metamorphic
Gneiss
Gneiss (High-grade Regional Metamorphic Rock)
Hardness: 6-7 on Mohs scale; Color: Distinct salt-and-pepper banding of light (feldspar/quartz) and dark (biotite/hornblende) minerals; Luster: Dull to vitreous; Crystal Structure: Coarse-grained with Gneissose banding; Cleavage: Poorly defined perpendicular to banding.
- Hardness
- 6-7 on Mohs scale
- Luster
- Dull to vitreous
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Physical properties
Hardness: 6-7 on Mohs scale; Color: Distinct salt-and-pepper banding of light (feldspar/quartz) and dark (biotite/hornblende) minerals; Luster: Dull to vitreous; Crystal Structure: Coarse-grained with Gneissose banding; Cleavage: Poorly defined perpendicular to banding.
Formation & geological history
Formed through high-grade regional metamorphism of pre-existing igneous rocks (like granite) or sedimentary rocks (like shale). Occurs at high temperatures and pressures deep in the earth\'s crust, typically during mountain-building events (orogeny).
Uses & applications
Commonly used as a crushed stone for road construction, concrete aggregate, and railroad ballast. High-quality slabs are used for building facades, countertops, and ornamental landscaping stones.
Geological facts
Gneiss contains some of the oldest rocks on Earth, such as the Acasta Gneiss in Canada, which is approximately 4.03 billion years old. The term comes from the Middle High German word \'gneist\' meaning \'to spark\' because the rock can glitter.
Field identification & locations
Identify in the field by looking for \'foliation\' or alternating bands of light and dark minerals. It will not split into thin sheets like schist. It is extremely common in the roots of mountain ranges and ancient continental shields like the Canadian Shield or the Appalachians.
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