What is Slate?

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. It is the finest grained foliated metamorphic rock. Foliation may not correspond to the original sedimentary layering, but instead is in planes perpendicular to the direction of metamorphic compression.
The foliation in slate is called "slaty cleavage". It is caused by strong compression causing fine grained clay flakes to regrow in planes perpendicular to the compression. When expertly "cut" by striking parallel to the foliation, with a specialised tool in the quarry, many slates will display a property called fissility, forming smooth flat sheets of stone which have long been used for roofing, floor tiles, and other purposes. 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 not to be confused with shale, from which it may be formed, or schist.
The word "slate" is also used for certain types of object made from slate rock. It may mean a single roofing tile made of slate, or a writing slate. This was traditionally a small smooth piece of the rock, often framed in wood, used with chalk as a notepad or noticeboard, and especially for recording charges in pubs and inns. The phrases "clean slate" and "blank slate" come from this usage.Slate is a low grade metamorphic rock which is formed by the alteration of shale or mudstone by regional metamorphism. Slate is a fine grained foliated rock and is the finest grained foliated metamorphic rock. Foliation is not formed along the original sedimentary layering but is the response of metamorphic compression. The strong foliation is called slaty cleavage which is the result of compression causing fine grained clay flakes to regrow in planes perpendicular to the compression.

Composition of slate

Slate is primarily composed of clay minerals or even micas depending upon the degree of metamorphism. The clay minerals which were originally deposited with temperature and pressure increasing level, it is altered into mica. Slate can also have abundant quartz and small amount of feldspar, calcite, pyrite, hematite and other minerals.

How slate forms?

Shale is deposited in a sedimentary basin where finer particles are transported by wind or water. These deposited fine grains are then compacted and lithified. Tectonic environments for producing slates are when this basin is involved in a convergent plate boundaries. The shale and mudstone in the basin is compressed by horizontal forces with minor heating. These forces and heat modify the clay minerals. Foliation develops at right angles to the compressive forces of the convergent plate boundaries.

Colour of slate

Most slates are grey in colour and from light to dark shades of grey can also be present. It also have green, red, black, purple and brown colour shades. The colour of slates are determined by amount of iron and organic material present.

Slaty cleavage

Foliations is slate is the result of parallel orientation of platy minerals in the rock such as grains of clay and mica. These parallel minerals alignment gives the rock ability to break smoothly along planes of foliation. 


Slates are mined to use as a roofing slates throughout the world. Slates are well used as it can be cut into thin sheets, absorbs minimal moisture and performs well when in contact with freezing water. Slates can also be used for interior flooring, exterior paving, dimension stone and decorative aggregates.


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