metamorphic
Banded Gneiss (with Quartz Vein)
Gneiss (SiO2 and varied Silicates)
Hardness: 6-7 on Mohs scale; Color: Black, white, and grey bands; Luster: Dull to vitreous; Crystal Structure: Coarse-grained foliated texture (gneissose banding); Cleavage: Poorly defined, breaks along foliation; Specific Gravity: 2.6-2.9.
- Hardness
- 6-7 on Mohs scale
- Color
- Black, white, and grey bands
- Luster
- Dull to vitreous
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Physical properties
Hardness: 6-7 on Mohs scale; Color: Black, white, and grey bands; Luster: Dull to vitreous; Crystal Structure: Coarse-grained foliated texture (gneissose banding); Cleavage: Poorly defined, breaks along foliation; Specific Gravity: 2.6-2.9.
Formation & geological history
Formed through high-grade regional metamorphism of pre-existing igneous or sedimentary rocks under intense heat and pressure. The white portion is likely a hydrothermal quartz vein or a leucosome from partial melting (migmatization).
Uses & applications
Used primarily as crushed stone for road construction, as dimension stone for building facades, countertops, and landscaping.
Geological facts
Gneiss is among the oldest known rocks on Earth, with some specimens like the Acasta Gneiss dating back over 4 billion years. The distinct 'zebra' banding is caused by the segregation of light (felsic) and dark (mafic) minerals.
Field identification & locations
Identify in the field by looking for distinct light and dark parallel banding (foliation) that is coarser than schist. It is very common in mountain belts and Precambrian shield areas like the Canadian Shield or the Appalachians.
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