metamorphic
Slate
Slate
Hardness: 2.5-4 Mohs; Color: Dark grey, black, green, or purple; Luster: Dull to pearly; Crystal structure: Microcrystalline; Cleavage: Perfect slaty cleavage; Specific gravity: 2.7-2.8
- Hardness
- 2
- Color
- Dark grey, black, green, or purple
- Luster
- Dull to pearly
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Physical properties
Hardness: 2.5-4 Mohs; Color: Dark grey, black, green, or purple; Luster: Dull to pearly; Crystal structure: Microcrystalline; Cleavage: Perfect slaty cleavage; Specific gravity: 2.7-2.8
Formation & geological history
Formed through the low-grade regional metamorphism of shale or mudstone under relatively low pressure and temperature conditions. It develops a foliated texture as micas minerals align perpendicularly to the source of pressure.
Uses & applications
Extensively used in construction for roofing shingles, flooring tiles, flagstones, and billiard table tops due to its durability and easy splitting into thin sheets.
Geological facts
Slate has been used as a writing surface for centuries (chalkboards). It is extremely waterproof and resistant to frost, making it one of the longest-lasting roofing materials available.
Field identification & locations
Identify in the field by its distinct ability to split into thin, flat plates (slaty cleavage). It is commonly found in fold mountain belts globally, such as the Appalachian Mountains or the Welsh mountains.
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