Slate
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Slate is a fine-grained, homogeneous, metamorphic rock derived from an original sedimentary rock composed of clay or volcanic ash through low grade regional metamorphism. The result is a foliated rock in which the foliation may not correspond to the original sedimentary layering.
Slate is mainly composed of quartz and muscovite or illite, often along with biotite, chlorite, hematite, and pyrite along with, less frequently, apatite, graphite, kaolin, magnetite, tourmaline, or zircon.
Slate can be made into roofing slates (sometimes called roofing shingles in the United States), because it has two lines of breakability: cleavage and grain. This makes it possible to split slate into thin sheets. Fine slate can also be used as a whetstone to hone knives. Because of its thermal stability and chemical inertness, slate has been used for laboratory bench tops and for billiard table tops. In 18th and 19th century schools, slate was extensively used for blackboards and individual writing slates for which slate pencils were used.
Slate-producing regions include Cornwall (famously the town of Delabole) and Wales in the United Kingdom (see Penrhyn Quarry), Portugal, the east coast of Newfoundland, and the Slate Valley of Vermont and New York. Granville, New York, of the Slate Valley claims to be the colored slate capital of the world.
Slate is also found in the Arctic and was used by the Inuit to make the blades for ulus.
See also
External links
- [History of the Welsh slate industry]
- [Slatesite] — bilingual site focusing on Welsh slate
- [John T F Turner - A Familiar Description of the Old Delabole Slate Quarries, 1865]
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