metamorphic
Slate
Foliated Metamorphic Slate
Hardness: 2.5-4 (Mohs); Color: Gray to black (carbonaceous), though can be green, red, or purple; Luster: Dull; Crystal Structure: Microcrystalline with strong slaty cleavage; Cleavage: Perfect planar cleavage; Specific Gravity: 2.7-2.8.
- Hardness
- 2
- Luster
- Dull
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Physical properties
Hardness: 2.5-4 (Mohs); Color: Gray to black (carbonaceous), though can be green, red, or purple; Luster: Dull; Crystal Structure: Microcrystalline with strong slaty cleavage; Cleavage: Perfect planar cleavage; Specific Gravity: 2.7-2.8.
Formation & geological history
Formed by low-grade regional metamorphism of shale or mudstone. It is subjected to low temperatures and pressures that cause clay minerals to transition into mica and chlorite. This typically occurs in sedimentary basins and plate tectonic convergent boundaries.
Uses & applications
Extensively used in construction for roofing shingles, floor tiles, and paving. Historically used for chalkboards, pool table beds, and laboratory bench tops because of its low water absorption and fire resistance.
Geological facts
Slate can split into very thin sheets while maintaining its structural integrity. It is often found in terrains that have undergone tectonic folding and mountain building. Some ancient slates contain fossils that were compressed and distorted during metamorphism.
Field identification & locations
Identify in the field by its distinct ability to split along smooth flat planes (slaty cleavage) and the ringing sound it makes when struck. Common in the Appalachian Mountains, Wales, and Brazil. Collectors look for flat, decorative slabs or pieces with unique mineral inclusions.
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