metamorphic
Banded Gneiss
Gneiss (High-grade Metamorphic Rock)
Hardness: 6-7 (Mohs scale); Color: Typically alternating light (quartz/feldspar) and dark (biotite/amphibole) bands; Luster: Dull to vitreous; Crystal Structure: Foliated with coarse grains; Cleavage: Poorly defined along bands.
- Hardness
- 6-7 (Mohs scale)
- Luster
- Dull to vitreous
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Physical properties
Hardness: 6-7 (Mohs scale); Color: Typically alternating light (quartz/feldspar) and dark (biotite/amphibole) bands; Luster: Dull to vitreous; Crystal Structure: Foliated with coarse grains; Cleavage: Poorly defined along bands.
Formation & geological history
Formed through high-grade regional metamorphism of pre-existing igneous or sedimentary rocks at extreme temperatures (over 600°C) and pressures. This causes the minerals to recrystallize and segregate into distinct layers (gneissic banding).
Uses & applications
Commonly used in the construction industry as crushed stone for road bases or as 'granite' slabs for countertops and flooring due to its durability. Small, uniquely banded river stones are popular with hobbyists for rock tumbling.
Geological facts
Gneiss contains some of the oldest rock formations on Earth, such as the Acasta Gneiss in Canada, which is roughly 4 billion years old. The term 'gneiss' comes from a German word meaning 'sparkling' or 'to glow.'
Field identification & locations
Identify in the field by looking for distinct 'gneissic banding' (parallel layers of different minerals) that do not split easily like slate or schist. Often found in mountainous regions or glacial deposits. Its water-worn, rounded shape suggests it was moved by a river or glacier.
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