metamorphic
Gneiss
Gneiss (Metamorphic Rock)
Hardness: 6-7 on Mohs scale. Color: Banded pink (potassium feldspar), gray/white (quartz), and black/dark green (biotite or hornblende). Luster: Dull to vitreous. Structure: Gneissic banding (foliated). Cleavage: Poor, typically breaks along plane of mica layers if present.
- Hardness
- 6-7 on Mohs scale
- Luster
- Dull to vitreous
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Physical properties
Hardness: 6-7 on Mohs scale. Color: Banded pink (potassium feldspar), gray/white (quartz), and black/dark green (biotite or hornblende). Luster: Dull to vitreous. Structure: Gneissic banding (foliated). Cleavage: Poor, typically breaks along plane of mica layers if present.
Formation & geological history
Formed through high-grade regional metamorphism of pre-existing igneous rocks like granite or sedimentary rocks like shale. Subjected to intense heat and pressure deep within the Earth's crust, often during mountain-building events (orogeny).
Uses & applications
Commonly used as a crushed stone for road construction, railroad ballast, and concrete aggregate. Polished slabs are used for architectural facing, flooring, and kitchen countertops due to its durability and attractive banding.
Geological facts
The oldest known rocks on Earth are the Acasta Gneiss in Canada, dated to approximately 4.03 billion years ago. The name 'gneiss' comes from an old German verb meaning 'to spark', referring to the way it glitters under light.
Field identification & locations
Identify in the field by looking for distinct alternating bands of light and dark minerals (foliation) where minerals have segregated into layers. It can be found globally in the core of mountain ranges and ancient continental shields.
