metamorphic
Slate
Foliated Metamorphic Rock (Slate)
Hardness: 2.5-4 on the Mohs scale; Color: Dark grey to black; Luster: Dull to slightly silky; Cleavage: Excellent slaty cleavage, splits easily into thin, flat sheets; Grain size: Very fine-grained; Specific Gravity: 2.7-2.8
- Hardness
- 2
- Color
- Dark grey to black
- Luster
- Dull to slightly silky
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Physical properties
Hardness: 2.5-4 on the Mohs scale; Color: Dark grey to black; Luster: Dull to slightly silky; Cleavage: Excellent slaty cleavage, splits easily into thin, flat sheets; Grain size: Very fine-grained; Specific Gravity: 2.7-2.8
Formation & geological history
Formed through low-grade regional metamorphism of shale, mudstone, or volcanic ash under low temperature and pressure conditions, causing clay minerals to align perpendicular to stress.
Uses & applications
Widely used for roofing tiles, flooring, flagstones, outdoor paving, blackboards, billiard table tops, and decorative landscaping.
Geological facts
Slate is renowned for its slaty cleavage, allowing it to be split into smooth, flat sheets. Some ancient slate formations date back over 500 million years to the Precambrian and Paleozoic eras.
Field identification & locations
Identified by its fine-grained, dark grey to black matte surface, distinctive layered splitting (slaty cleavage), and dull ring when struck. Commonly found in mountainous or regionally metamorphosed terranes like Wales, Spain, Brazil, and the Appalachian region of North America.
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