metamorphic
Slate
Slate (foliated metamorphic rock rich in quartz, sericite, and chlorite)
Hardness: 2.5–4 on Mohs scale; Color: typically dark gray, but can be black, green, or purple; Luster: dull to slightly satiny; Structure: very fine-grained and foliated with slaty cleavage; Gravity: 2.7–2.8.
- Hardness
- 2
- Color
- typically dark gray, but can be black, green, or purple
- Luster
- dull to slightly satiny
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Physical properties
Hardness: 2.5–4 on Mohs scale; Color: typically dark gray, but can be black, green, or purple; Luster: dull to slightly satiny; Structure: very fine-grained and foliated with slaty cleavage; Gravity: 2.7–2.8.
Formation & geological history
Formed through low-grade regional metamorphism of shale or mudstone under low pressure and temperature conditions. It is typically associated with tectonic plate boundaries and mountain-building events (orogenies).
Uses & applications
Widely used in the construction industry for roofing shingles, floor tiles, and flagstones. Historically used for blackboards, writing slates, and billiard table tops.
Geological facts
Slate has a unique property called 'slaty cleavage' which allows it to be split into thin, flat, durable sheets. It is the first step in the metamorphic transition from shale to phyllite, schist, and finally gneiss.
Field identification & locations
Identify in the field by its thin, plate-like layers and the 'clinking' sound it makes when struck. Common in the Appalachian Mountains, Wales, and Brazil. Collectors look for pieces with fossils or unique color banding.
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