metamorphic
Garnet Schist
Almandine Garnet in Mica Schist (Fe3Al2(SiO4)3)
Hardness: 6.5-7.5 (Garnet) / 2-2.5 (Mica matrix); Color: Deep red to brownish-red crystals in a silver-grey or bronze matrix; Luster: Vitreous (Garnet) to Pearly (Mica); Crystal Structure: Isometric (Garnet) / Monoclinic (Schist).
- Hardness
- 6
- Luster
- Vitreous (Garnet) to Pearly (Mica)
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Physical properties
Hardness: 6.5-7.5 (Garnet) / 2-2.5 (Mica matrix); Color: Deep red to brownish-red crystals in a silver-grey or bronze matrix; Luster: Vitreous (Garnet) to Pearly (Mica); Crystal Structure: Isometric (Garnet) / Monoclinic (Schist).
Formation & geological history
Formed under regional metamorphism of clay-rich sedimentary rocks (shales) at high temperatures and pressures, typically in orogenic belts. Age varies by location but involves deep crustal burial.
Uses & applications
Used as an abrasive in industrial sandblasting, water filtration, and occasionally as a semi-precious gemstone (garnet). The schist itself is often used for decorative stone and pavers.
Geological facts
Garnets are index minerals used by geologists to determine the degree of metamorphism (grade) a rock has experienced. The transition of mica into large garnet crystals indicates mid-to-high grade metamorphic zones.
Field identification & locations
Identify in the field by the 'porphyroblastic' texture—large, hard, dark red dodecahedral crystals embedded in a flaky, sub-parallel foliated mica matrix that glitters in sunlight. Found worldwide in mountain ranges like the Appalachians or Alps.
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