EN 12326 - terminology

The European standard defines a slate in commercial terms as a "... rock which is easily split into thin sheets along a plane of cleavage resulting from a schistosity flux caused by very low or low grade metamorphism due to tectonic compression. [...] and belongs petrogaphically to a range which begins at the boundary between sedimentary and metamorphic formations and ends at the end of the epizonal-metamorphic phyllite formation." Thus, the term slate is clearly distinguished from sedimentary stones, which invariably split along a bedding or sedimentation plane such as shales.

In its second definition is the term roofing slate which is a "rock used for roofing and cladding, in which phyllosilicates are the predominant and most important components and exhibiting a prominent slaty cleavage."

And finally the third term is roofing carbonate slate which is a "rock used for roofing and cladding, containing phyllosilicates and a minimum carbonate content of 20 % and exhibiting a prominent slaty cleavage."