metamorphic
Slate
Foliated Metamorphic Rock (primarily composed of quartz, illite, and chlorite)
Hardness: 2.5-4 (Mohs); Color: Dark grey to charcoal, sometimes green or purple; Luster: Dull to satiny; Structure: Foliated (slaty cleavage); Cleavage: Perfect planar cleavage; Specific Gravity: 2.7-2.8
- Hardness
- 2
- Color
- Dark grey to charcoal, sometimes green or purple
- Luster
- Dull to satiny
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Physical properties
Hardness: 2.5-4 (Mohs); Color: Dark grey to charcoal, sometimes green or purple; Luster: Dull to satiny; Structure: Foliated (slaty cleavage); Cleavage: Perfect planar cleavage; Specific Gravity: 2.7-2.8
Formation & geological history
Formed by the low-grade regional metamorphism of shale or mudstone under relatively low pressure and temperature conditions. It originates from the deep burial of clay-rich sedimentary basins.
Uses & applications
Extensively used in construction for roofing shingles, flooring, and paving. Historically used for writing blackboards and billiard table tops due to its thermal stability and flat cleavage.
Geological facts
The term 'clean slate' comes from the historical use of slate as a reusable writing surface. Its ability to be split into extremely thin, durable sheets makes it unique among rocks.
Field identification & locations
Identified in the field by its tendency to break into flat, thin sheets (slaty cleavage) and its 'clink' sound when struck. Common in fold mountain belts like the Appalachians or North Wales.
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