Slate is a fine-grained, foliated, ,
metamorphic rockderived from an original shale-type sedimentary rockcomposed of clayor 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 frequently grey in colour especially when seen en masse covering roofs. However, slate occurs in a variety of colours even from a single locality. For example slate from North Wales can be found in many shades of grey from pale to dark and may also be purple, green or cyan.
Slate is mainly composed of
quartzand muscoviteor illite, often along with biotite, chlorite, hematite, and pyritealong with, less frequently, apatite, graphite, kaolin, magnetite, tourmaline, or zirconas well as feldspar. Occasionally, as in the purple slates of North Wales, ferrous reduction spheres form around iron nuclei, leaving a light green spotted texture. These spheres are sometimes deformed by a subsequent applied stress field to ovoids, which appear as ellipses when viewed on a cleavage planeof the specimen.
Uses of slate
Slate can be made into
roofing slates, also called "roofing shingles", installed by a slater. Slate 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. Due to its thermal stability and chemical inertness, slate has been used for laboratory bench tops and for billiard tabletops. In 18th and 19th century schools, slate was extensively used for blackboards and individual writing slates for which chalk pencils were used. Because it is a good electrical insulatorand fireproof, it was used to construct early 20th century electric switchboards and relaycontrols for large electric motors. British sculptor Stephen Kettleis notable for his use of slate to create statues housed in the Science Museumin London.
Slate tiles are often used for interior and exterior flooring or wall cladding. Tiles are installed and set on mortar and grouted along the edges. Chemical sealants are often used on tiles to improve durability and appearance, increase stain resistance, reduce
efflorescence, and increase or reduce surface smoothness. Tiles are often sold gauged, meaning that the back surface is ground for ease of installation.
Slate tiles were used in 19th century UK building construction (apart from roofs). They can be set into the walls to provide a rudimentary
damp-proof membrane. Small offcuts are used as shims to level floor joists.
Slate is often used as a decor in freshwater aquariums. Slate will not alter the chemistry of water (except in the slate containing feldspar which may leach silicates into the water resulting in excess diatom growth in marine aquaria). When broken, slate produces a natural appearance while remaining relatively flat and can be easily stacked. Silicone glue adheres to slate, creating a non-toxic bond to secure it. It is also used in stairs and pathways for the same reasons.
Traditional Japanese Go equipment uses slate for the black pieces.
Slate-producing regions in Europe include
Wales(see slate industry in Wales), Cornwall(famously the town of Delabole), and Cumbria (see Burlington Slate Quarries, Honister Slate Mineand Skiddaw Slate) in the United Kingdom; parts of France (Angers, Anjou and in the Maritime Alps); Belgium (formerly); Liguriain northern Italyespecially between the town of Lavagna (which means chalkboard in Italian) and Fontanabuona valley; Portugalespecially around Valongoin the north of the country; Germany's ( Moselle River-region, Hunsrück, Eifel, Westerwald, Thuringiaand north- Bavaria); Alta, Norway(actually schistnot a true slate) and Galicia. In the Americas, slate is found in Brazil (the second biggest producer of slate) around Papagaioin Minas Gerais(responsible for 95% of the extraction of slate in Brazil), the east coast of Newfoundland, the Slate Belt of Eastern Pennsylvania, and the Slate Valleyof Vermontand New York. The area around Granville, NY, is one place where colored slate(non-blue) is mined. Others include Wales (purple and formerly green) and Cumbria (green) in the UK; Brazil (green); China (many colors); and Newfoundland. Fact|date=November 2007
There was also a major slating operation in
Monson, Maineduring the late 19th and early 20th centuries. The slate found in Monson is usually a dark purple to blackish color, and many local structures are still roofed with slate tiles. Of many operations there is only one business currently operating. The output was so great it formed a train route throughout the woods of Monson and as many as 18 quarries were made. The roof of St. Patrick's Cathedral was made of roofing slate from Monson, as was the Headstone of John F. Kennedy.
Slate is also found in the
Arcticand was used by the Inuitto make the blades for ulus. China has vast slate deposits; in recent years its export of finished and unfinished slate has increased.
List of rock types
National Slate Museum
Fell Exhibition Slate Mine
Slate industry in Wales
* [http://www.aditnow.co.uk/ AditNow] - Photographic database of mines
* [http://www.slateroof.co.uk/ddpint.html History of the Welsh slate industry]
* [http://cornovia.org.uk/htexts/turner01.html John T F Turner - A Familiar Description of the Old Delabole Slate Quarries, 1864]
* [http://www.slate.org.uk/ History of the Scottish Slate Islands]
*Granville Slate Museum [http://www.slatevalleymuseum.org/]
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