metamorphic
Schist
Mica Schist (primarily consisting of muscovite KAl2(AlSi3O10)(OH)2 or biotite K(Mg,Fe)3(AlSi3O10)(OH)2)
Hardness: 3.5-4 (on Mohs scale); Color: Silvery-grey to brownish-grey; Luster: Pearly to sub-metallic due to mica content; Crystal Structure: Predominantly platy/foliated; Cleavage: High fissility (splits into thin plates); Specific Gravity: 2.7-2.8
- Hardness
- 3
- Color
- Silvery-grey to brownish-grey
- Luster
- Pearly to sub-metallic due to mica content
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Physical properties
Hardness: 3.5-4 (on Mohs scale); Color: Silvery-grey to brownish-grey; Luster: Pearly to sub-metallic due to mica content; Crystal Structure: Predominantly platy/foliated; Cleavage: High fissility (splits into thin plates); Specific Gravity: 2.7-2.8
Formation & geological history
Formed through the regional metamorphism of shale or mudstone under intermediate-grade heat and pressure. This occurs typically in the roots of mountain belts during tectonic plate collisions.
Uses & applications
Used historically as a building stone, for decorative flagstones, and in some architectural cladding. It is also used in the manufacturing of fillers for plastics and paints when ground down.
Geological facts
The characteristic 'schistosity' (alignment of mineral grains) is what gives the rock its reflective, glittery appearance. It is a transition stage between phyllite and gneiss.
Field identification & locations
Identify by looking for a 'glittery' reflection and a visible layered texture (foliation) where the rock naturally splits. It is commonly found in the Appalachian Mountains, the Scottish Highlands, and the Alps. Collectors should look for specimens containing accessory minerals like Garnet.
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