metamorphic
Slate
Foliated Metamorphic Rock (composed primarily of Quartz, Muscovite, and Illite)
Hardness: 2.5-4 (Mohs scale); Color: Ash grey to dark grey/black; Luster: Dull/Matte; Structure: Fine-grained, foliated with slaty cleavage; Physical: Splits easily into thin, flat sheets.
- Hardness
- 2
- Color
- Ash grey to dark grey/black
- Luster
- Dull/Matte
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Physical properties
Hardness: 2.5-4 (Mohs scale); Color: Ash grey to dark grey/black; Luster: Dull/Matte; Structure: Fine-grained, foliated with slaty cleavage; Physical: Splits easily into thin, flat sheets.
Formation & geological history
Formed through low-grade regional metamorphism of shale or mudstone. Under relatively low pressure and temperature conditions, the clay minerals in the original sedimentary rock align perpendicularly to the direction of pressure.
Uses & applications
Commonly used in construction for roofing shingles, flooring tiles, and flagstones. Historically used for chalkboards and billiard table tops due to its natural flatness and thermal stability.
Geological facts
Slate is the finest-grained foliated metamorphic rock. Some specimens contain tiny crystals of pyrite ('fool's gold') which can weather and cause staining, though high-quality slate is exceptionally durable and can last over a century as a roof.
Field identification & locations
Identify in the field by its distinct tendency to break into flat, smooth slabs (slaty cleavage) and its 'clink' sound when struck. Found in mountainous or formerly mountainous regions like the Appalachians (USA), Wales (UK), and Brazil.
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