sepiolite, (German: “sea-foam”), a fibrous hydrated magnesium silicate, Mg₄Si₆O₁₅(OH)₂·6H₂O, that is opaque and white, grey, or cream in colour. It may resemble the bones of the cuttlefish Sepia, from which the name derives. In the Black Sea region, where the light, porous clay mineral is abundant, it is said to resemble sea-foam, hence the German name. For its chemical formula, structure, and physical properties, see clay mineral (table).

As in palygorskite, the structure of sepiolite contains extended silicon-oxygen sheets, so that the mineral belongs to the layer silicate family, but the tetrahedral SiO₄ groups forming these sheets are oriented so as to develop extended lathlike features that give rise to the fibrous character of the mineral.

Sepiolite’s chief use is for tobacco pipes. The most important commercial deposit is the plain of Eskişehir, Tur., where it is found as irregular nodules in alluvial deposits; it is an alteration product of serpentine. When first extracted, sepiolite is soft, but it hardens on drying. Sepiolite also occurs in France, Greece, the Czech Republic, and the United States.

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