metamorphic
Gneiss
Foliated Metamorphic Rock (Gneiss)
Hardness: 6-7 on Mohs scale; Color: typically banded with alternating light (quartz/feldspar) and dark (biotite/amphibole) layers; Luster: vitreous to dull; Crystal structure: coarse-grained crystalline; Cleavage: poor, but tends to split along foliation planes.
- Hardness
- 6-7 on Mohs scale
- Luster
- vitreous to dull
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Physical properties
Hardness: 6-7 on Mohs scale; Color: typically banded with alternating light (quartz/feldspar) and dark (biotite/amphibole) layers; Luster: vitreous to dull; Crystal structure: coarse-grained crystalline; Cleavage: poor, but tends to split along foliation planes.
Formation & geological history
Formed through high-grade regional metamorphism of pre-existing igneous rocks like granite or sedimentary rocks like shale. It is subjected to intense heat and pressure deep within the crust, often associated with mountain-building events (orogenies).
Uses & applications
Commonly used as a dimensional stone in construction for flooring, countertops, and facing on buildings. It is also crushed for use as road base and railway ballast. Polished specimens are sold in the lapidary trade.
Geological facts
Gneiss is one of the oldest rock types on Earth; the Acasta Gneiss in Northwest Territories, Canada, is over 4 billion years old. The term 'gneiss' comes from a Middle High German verb meaning 'to spark' because the rock glitters.
Field identification & locations
Identified in the field by its characteristic 'gneissic banding' where minerals are segregated into visible layers. Found in the deeply eroded cores of mountain ranges and ancient continental shields like the Canadian Shield or Appalachian Mountains.
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