metamorphic
Banded Gneiss
Gneiss
Hardness: 6-7 on Mohs scale. Color: Alternating dark (mafic) and light (felsic) bands of black, grey, white, and tan. Luster: Dull to vitreous. Structure: Foliated with distinct compositional banding. Cleavage: Poor, typically breaks into irregular chunks.
- Hardness
- 6-7 on Mohs scale
- Luster
- Dull to vitreous
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Physical properties
Hardness: 6-7 on Mohs scale. Color: Alternating dark (mafic) and light (felsic) bands of black, grey, white, and tan. Luster: Dull to vitreous. Structure: Foliated with distinct compositional banding. Cleavage: Poor, typically breaks into irregular chunks.
Formation & geological history
Formed from high-grade regional metamorphism of igneous (granite) or sedimentary (shale/mudstone) protoliths. Exposed to extreme heat and pressure, causing minerals to segregate into bands. Often found in Precambrian shields.
Uses & applications
Used as crushed stone for road construction and aggregate. High-quality slabs are used for countertops, flooring, and decorative building facades.
Geological facts
Gneiss includes some of the oldest known rocks on Earth, such as the Acasta Gneiss in Canada, which is over 4 billion years old. The term 'gneiss' comes from an old German verb meaning 'to spark' or 'to glow'.
Field identification & locations
Identify by looking for distinct 'gneissic banding' where minerals are separated into stripes. It will not split along flat planes like slate or schist. Often found in riverbeds as rounded cobbles after being eroded from mountain ranges.
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