metamorphic
Slate
Slate (foliated metamorphic rock, primarily composed of quartz, illite, chlorite, and micas)
Hardness: 2.5-4 (Mohs); Color: grey to black (dark due to carbon/organic material); Luster: dull to slightly pearly; Crystal structure: microcrystalline foliated (slaty cleavage); Cleavage: perfect planar; Specific gravity: 2.7-2.8
- Hardness
- 2
- Color
- grey to black (dark due to carbon/organic material)
- Luster
- dull to slightly pearly
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Physical properties
Hardness: 2.5-4 (Mohs); Color: grey to black (dark due to carbon/organic material); Luster: dull to slightly pearly; Crystal structure: microcrystalline foliated (slaty cleavage); Cleavage: perfect planar; Specific gravity: 2.7-2.8
Formation & geological history
Formed through low-grade regional metamorphism of shale or mudstone under relatively low pressure and temperature. Large deposits are found from the Paleozoic era (approx. 300-500 million years ago) during mountain-building events.
Uses & applications
Extensively used in construction for roofing shingles, flooring, and billiard table tops. It is also used for chalkboards, headstones, and decorative laboratory countertops due to its durability and chemical resistance.
Geological facts
Slate's most distinctive feature is its 'slaty cleavage,' which allows it to be split into thin, flat, durable sheets. Historically, it was the primary material for writing tablets in schools before the mass production of paper.
Field identification & locations
Identify in the field by its distinct layering and tendency to snap into flat plates; it will sound like a metallic ring when tapped but a dull thud if weathered. Commonly found in folded mountain belts such as the Appalachian Mountains in the US and North Wales in the UK.
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