metamorphic
Slate
Foliated Metamorphic Rock (composed mainly of clay minerals or micas)
Hardness: 2.5-4 Mohs; Color: grey, bluish-grey, dark grey to black; Luster: dull to slightly pearly; Structure: fine-grained, foliated with slaty cleavage; Cleavage: perfect planar cleavage into thin sheets; Specific Gravity: 2.7-2.8
- Hardness
- 2
- Color
- grey, bluish-grey, dark grey to black
- Luster
- dull to slightly pearly
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Physical properties
Hardness: 2.5-4 Mohs; Color: grey, bluish-grey, dark grey to black; Luster: dull to slightly pearly; Structure: fine-grained, foliated with slaty cleavage; Cleavage: perfect planar cleavage into thin sheets; Specific Gravity: 2.7-2.8
Formation & geological history
Formed by the low-grade regional metamorphism of shale or mudstone under relatively low pressure and temperature. This process aligns clay mineral flakes into parallel layers. Geological age varies greatly from Precambrian to Cenozoic depending on the tectonic history of the region.
Uses & applications
Used extensively in construction for roofing shingles, floor tiles, and billiard table tops due to its durability and planar cleavage. Historically used for writing slates and blackboards. Also used in landscaping and as a decorative stone.
Geological facts
Slate is famous for its 'slaty cleavage' which allows it to be split into very thin, flat sheets. This characteristic is a result of the alignment of microscopic mica crystals during metamorphism. It is one of the lowest-grade metamorphic rocks, representing the first step in the transformation from shale to schist.
Field identification & locations
Identify in the field by its tendency to split into thin, flat plates with a dull luster and a 'clink' sound when struck. It is commonly found in mountainous regions or former tectonic plate boundaries. Collectors should look for evenness of the splitting planes and lack of visible large crystals compared to phyllite.
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