metamorphic
Slate
Foliated Metamorphic Rock (primarily clay minerals and micas)
Hardness: 2.5-4 (Mohs scale), Color: shades of gray, green, purple, or brown. Luster: Dull. Crystal structure: Microcrystalline. Cleavage: Perfect slaty cleavage, splitting into thin, flat sheets. Specific gravity: 2.7-2.8.
- Hardness
- 2
- Color
- shades of gray, green, purple, or brown
- Luster
- Dull
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Physical properties
Hardness: 2.5-4 (Mohs scale), Color: shades of gray, green, purple, or brown. Luster: Dull. Crystal structure: Microcrystalline. Cleavage: Perfect slaty cleavage, splitting into thin, flat sheets. Specific gravity: 2.7-2.8.
Formation & geological history
Formed by low-grade regional metamorphism of shale or mudstone under high pressure and low temperature, typically during mountain-building events. Geological age varies widely from Precambrian to Cenozoic.
Uses & applications
Extensively used in construction for roofing shingles, flooring, and flagstones. Also used for billiard table tops, blackboards, and decorative stone in gardens.
Geological facts
Slate is renowned for its ability to be split into incredibly thin, durable sheets due to its foliation. Famous historic quarries exist in Wales and the 'Slate Belt' of the Northeastern United States.
Field identification & locations
Identify in the field by its distinct foliation (splitting into flat planes) and a 'clink' sound when struck. Commonly found in folded mountain belts like the Appalachians, the Alps, and the Pyrenees.
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