metamorphic
Gneiss (with Quartz and Mica)
Gneiss
Hardness: 6-7 on Mohs scale; Color: Gray, tan, and reddish-brown (likely due to iron oxidation); Luster: Sub-vitreous to dull; Crystal structure: Coarse-grained with visible foliation; Cleavage: Poor; Specific gravity: 2.6-2.9.
- Hardness
- 6-7 on Mohs scale
- Color
- Gray, tan, and reddish-brown (likely due to iron oxidation)
- Luster
- Sub-vitreous to dull
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Physical properties
Hardness: 6-7 on Mohs scale; Color: Gray, tan, and reddish-brown (likely due to iron oxidation); Luster: Sub-vitreous to dull; Crystal structure: Coarse-grained with visible foliation; Cleavage: Poor; Specific gravity: 2.6-2.9.
Formation & geological history
Formed through high-grade regional metamorphism of pre-existing igneous or sedimentary rocks. This specimen likely developed under intense heat and pressure deep within the crust, often associated with Precambrian or Paleozoic mountain-building events.
Uses & applications
Primarily used as crushed stone in construction for road bases and concrete aggregate. Select decorative varieties are used for countertops, floor tiles, and grave markers.
Geological facts
Gneiss is one of the oldest known rock types on Earth, with the Acasta Gneiss in Canada dating back over 4 billion years. Its characteristic wavy banding is called 'gneissose banding'.
Field identification & locations
Identify in the field by looking for distinct compositional banding or 'stripes' and a coarse crystalline texture. Commonly found in ancient shield areas like the Canadian Shield or the Appalachian Mountains. Used by geologists to map historical tectonic pressure lines.
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