metamorphic
Schist (Biotite/Mica Schist)
Medium-grade regional metamorphic rock (rich in Phyllosilicates)
Hardness: 2-4 (Mohs scale); Color: Dark grey, black, or silvery-green; Luster: Pearly to sub-metallic; Crystal structure: Foliated (schistosity); Cleavage: Perfect basal cleavage in individual mineral grains; Specific gravity: 2.7-3.0
- Hardness
- 2-4 (Mohs scale)
- Color
- Dark grey, black, or silvery-green
- Luster
- Pearly to sub-metallic
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Physical properties
Hardness: 2-4 (Mohs scale); Color: Dark grey, black, or silvery-green; Luster: Pearly to sub-metallic; Crystal structure: Foliated (schistosity); Cleavage: Perfect basal cleavage in individual mineral grains; Specific gravity: 2.7-3.0
Formation & geological history
Formed through the regional metamorphism of shale or mudstone at medium temperatures and pressures. This transformation typically occurs at convergent plate boundaries over millions of years (often Proterozoic to Paleozoic ages).
Uses & applications
Mostly used as decorative stone in landscaping, flagstone for walkways, and occasionally as a source for graphite or high-grade mica. In historical contexts, it was used for building foundations and walls.
Geological facts
The word 'Schist' is derived from the Greek 'schistos,' meaning 'easy to split.' It is defined by having more than 50% platy and elongated minerals aligned in a parallel fashion, a texture known as schistosity.
Field identification & locations
Identify in the field by its distinct 'glittery' appearance when held in the sun and its tendency to flake or split into thin layers. Commonly found in Appalachian Mountains (USA), the Alps, and the Scottish Highlands. Collectors look for garnet inclusions common in this matrix.
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