metamorphic
Mica Schist with Garnet
Garnetiferous Mica Schist
Hardness: 2.5-3 for mica matrix, 6.5-7.5 for garnet inclusions. Color: Silvery-gray, bronze, or greenish-brown with reddish-brown gems. Luster: Pearly to sub-metallic (mica) and vitreous (garnet). Crystal structure: Monoclinic (mica) and Isometric (garnet). Cleavage: Perfect basal cleavage in mica layers.
- Hardness
- 2
- Luster
- Pearly to sub-metallic (mica) and vitreous (garnet)
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Physical properties
Hardness: 2.5-3 for mica matrix, 6.5-7.5 for garnet inclusions. Color: Silvery-gray, bronze, or greenish-brown with reddish-brown gems. Luster: Pearly to sub-metallic (mica) and vitreous (garnet). Crystal structure: Monoclinic (mica) and Isometric (garnet). Cleavage: Perfect basal cleavage in mica layers.
Formation & geological history
Formed through regional metamorphism of shale or mudstone under intermediate to high pressure and temperature. The mica grains align to create schistosity (layering), and garnets grow as porphyroblasts within the matrix.
Uses & applications
Mica is used in electronics for insulation, as a filler in paints and plastics, and in cosmetics. Garnets are used as abrasives (sandblasting) and as semi-precious gemstones. Large specimens are popular for geological collections.
Geological facts
Mica was used in ancient times for windows before the invention of glass because it can be split into thin, transparent sheets. The reddish spots often seen in these rocks are the mineral Almandine, the most common type of garnet.
Field identification & locations
Identify by its 'shimmery' or 'glittery' appearance due to mica alignment. It often feels slightly greasy or flaky to the touch. Look for dark, rounded nodules (garnets) embedded within the flaky layers. Found in mountain ranges like the Appalachians, Alps, and Himalayas.
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