metamorphic
Gneiss with Quartz Veining
Gneiss (High-grade regional metamorphic rock)
Hardness: 6-7 on Mohs scale; Color: Dark grey or black host rock with white to tan veins; Luster: Dull to vitreous; Structure: Foliated (banded); Cleavage: Poorly defined, tends to break along foliation planes; SG: 2.6 - 2.9.
- Hardness
- 6-7 on Mohs scale
- Color
- Dark grey or black host rock with white to tan veins
- Luster
- Dull to vitreous
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Physical properties
Hardness: 6-7 on Mohs scale; Color: Dark grey or black host rock with white to tan veins; Luster: Dull to vitreous; Structure: Foliated (banded); Cleavage: Poorly defined, tends to break along foliation planes; SG: 2.6 - 2.9.
Formation & geological history
Formed through high-pressure, high-temperature metamorphosis of igneous (granite) or sedimentary (shale) protoliths. The white veins are typically quartz or feldspar deposited by hydrothermal fluids in fractures during later phases of tectonic activity.
Uses & applications
Used primarily as crushed stone for road construction, concrete aggregate, and sometimes as dimension stone for counter tops or building facades if the pattern is particularly aesthetic.
Geological facts
Gneiss can be among the oldest rocks on Earth; the Acasta Gneiss in Canada is approximately 4 billion years old. The distinct layering is called 'gneissic banding' and results from the segregation of different mineral types.
Field identification & locations
Identify in the field by looking for distinct bands of light and dark minerals and a 'gritty' texture. The presence of cross-cutting white veins suggests a history of brittle deformation and fluid movement. Found globally in continental shields and mountain belts.
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