metamorphic
Garnet Schist
Almandine Garnet-Mica Schist (X₃Y₂(SiO₄)₃)
Hardness: 6.5-7.5 (garnets), 2.5 (mica); Color: Red/brown crystals in a silver/grey/white matrix; Luster: Vitreous to sub-vitreous; Structure: Foliated matrix with porphyroblastic crystals; Cleavage: None in garnet, perfect basal in mica.
- Hardness
- 6
- Color
- Red/brown crystals in a silver/grey/white matrix
- Luster
- Vitreous to sub-vitreous
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Physical properties
Hardness: 6.5-7.5 (garnets), 2.5 (mica); Color: Red/brown crystals in a silver/grey/white matrix; Luster: Vitreous to sub-vitreous; Structure: Foliated matrix with porphyroblastic crystals; Cleavage: None in garnet, perfect basal in mica.
Formation & geological history
Formed through regional metamorphism of clay-rich sedimentary rocks (shales) under high pressure and temperature conditions, typically during mountain-building events (orogenies). Most specimens range from several hundred million to over a billion years old.
Uses & applications
Used primarily for geological study, education, and as mineral specimens for collectors. Industrial garnets are used as abrasives, while high-quality individual crystals may be used in jewelry.
Geological facts
Garnets are known as 'porphyroblasts' because they grow larger than the surrounding mineral matrix during metamorphism. They are often used by geologists as 'geothermometers' to calculate the temperature at which the rock formed.
Field identification & locations
Identify by looking for distinct, dark red dodecahedron crystals embedded within a shiny, flaky, or layered 'schistose' matrix. Common in metamorphic belts like the Appalachians, Alps, and Himalayas. Check for the characteristic sparkle of mica flakes.
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