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Slate Macro 1

Slate Macro (~ 6 cm long and ~ 4 cm high)

Slate is a fine-grained, foliated, homogeneous metamorphic rock derived from an original shale-type 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 frequently grey in color, especially when seen en masse covering roofs. However, slate occurs in a variety of colors 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 not to be confused with shale, from which it may be formed, or schist.

Ninety percent of Europe's natural slate used for roofing originates from the slate industry in Spain.[1]

Historical mining terminologyEdit

Before the mid-19th century, the terms slate, shale and schist were not sharply distinguished.[2] In the context of underground coal mining, the term slate was commonly used to refer to shale well into the 20th century.[3] For example, roof slate refers to shale above a coal seam, and draw slate refers to roof slate that falls from the mine roof as the coal is removed.[4]

Mineral composition Edit

Slate is mainly composed of quartz and muscovite or illite, often along with biotite, chlorite, hematite, and pyrite and, 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 Edit

St Fagans Tannery 7

Slate roof.

Slate in buildings Edit

Slate can be made into roofing slates, also called roofing shingles, installed by a slater.[5] Slate has two lines of breakability: cleavage and grain, which make it possible to split the stone into thin sheets. When broken, slate retains a natural appearance while remaining relatively flat and easily stackable.

Slate is particularly suitable as a roofing material as it has an extremely low water absorption index of less than 0.4%. Its low tendency to absorb water also makes it very resistant to frost damage and breakage due to freezing.

Slate roof tiles are usually fixed using either nail fixing, or the hook fixing method as is common with Spanish slate. Using hook fixing means that [6]:-

  • Areas of weakness on the tile are fewer since no holes have to be drilled
  • Roofing features such as valleys and domes are easier to create since narrow tiles can be used
  • Hook fixing is particularly suitable in regions subject to severe weather conditions, such as Scotland and parts of Wales, since there is a greater resistance to wind uplift as the lower edge of the slate is secured.

Slate tiles are often used for interior and exterior flooring, stairs, walkways and 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 flooring can be slippery when used in external locations subject to rain. Slate tiles were used in 19th century UK building construction (apart from roofs) and in slate quarrying areas such as Bethesda, Wales there are still many buildings wholly constructed of slate. Slates can also be set into walls to provide a rudimentary damp-proof membrane. Small offcuts are used as shims to level floor joists. In areas where slate is plentiful it is also used in pieces of various sizes for building walls and hedges, sometimes combined with other kinds of stone.

DeborahLeavitt

Slate gravestone in Hingham, Massachusetts

Other uses Edit

Betjeman memorial

A grave with inscription on slate.

Because it is a good electrical insulator and fireproof, it was used to construct early-20th century electric switchboards and relay controls for large electric motors. 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 table tops. In 18th- and 19th-century schools, slate was extensively used for blackboards and individual writing slates for which slate or chalk pencils were used.

In areas where it is available, high-quality slate is used for tombstones, commemorative tablets and by artists in various genres.

See alsoEdit

ReferencesEdit

  1. European Association of Mining Industries website retrieved on 26/01/2010
  2. R. W. Raymond, Slate, A Glossary of Mining and Metallurigical Terms, American Institute of Mining Engineers, 1881; page 78.
  3. Albert H. Fay, Slate, A Glossary of the Mining and Mineral Industry, United States Bureau of Mines, 1920; page 622.
  4. J. Marvin Weller, ed.,Supplement to the Glossary of Geology and Related Sciences, American Geological Institute, 1960; page 18.
  5. Hart, Diane (1991) The building slates of the British Isles. Watford: Building Research Establishment ISBN 0851254837
  6. Galician and Spanish Slate website “Hook Fixing” . Retrieved on 26/01/2010
  • Page, William (ed.) (1906) The Victoria History of the County of Cornwall; vol. I. (Chapter on quarries.) Westminster: Constable

Further readingEdit

  • Hudson, Kenneth (1972) Building Materials; chap. 2: Stone and slate. London: Longman ISBN 0582127912; pp. 14-27

External linksEdit

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