metamorphic
Garnet-Mica Schist
Schist (variety with Almandine Garnet, Fe3Al2(SiO4)3)
Mohs Hardness: 6.5-7.5 (garnets) and 2-3 (mica/matrix); Color: Silvery-gray matrix with deep reddish-brown inclusions; Luster: Vitreous to sub-vitreous (garnets), Pearly to metallic (mica); Structure: Foliated or schistose texture with porphyroblastic crystals; Specific Gravity: 3.5-4.3.
- Hardness
- 6
- Color
- Silvery-gray matrix with deep reddish-brown inclusions
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Physical properties
Mohs Hardness: 6.5-7.5 (garnets) and 2-3 (mica/matrix); Color: Silvery-gray matrix with deep reddish-brown inclusions; Luster: Vitreous to sub-vitreous (garnets), Pearly to metallic (mica); Structure: Foliated or schistose texture with porphyroblastic crystals; Specific Gravity: 3.5-4.3.
Formation & geological history
Formed through regional metamorphism of shale or mudstone under intermediate to high temperatures and pressures. This creates a foliated texture as mica minerals align, with garnets growing as porphyroblasts during the process. Usually dating from the Proterozoic to Paleozoic eras depending on the orogeny.
Uses & applications
Garnets are used as industrial abrasives (sandblasting, waterjet cutting) and in jewelry as gemstones. Schist is occasionally used as decorative stone in construction or flagstone.
Geological facts
Garnets are the birthstone for January and were historically used as bullets by some Asiatic tribes who believed the deep red color would cause more intense wounds. Schist is the primary rock type of the Manhattan Schist which provides the bedrock stability for NYC skyscrapers.
Field identification & locations
Look for a 'shiny' or 'glittery' foliation caused by mica flakes and embedded dark red, dodecahedral crystals. Found commonly in mountain belts like the Appalachians, Alps, or Himalayas. Collectors should look for sharp crystal faces and high contrast between the matrix and garnet.
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