metamorphic
Banded Gneiss
Gneiss (High-grade regional metamorphic rock)
Hardness: 6-7 on Mohs scale; Color: Alternating dark and light bands (mafic and felsic); Luster: Dull to vitreous; Structure: Foliated with distinct compositional banding (gneissic banding); Cleavage: Poor, usually breaks along irregular surfaces; Specific Gravity: 2.6-2.9.
- Hardness
- 6-7 on Mohs scale
- Color
- Alternating dark and light bands (mafic and felsic)
- Luster
- Dull to vitreous
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Physical properties
Hardness: 6-7 on Mohs scale; Color: Alternating dark and light bands (mafic and felsic); Luster: Dull to vitreous; Structure: Foliated with distinct compositional banding (gneissic banding); Cleavage: Poor, usually breaks along irregular surfaces; Specific Gravity: 2.6-2.9.
Formation & geological history
Formed through high-grade regional metamorphism of pre-existing igneous or sedimentary rocks (protoliths) under extreme temperature and pressure. This specimen shows a 'rolled' or river-worn texture, indicating it was likely eroded from a metamorphic mountain belt and shaped by water.
Uses & applications
Commonly used in the construction industry as crushed stone for road bases and concrete aggregate. High-quality specimens with attractive banding are used as decorative building stone, countertops, and landscaping rock.
Geological facts
Gneiss contains some of the oldest known crustal rocks on Earth, such as the Acasta Gneiss in Canada, which is over 4 billion years old. The distinct banding is caused by 'metamorphic differentiation,' where minerals segregate into layers during intense squeezing.
Field identification & locations
Identify in the field by looking for distinct 'zebra-stripe' layers of different minerals (usually white quartz/feldspar and black biotite/hornblende) that do not easily split like schist. Commonly found in the roots of ancient mountain ranges or glacial till in the Northern Hemisphere.
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