metamorphic
Garnet-bearing Quartzite
Metamorphic rock containing Almandine Garnet (Fe3Al2(SiO4)3) and Quartz (SiO2)
Hardness: 6.5-7.5 (depending on mineral ratio); Color: Rusty orange to yellow-brown matrix with dark reddish-brown to purple garnet inclusions; Luster: Vitreous to greasy; Structure: Granoblastic with porphyroblastic garnets.
- Hardness
- 6
- Luster
- Vitreous to greasy
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Physical properties
Hardness: 6.5-7.5 (depending on mineral ratio); Color: Rusty orange to yellow-brown matrix with dark reddish-brown to purple garnet inclusions; Luster: Vitreous to greasy; Structure: Granoblastic with porphyroblastic garnets.
Formation & geological history
Formed through the regional metamorphism of clay-rich sandstones or schistose rocks at high pressure and temperature. The garnets grew as new minerals (porphyroblasts) within the quartz matrix during the recrystallization process.
Uses & applications
Predominantly used as architectural stone or decorative landscaping. Low-quality garnets are used as industrial abrasives (sandblasting and water-jet cutting).
Geological facts
The almandine garnets found in such rocks are the most common type of garnet and were widely used in ancient Roman jewelry. The 'rusty' color of the matrix is often due to iron oxide staining from weathered pyrite or magnetite.
Field identification & locations
Identify by looking for hard, rounded reddish 'seeds' embedded in a granular quartz-rich rock. Common in metamorphic belts such as the Appalachian or Himalayan mountains. It will easily scratch glass.
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