metamorphic
Banded Gneiss
Gneiss (High-grade Regional Metamorphic Rock)
Hardness: 6-7 on Mohs scale; Color: Alternating bands of dark (biotite/amphibole) and light (quartz/feldspar) minerals; Luster: Vitreous to dull; Crystal structure: Foliated with distinct mineral segregation; Cleavage: None (fractures irregularly); Specific gravity: 2.6-2.9.
- Hardness
- 6-7 on Mohs scale
- Luster
- Vitreous to dull
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Physical properties
Hardness: 6-7 on Mohs scale; Color: Alternating bands of dark (biotite/amphibole) and light (quartz/feldspar) minerals; Luster: Vitreous to dull; Crystal structure: Foliated with distinct mineral segregation; Cleavage: None (fractures irregularly); 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 pressure and temperature. This process leads to recrystallization and mineral segregation into bands. Gneiss is often associated with mountain-building events and is typically Proterozoic to Archean in age.
Uses & applications
Used primarily in building construction as crushed stone or decorative dimension stone (countertops and flooring). It is also valued as a educational geological specimen for demonstrating foliation.
Geological facts
Gneiss contains some of the oldest known crustal material on Earth, such as the Acasta Gneiss in Canada, which is roughly 4 billion years old. The term 'gneiss' comes from a Middle High German verb meaning to spark or glisten.
Field identification & locations
Identify in the field by looking for distinct color banding (gneissic banding) and a coarse-grained texture. It does not split along planes as easily as schist. Commonly found in the crystalline cores of mountain ranges and shield areas like the Canadian Shield or the Appalachian Mountains.
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