metamorphic
Gneiss (Metamorphic Rock)
Gneiss
Hardness: 6-7 on Mohs scale; Color: Gray, pink, white, and black banded patterns; Luster: Dull to vitreous; Crystal Structure: Coarse-grained with foliation; Cleavage: Poorly developed (tends to break across bands); Specific Gravity: 2.6-2.9.
- Hardness
- 6-7 on Mohs scale
- Color
- Gray, pink, white, and black banded patterns
- Luster
- Dull to vitreous
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Physical properties
Hardness: 6-7 on Mohs scale; Color: Gray, pink, white, and black banded patterns; Luster: Dull to vitreous; Crystal Structure: Coarse-grained with foliation; Cleavage: Poorly developed (tends to break across bands); Specific Gravity: 2.6-2.9.
Formation & geological history
Formed by high-grade regional metamorphism of pre-existing igneous rocks (like granite) or sedimentary rocks (like shale). It is created under extreme temperature and pressure deep within the Earth's crust, often during mountain-building events (orogeny). Geological ages vary from over 4 billion years (Acasta Gneiss) to relatively recent eras.
Uses & applications
Commonly used as a dimension stone for construction, paving, countertops, and crushed stone for road base. Due to its durability and attractive banding, it is often used in architectural landscaping and cemetery monuments.
Geological facts
Gneiss is one of the most common and widely distributed rocks in the Earth's crust. The 'Acasta Gneiss' in Canada is one of the oldest known intact crustal rocks on Earth, dated at approximately 4.03 billion years old.
Field identification & locations
Identify in the field by looking for 'gneissose banding,' which are distinct alternating layers of light-colored minerals (quartz/feldspar) and dark-colored minerals (mica/amphibole). Unlike schist, gneiss does not split easily along planes. It is found globally in the core of ancient mountain ranges and Precambrian shields.
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