metamorphic
Slate
Slate (Metamorphic silicate rock)
Hardness: 2.5-4 on the Mohs scale. Color: Dark gray to black, charcoal, or bluish-gray. Luster: Dull to slightly satin sheen. Structure: Fine-grained, foliated with slaty cleavage. Cleavage: Perfect slaty cleavage, splitting cleanly along smooth parallel planes.
- Hardness
- 2
- Color
- Dark gray to black, charcoal, or bluish-gray
- Luster
- Dull to slightly satin sheen
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Physical properties
Hardness: 2.5-4 on the Mohs scale. Color: Dark gray to black, charcoal, or bluish-gray. Luster: Dull to slightly satin sheen. Structure: Fine-grained, foliated with slaty cleavage. Cleavage: Perfect slaty cleavage, splitting cleanly along smooth parallel planes.
Formation & geological history
Formed through low-grade regional metamorphism of shale or mudstone under relatively low heat and pressure. The original clay minerals transform into microscopic flakes of mica and chlorite aligned perpendicular to the directional pressure.
Uses & applications
Widely used in building construction for roofing tiles, flagstone flooring, pool table tops, blackboards, coasters, garden landscaping, and decorative stonework.
Geological facts
Slate produces a distinct ringing sound when lightly tapped with a hard object, which can help distinguish it from softer shale. Because of its impermeable microcrystalline structure, slate resists water absorption and freeze-thaw cycles exceptionally well.
Field identification & locations
Identify in the field by its distinct parallel cleavage planes, flat planar splitting, dull earthy dark color, and fine grain size without visible large crystals. Common in mountain belts, slate quarries, and ancient seabed formations globally.
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