metamorphic
Gneiss
Gneiss (High-grade Metamorphic Rock)
Hardness: 6-7 on Mohs scale; Color: Distinct alternating bands of light (quartz, feldspar) and dark (biotite, amphibole) minerals; Luster: Vitreous to dull; Crystal structure: Coarse-grained foliated texture (gneissose banding).
- Hardness
- 6-7 on Mohs scale
- Luster
- Vitreous to dull
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Physical properties
Hardness: 6-7 on Mohs scale; Color: Distinct alternating bands of light (quartz, feldspar) and dark (biotite, amphibole) minerals; Luster: Vitreous to dull; Crystal structure: Coarse-grained foliated texture (gneissose banding).
Formation & geological history
Formed through high-grade regional metamorphism of pre-existing igneous or sedimentary rocks (protoliths) under intense heat and pressure. This occurs deep within the Earth's crust, often during mountain-building events (orogenies).
Uses & applications
Used primarily as a dimension stone for architectural facings, flooring, and gravestones. Crushed stone is used in construction for road base and concrete aggregate.
Geological facts
Gneiss contains some of the oldest known rocks on Earth, such as the Acasta Gneiss in Canada, which is approximately 4 billion years old. The term 'gneiss' comes from an Old High German verb meaning 'to spark' or 'glitter'.
Field identification & locations
Identify in the field by looking for 'gneissose banding', where minerals are segregated into distinct dark and light layers that are wider than schistosity. Commonly found in the crystalline cores of mountain ranges and ancient continental shields.
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