metamorphic
Slate
Foliated Metamorphic Rock (composed mainly of Quartz, Illite, and Chlorite)
Hardness: 2.5–4.0 on the Mohs scale; Color: typically shades of gray, but can be black, green, or purple; Luster: dull to slightly pearly; Crystal structure: fine-grained (aphanitic), exhibiting slaty cleavage; Physical property: splits into thin, durable flat sheets.
- Hardness
- 2
- Color
- typically shades of gray, but can be black, green, or purple
- Luster
- dull to slightly pearly
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Physical properties
Hardness: 2.5–4.0 on the Mohs scale; Color: typically shades of gray, but can be black, green, or purple; Luster: dull to slightly pearly; Crystal structure: fine-grained (aphanitic), exhibiting slaty cleavage; Physical property: splits into thin, durable flat sheets.
Formation & geological history
Formed through low-grade regional metamorphism of shale or mudstone under relatively low pressure and temperature. It is commonly found in regions with historical tectonic activity and can date back to various geological eras including the Paleozoic.
Uses & applications
Extensively used in construction for roofing tiles, flooring, and wall cladding. Historically used for blackboards and writing slates. In industry, it is used for electrical insulation and laboratory bench tops.
Geological facts
Slate is incredibly durable and fire-resistant, making slate roofs last for hundreds of years. The process of splitting slate along its layers is known as 'riving'. One of the most famous slate sources is North Wales, which was once the slate-producing capital of the world.
Field identification & locations
Identify in the field by its distinct foliation (thin parallel layers) and the way it rings when lightly struck with a hammer. Found commonly in mountain ranges like the Appalachians in the US or the Alps in Europe. Collectors look for pieces with fossils preserved between layers.
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