneiss in Canada is dated at roughly 4 billion years old. The term 'gneiss' comes from an old German word meaning 'sparkling' or 'to glow'.
Field identification & locations
Identify in the field by looking for 'compositional banding' where minerals are sorted into distinct layers rather than being randomly mixed. Commonly found in the roots of mountain ranges or in glacial deposits. It is more resistant to weathering than many other rocks.
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