metamorphic
Slate
Metamorphic rock (fine-grained, foliated)
Hardness: 2.5–4 on Mohs scale. Color: Dark gray to black (can also be green, red, or purple). Luster: Dull to sub-vitreous. Cleavage: Excellent slaty cleavage (breaks into thin, flat sheets). Grain size: Very fine-grained.
- Hardness
- 2
- Color
- Dark gray to black (can also be green, red, or purple)
- Luster
- Dull to sub-vitreous
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Physical properties
Hardness: 2.5–4 on Mohs scale. Color: Dark gray to black (can also be green, red, or purple). Luster: Dull to sub-vitreous. Cleavage: Excellent slaty cleavage (breaks into thin, flat sheets). Grain size: Very fine-grained.
Formation & geological history
Formed through low-grade regional metamorphism of shale or mudstone under low pressure and temperature conditions. Typically originates in sedimentary basins subject to tectonic compression.
Uses & applications
Widely used in roofing tiles, flooring, flagstones, billiard table tops, blackboards, and decorative stone items.
Geological facts
Slate's property of splitting into thin, smooth sheets is known as slaty cleavage. It is extremely durable and weather-resistant, making slate roofs last well over a century.
Field identification & locations
Identify in the field by its very fine-grained texture, dark slate-gray color, and tendency to split easily into thin, flat slabs with a dull surface ring when struck lightly. Commonly found in mountainous regions worldwide such as Wales, Spain, Brazil, and the Appalachian region in North America.
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