metamorphic
Gneiss
Gneiss (High-grade Metamorphic Rock)
Hardness: 6-7 on Mohs scale; Color: Gray, salt-and-pepper, or tan with dark bands; Luster: Dull to sub-vitreous; Crystal structure: Phaneritic (coarse-grained) with gneissic banding (foliation); Cleavage: None (fractures unevenly); Specific gravity: 2.6 to 2.9.
- Hardness
- 6-7 on Mohs scale
- Color
- Gray, salt-and-pepper, or tan with dark bands
- Luster
- Dull to sub-vitreous
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Physical properties
Hardness: 6-7 on Mohs scale; Color: Gray, salt-and-pepper, or tan with dark bands; Luster: Dull to sub-vitreous; Crystal structure: Phaneritic (coarse-grained) with gneissic banding (foliation); Cleavage: None (fractures unevenly); Specific gravity: 2.6 to 2.9.
Formation & geological history
Formed by high-pressure and high-temperature regional metamorphism of pre-existing igneous or sedimentary rocks (like granite or shale). The banding is caused by the segregation of minerals during intense heat and pressure, often dating back to the Precambrian or Paleozoic eras.
Uses & applications
Used frequently as a dimension stone for flooring, facing, and cemetery monuments; also crushed for use in road construction, railroad ballast, and landscaping.
Geological facts
Gneiss is one of the oldest rock types on Earth, with some specimens from the Acasta Gneiss complex in Canada dated to over 4 billion years old. The term comes from the Middle High German verb 'gneist', meaning 'to spark', because the rock often glitters due to mica content.
Field identification & locations
Identify in the field by looking for distinct, thick bands of light and dark minerals (maca, quartz, feldspar, and amphiboles). Commonly found in the crystalline cores of mountain ranges and stable continental shields.
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