Chemical composition

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 as 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 plane of the specimen.

Uses of slate

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 extraction

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 Mine and Skiddaw Slate) in the United Kingdom; parts of France (Angers, Anjou and in the Maritime Alps); Belgium (formerly); Liguria in northern Italy especially between the town of Lavagna (which means chalkboard in Italian) and Fontanabuona valley; Portugal especially around Valongo in the north of the country; Germany's (Moselle River-region, Hunsrück, Eifel, Westerwald, Thuringia and north-Bavaria); Alta, Norway (actually schist not a true slate) and Galicia. In the Americas, slate is found in Brazil (the second biggest producer of slate) around Papagaio in 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 Valley of Vermont and 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