metamorphic
Gneiss
Gneiss
Hardness: 6-7 on Mohs scale; Color: Banded appearance with white, pink, gray, and black minerals; Luster: Vitreous to dull; Crystal structure: Coarse-grained, foliated with distinct banding (gneissic foliation); Cleavage: None (fractures irregularly); Specific gravity: 2.6-2.9.
- Hardness
- 6-7 on Mohs scale
- Color
- Banded appearance with white, pink, gray, and black minerals
- Luster
- Vitreous to dull
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Physical properties
Hardness: 6-7 on Mohs scale; Color: Banded appearance with white, pink, gray, and black minerals; Luster: Vitreous to dull; Crystal structure: Coarse-grained, foliated with distinct banding (gneissic foliation); Cleavage: None (fractures irregularly); Specific gravity: 2.6-2.9.
Formation & geological history
Formed through high-grade regional metamorphism of pre-existing igneous rocks (like granite) or sedimentary rocks (like shale). It is subjected to intense heat and pressure, often at depths of 10-45 km, resulting in the segregation of minerals into bands. These can range from Precambrian to much younger geological ages.
Uses & applications
Used primarily as a crushed stone in construction for road base and concrete aggregate. High-quality varieties are used as dimension stone for architectural facings, flooring, and decorative landscaping. Occasionally used for curbstones and paving.
Geological facts
Gneiss is one of the oldest rock types on Earth; the Acasta Gneiss in Canada is approximately 4.03 billion years old. The term comes from an Old High German verb meaning 'to spark' because the rock can glint in the light.
Field identification & locations
Identify in the field by looking for 'gneissic banding'—alternating layers of light-colored minerals (quartz, feldspar) and dark-colored minerals (biotite, amphibole). It is commonly found in the cores of mountain ranges and in ancient continental shields (e.g., Canada, Scandinavia, Brazil).
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