metamorphic
Gneiss
Gneiss (High-grade metamorphic rock composed primarily of Quartz, Feldspar, and various Micas)
Hardness: 6-7 on Mohs scale; Color: Characteristic 'gneissose banding' with alternating light (felsic) and dark (mafic) layers; Luster: Dull to vitreous; Structure: Foliated, medium to coarse-grained; Cleavage: Often exhibits poor cleavage along foliated planes.
- Hardness
- 6-7 on Mohs scale
- Luster
- Dull to vitreous
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Physical properties
Hardness: 6-7 on Mohs scale; Color: Characteristic 'gneissose banding' with alternating light (felsic) and dark (mafic) layers; Luster: Dull to vitreous; Structure: Foliated, medium to coarse-grained; Cleavage: Often exhibits poor cleavage along foliated planes.
Formation & geological history
Formed through high-grade regional metamorphism of pre-existing igneous (granite) or sedimentary (shale/mudstone) rocks under conditions of extreme temperature and pressure. It is often associated with mountain-building processes and can range in age from pre-Cambrian to relatively recent geological eras.
Uses & applications
Commonly used in the construction industry as crushed stone for road bases and concrete aggregate. It is also used as dimension stone for architectural facings, counter tops, and flooring when it displays attractive banding.
Geological facts
Gneiss is one of the oldest known rock types on Earth; the Acasta Gneiss in Canada is approximately 4 billion years old. The name comes from an old German mining term meaning 'sparkling' or 'to glisten.'
Field identification & locations
Identify in the field by looking for distinct mineral banding (foliation) where minerals are segregated into layers rather than being evenly mixed. It is found in the 'roots' of ancient mountain ranges. Collectors look for specimens with sharp, high-contrast color bands.
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