Slate is an ultrafine/very fine-grained schistose metamorphic rock characterized by a slaty cleavage and, consequently, tends to split easily along foliation planes. The name itself derives from the German schleissen, ‘to split’, in reference to the high degree of fissility of the rock. The related term ‘clay slate’ is a direct translation of the German Thonschiefer and the French schiste argileux, which is redundant in English. Slate form from very low-grade metamorphism of clay-rich sedimentary rocks (shales, mudrocks, siltstone to very fine-grained and matrix-rich sandstone) or volcanic ash deposits. The preferred orientation of platy phyllosilicates, resulting from the metamorphism of clay minerals, determines the penetrative foliation characterizing slates. Such foliation does not necessarily correspond to the original bedding or layering and rather represent tectonic surfaces, often perpendicular to the direction of maximum compression. The minerals present in slates are white mica (illite, smectite, pyrophyllite), chlorite, graphite, kaolinite, quartz, feldspars, and oxides, representing a mixture of metamorphic and detrital grains that are largely too fine-grained to be seen with the unaided eye. Slate colors are highly variable depending on the mineral content: dark varieties generally containing much graphite and green varieties containing abundant chlorite. With increasing metamorphic grade, slate develops a lustrous sheen related to coarser white mica grains that are able to reflect light, thus becoming phyllite.
Slates have been used historically as building stone, since, due to their strong fissility, they make excellent roof tiles. They have also been used as writing slates, i.e. as blackboards or notepads, as chopping boards, tombstones, and as thermal/electric insulators.
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