metamorphic
Slate
Slate (Regional metamorphic rock composed primarily of quartz, illite, and chlorite)
Hardness: 2.5–4 on Mohs scale. Color: Dark gray to black (due to carbonaceous material). Luster: Dull to slightly silky. Structure: Foliated with distinct slaty cleavage. Specific gravity: 2.7–2.8.
- Hardness
- 2
- Color
- Dark gray to black (due to carbonaceous material)
- Luster
- Dull to slightly silky
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Physical properties
Hardness: 2.5–4 on Mohs scale. Color: Dark gray to black (due to carbonaceous material). Luster: Dull to slightly silky. Structure: Foliated with distinct slaty cleavage. Specific gravity: 2.7–2.8.
Formation & geological history
Formed through the low-grade regional metamorphism of shale or mudstone. It is subjected to tectonic pressure and heat, typically found in Fold Mountains. Geologically, many deposits are from the Paleozoic era.
Uses & applications
Used extensively in construction for roofing shingles, flooring tiles, and billiard table tops. Historically used for blackboards and writing slates. High-quality specimens are used for landscaping and garden ornaments.
Geological facts
Slate is unique for its fissility, meaning it can be split into very thin, flat sheets. Some slates from Wales and the Appalachian Mountains are over 500 million years old and still maintain their structural integrity.
Field identification & locations
Identify in the field by its ability to break along flat parallel planes (slaty cleavage) and its 'clinking' sound when struck compared to the 'thud' of shale. Commonly found in mountainous regions like Vermont, Pennsylvania, North Wales, and Brazil.
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