metamorphic
Slate
Foliated Metamorphic Rock (composed primarily of Quartz, Muscovite, and Illite)
Hardness: 2.5-4 Mohs; Color: Typically grey to dark blue-grey; Luster: Dull to satiny; Crystal Structure: Microcrystalline with excellent slaty cleavage; Specific Gravity: 2.7–2.8.
- Hardness
- 2
- Color
- Typically grey to dark blue-grey
- Luster
- Dull to satiny
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Physical properties
Hardness: 2.5-4 Mohs; Color: Typically grey to dark blue-grey; Luster: Dull to satiny; Crystal Structure: Microcrystalline with excellent slaty cleavage; Specific Gravity: 2.7–2.8.
Formation & geological history
Formed from low-grade regional metamorphism of shale or mudstone under relatively low pressure and temperature. Foundations of slate often date to the Paleozoic or Precambrian eras, formed in marine basins.
Uses & applications
Commonly used for roofing tiles, flooring, chalkboards, billiard table tops, and laboratory countertops due to its durability and low water absorption.
Geological facts
Slate is the finest-grained foliated metamorphic rock. It is famous for its 'slaty cleavage,' which allows it to be split into thin, flat sheets, a property widely used in the construction industry for centuries.
Field identification & locations
Identify in the field by its tendency to break into thin, flat planes and its 'tink' sound when struck. Common in the Appalachian Mountains, Wales, and Vermont. Collectors look for pieces containing pyrite inclusions or fossils.
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