metamorphic
Banded Gneiss
Gneiss (High-grade Metamorphic Rock)
Hardness: 6-7 on Mohs scale; Color: Alternating light (quartz/feldspar) and dark (biotite/hornblende) bands; Luster: Dull to vitreous; Structure: Foliated with distinct banding (gneissic foliation); Cleavage: Poor, usually breaks along uneven surfaces.
- Hardness
- 6-7 on Mohs scale
- Luster
- Dull to vitreous
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Physical properties
Hardness: 6-7 on Mohs scale; Color: Alternating light (quartz/feldspar) and dark (biotite/hornblende) bands; Luster: Dull to vitreous; Structure: Foliated with distinct banding (gneissic foliation); Cleavage: Poor, usually breaks along uneven surfaces.
Formation & geological history
Formed through high-grade regional metamorphism of pre-existing igneous rocks (like granite) or sedimentary rocks (like shale). It subjects the parent rock to high temperatures and pressures, causing minerals to segregate into bands. Origin dates can range from hundreds of millions to billions of years (Precambrian).
Uses & applications
Used in construction as crushed stone for road bases, as dimension stone for building facades, floor tiles, and occasionally for gravestones or kitchen countertops (often sold as 'granite').
Geological facts
Gneiss is among the oldest known rocks on Earth, with the Acasta Gneiss in Canada dated at approximately 4.03 billion years old. The term 'gneiss' comes from an Old High German word meaning 'to spark' or 'bright.'
Field identification & locations
Identify by the 'gneissic banding' where minerals are separated into distinct layers. Common in the roots of mountain ranges and ancient continental shields like the Canadian Shield or the Appalachian Mountains. Look for the visible segregation of minerals and its characteristic toughness.
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