metamorphic
Garnet-bearing Schist
Almandine Garnet (Fe3Al2(SiO4)3) in Mica Schist
Hardness: 6.5-7.5 (garnet) vs 2-3 (mica matrix). Color: Deep reddish-brown porphyroblasts in a dark grey/black groundmass. Luster: Vitreous to resinous for the crystal; silky or pearly for the matrix. Crystal Structure: Isometric dodecahedral crystals.
- Hardness
- 6
- Luster
- Vitreous to resinous for the crystal
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Physical properties
Hardness: 6.5-7.5 (garnet) vs 2-3 (mica matrix). Color: Deep reddish-brown porphyroblasts in a dark grey/black groundmass. Luster: Vitreous to resinous for the crystal; silky or pearly for the matrix. Crystal Structure: Isometric dodecahedral crystals.
Formation & geological history
Formed under high temperature and pressure during regional metamorphism of clay-rich sedimentary rocks (pelites). This specific sample shows garnet porphyroblasts that grew within the solid-state rock during the metamorphic event, typically during mountain-building processes.
Uses & applications
Garnets are used as industrial abrasives (sandblasting, waterjet cutting) and as gemstones when transparent. The schist matrix is occasionally used as dimension stone or for decorative purposes in landscaping.
Geological facts
Garnets are often used by geologists as 'geothermometers' to calculate the depth and temperature at which a mountain range originally formed. Many red garnets found in schist are Almandine, the most common member of the garnet group.
Field identification & locations
Identified in the field by the presence of hard, round, reddish 'knots' protruding from a foliated (layered) rock surface. Commonly found in metamorphic belts like the Appalachians, Alps, or Himalayas. Collectors look for well-defined dodecahedral crystal faces.
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