metamorphic
Slate
Foliated Metamorphic Rock (composed of quartz, illite, and chlorite)
Hardness: 2.5-4 Mohs; color: grey, black, purple, green, and rusty brown; luster: dull to slightly pearly; crystal structure: microcrystalline; cleavage: perfect slaty cleavage; specific gravity: 2.6-2.9.
- Hardness
- 2
- Color
- grey, black, purple, green, and rusty brown
- Luster
- dull to slightly pearly
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Physical properties
Hardness: 2.5-4 Mohs; color: grey, black, purple, green, and rusty brown; luster: dull to slightly pearly; crystal structure: microcrystalline; cleavage: perfect slaty cleavage; specific gravity: 2.6-2.9.
Formation & geological history
Formed through low-grade regional metamorphism of shale or mudstone under relatively low pressure and temperature. Primarily dates from various geological ages including the Paleozoic era.
Uses & applications
Widely used for roofing shingles, flooring tiles, laboratory bench tops, billiard tables, and as high-quality decorative building stone.
Geological facts
Slate can be split into very thin sheets while maintaining its strength. It has been used for hundreds of years as a writing surface (chalkboards) and is fireproof and waterproof.
Field identification & locations
Identified in the field by its characteristic slaty cleavage (breaking into flat sheets) and a distinctive 'clink' sound when struck. Common in mountain building regions like Wales, Vermont, and Pennsylvania.
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