ARGILLITE (from the Greek words "argillos" (clay) and "lithos" (stone)Argillite * EN: argillite, mudstone, claystone; DE: Tonschiefer; FR: argilite; ES: argilita; RU: аргиллит) is the pelitic sedimentary rock, which has formed itself as the result of the compaction, dehydration, and cementation of clays (with admixture of the particles of the non-clay minerals with the size of 0.01 millimetres); it differs from the latter ones with the greater hardness and with the inability to macerate within water. It is used as the raw material for the production of cement, expanded clay, and (rarely) building ceramics. Kaolinic argillites with the admixture of gibbeite are used as refractories (flint clay). See also the "Clays" article.