Metamorphic
Gneiss
Gneiss (High-grade regional metamorphic rock)
Hardness: 6-7 (Mohs); Color: Banded grey, white, pink, and black; Luster: Pearly to dull; Structure: Foliated with distinct compositional banding (gneissic banding); Cleavage: Poor, splits along planes of high mica content; Specific Gravity: 2.6 - 3.0.
- Hardness
- 6-7 (Mohs)
- Color
- Banded grey, white, pink, and black
- Luster
- Pearly to dull
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Physical properties
Hardness: 6-7 (Mohs); Color: Banded grey, white, pink, and black; Luster: Pearly to dull; Structure: Foliated with distinct compositional banding (gneissic banding); Cleavage: Poor, splits along planes of high mica content; Specific Gravity: 2.6 - 3.0.
Formation & geological history
Formed through high-grade regional metamorphism of igneous rocks (granite) or sedimentary rocks (shale). Subjected to extreme heat and pressure, typically during mountain-building events (orogeny). Geological age varies from Archean to more recent tectonic events.
Uses & applications
Used primarily in the construction industry as crushed stone for road bases and concrete aggregate. High-quality slabs are used for architectural facings, paving stones, and sometimes kitchen countertops ('granite' trade name).
Geological facts
Gneiss includes some of the oldest known rocks on Earth, such as the Acasta Gneiss in Canada, dated to approximately 4.03 billion years old. The term comes from an old German word meaning 'sparkling' or 'to glisten.'
Field identification & locations
Identify by the presence of 'gneissic banding'—alternating layers of light minerals (quartz/feldspar) and dark minerals (biotite/amphibole). It is found globally in the cores of mountain ranges and ancient continental shields like the Canadian Shield or the Appalachians.
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