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Geology is the Way

Slate

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.

Black slate. The prominent surface is a slaty cleavage plane, representing the main foliation in this rock. The alternation in color from dark grey to light grey mark the original bedding. Width: 8 cm across. Photo by James St. John.

Red slate. The red color is likely due to the presence of iron oxides and hydroxides. About 6 cm across. Photo by James St. John.

Outcrop of quartz-rich slate from Alta, Norway. The rock breaks along the horizontal, parallel foliation planes. Photo by Siim Sepp.

Slate from Alta, Norway, breaking as ‘platy ‘ fragments due to the strong fissility of the rock. Photo by Siim Sepp.

Dark slate interlayered with prominent metasandstone layers. Siamo Slate Fm. (Paleoproterozoic, Negaunee, Upper Peninsula of Michigan, USA). Photo by James St. John.

Slate. Note the strong slaty cleavage, along which the rock splits. Knife Lake Formation, Neoarchean. Gilbert, Minnesota, USA. Photo by James St. John.

References
De Segonzac, G. D. (1970). The transformation of clay minerals during diagenesis and low‐grade metamorphism: a review. Sedimentology15(3‐4), 281-346.
Frey, M., & Robinson, D. (Eds.). (2009). Low-grade metamorphism. John Wiley & Sons.
Kisch, H. J. (1991). Development of slaty cleavage and degree of very‐low‐grade metamorphism: a review. Journal of Metamorphic Geology9(6), 735-750.
Lombardero, M., & García-Guinea, J. (2001, June). The geology of roofing slate. Geological Society of London.
Sassi, F. P., & Scolari, A. (1974). The b 0 value of the potassic white micas as a barometric indicator in low-grade metamorphism of pelitic schists. Contributions to Mineralogy and Petrology45(2), 143-152.
Wichert, J. (2020). Slate as Dimension Stone: Origin, Standards, Properties, Mining and Deposits.
Wood, D. S. (1974). Current views of the development of slaty cleavage. Annual Review of Earth and Planetary Sciences2(1), 369-401.
        

See also
Slate – Alexstrekeisen.it
Slate – photobook by James St. John
Rocks and Minerals in Thin Section – a Colour Atlas

it_IT Italiano
Metamorphic Minerals
Metamorphic Structures
Metamorphic Rocks

 

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