metamorphic
Gneiss (Schistose)
Gneiss
Hardness: 6-7 on Mohs scale. Color: Gray, black, and silver/white. Luster: Vitreous to pearly due to mica. Crystal structure: Coarse-grained with foliation. Cleavage: Poor, but breaks along foliation planes. Specific Gravity: 2.6-2.9.
- Hardness
- 6-7 on Mohs scale
- Color
- Gray, black, and silver/white
- Luster
- Vitreous to pearly due to mica
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Physical properties
Hardness: 6-7 on Mohs scale. Color: Gray, black, and silver/white. Luster: Vitreous to pearly due to mica. Crystal structure: Coarse-grained with foliation. Cleavage: Poor, but breaks along foliation planes. Specific Gravity: 2.6-2.9.
Formation & geological history
Formed through high-grade regional metamorphism of pre-existing igneous or sedimentary rocks. Subjection to intense heat and pressure causes recrystallization and alignment of minerals into parallel bands or layers.
Uses & applications
Commonly used as crushed stone for road construction and aggregate. High-quality slabs are used for countertops, building facades, and decorative landscaping.
Geological facts
Gneiss is one of the oldest known rock types on Earth, with some specimens in the Acasta Gneiss complex dated at over 4 billion years old. It often shows 'augen' (eye-like) structures where large crystals are wrapped by finer mineral bands.
Field identification & locations
Identify in the field by looking for distinct mineral banding or 'foliation' consisting of alternating light and dark layers. It is ubiquitous in mountainous regions and precambrian shield areas globally.
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