Gypsum (from the Greek gýpsos - chalk, lime) mineral, aqueous calcium sulfate salt CaSO 4 · 2H 2 O; in its pure form it contains 32.56% CaO, 46.51% SO 3 and 20.93% H 2 O. Crystallizes in the monoclinic system. The structure of the gypsum crystal lattice is of the layered type. The double layers consist of [SO 4 ] tetrahedra linked through calcium. The crystals are lamellar, columnar (single or fused in the form of twins - the so-called dovetail), needle-shaped and fibrous. It is found mainly in the form of solid granular (alabaster) and fibrous (selenite) masses, as well as various crystalline groups (gypsum flowers, etc.). Pure gypsum is colorless and transparent; in the presence of impurities, it has gray, yellowish, pinkish, brown and other colors. Hardness on the mineralogical scale 1.5; density 2300 kg/m 3 , solubility 2.05 g/l at 20°C (highest - between 32 and 41°C). Precipitates from aqueous solutions rich in sulfate salts (during the drying out of sea lagoons and salt lakes). Gypsum precipitates at relatively low salinity; when salinity increases, anhydrous calcium sulfate, anhydrite, and then salts begin to precipitate instead of gypsum. As a result, gypsum is often found together with anhydrite, less often with halite and other salts. Many deposits were formed by the hydration of anhydrite. The main deposits of gypsum are of the sedimentary type and are widespread in sediments of various ages. Gypsum is widely used to produce binding materials; for the production of gypsum concrete, gypsum and gypsum concrete products; as an ornamental (selenite) and facing stone; in the production of paints, enamel, glazes; for gypsuming soil; in medicine, optics. Gypsum serves as the starting material in solutions designed to create hollow forms from a sculptural original; in these molds copies identical to the original are cast from bronze, porcelain and other materials. Gypsum is a component of Gancha and Stucco and lends itself well to tinting and coloring. According to TSB.