metamorphic
Banded Gneiss
Gneiss (High-grade Regional Metamorphic Rock)
Hardness: 6-7 on Mohs scale; Color: Alternating light (quartz/feldspar) and dark (biotite/amphibole) bands; Luster: Dull to vitreous; Structure: Foliated with distinct banding called 'gneissic banding'; Cleavage: Poor, usually breaks along uneven surfaces.
- Hardness
- 6-7 on Mohs scale
- Luster
- Dull to vitreous
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Physical properties
Hardness: 6-7 on Mohs scale; Color: Alternating light (quartz/feldspar) and dark (biotite/amphibole) bands; Luster: Dull to vitreous; Structure: Foliated with distinct banding called 'gneissic banding'; Cleavage: Poor, usually breaks along uneven surfaces.
Formation & geological history
Formed through high-grade regional metamorphism of pre-existing igneous or sedimentary rocks (protoliths) under extreme pressure and temperature (up to 700°C). This specimen represents the separation of minerals into layers. Geological ages vary from Precambrian to Cenozoic.
Uses & applications
Used in the construction industry as crushed stone for road bases, concrete aggregate, and as dimension stone for building facades, countertops, and curbstone. Polished specimens are used in landscaping and as minor decorative items.
Geological facts
Gneiss is one of the oldest known rock types on Earth; the Acasta Gneiss in Canada is approximately 4.03 billion years old. The banding is a result of chemical migration and recrystallization rather than original sedimentary layering.
Field identification & locations
Identify in the field by looking for 'gneissic banding' where minerals are segregated into light and dark stripes. It is commonly found in the crystalline cores of mountain ranges and stable continental shields (cratons). Hardness and density allow it to remain as rounded 'river rocks' or beach pebbles like the one pictured.
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