Slate

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For other uses, see Slate (disambiguation).

Image:SlateUSGOV.jpg Slate is a fine-grained, homogeneous, metamorphic rock which was 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, Portugal, the east coast of Newfoundland, and the Slate Valley of Vermont and New York.

Slate is also found in the Arctic and was used by the Inuit to make the blades for ulus.

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