metamorphic
Garnet Schist
Garnet-bearing Mica Schist (variety of compositions, often Al2SiO5 with X3Y2(SiO4)3)
Hardness: 6.5–7.5 (garnets) vs 2–3 (matrix); Color: Dark brown/grey matrix with reddish-pink crystals; Luster: Dull/earthy matrix, vitreous garnets; Structure: Foliated schistose texture; Cleavage: Perfect in mica, none in garnets.
- Hardness
- 6
- Color
- Dark brown/grey matrix with reddish-pink crystals
- Luster
- Dull/earthy matrix, vitreous garnets
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Physical properties
Hardness: 6.5–7.5 (garnets) vs 2–3 (matrix); Color: Dark brown/grey matrix with reddish-pink crystals; Luster: Dull/earthy matrix, vitreous garnets; Structure: Foliated schistose texture; Cleavage: Perfect in mica, none in garnets.
Formation & geological history
Formed through the regional metamorphism of clay-rich sedimentary rocks (pelites) such as shale or mudstone under high temperature and pressure conditions, typical of mountain-building events.
Uses & applications
Used as an abrasive in industry, a specimen for geological education, and occasionally the garnets are extracted for low-grade gemstones or sandpaper production.
Geological facts
The garnets in these rocks are known as 'porphyroblasts' because they grew larger than the surrounding mineral grains during the metamorphic process. They provide a record of the peak pressure/temperature the rock experienced.
Field identification & locations
Identify in the field by looking for 'sparkly' mica layers and distinct red/brown dodecahedral crystal bumps. Commonly found in metamorphic belts like the Alps, Himalayas, or Appalachian Mountains.
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