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gypsum(jĭp`səm), mineral composed of calcium sulfate (calcium, sulfur, and oxygen) with two molecules of water, CaSO4·2H2O. It is the most common sulfate mineral, occurring in many places in a variety of forms. A transparent crystalline variety is selenite. A massive gypsum of delicate color and texture, readily worked into ornamental vases, boxes, and the like, is called alabaster. A lustrous gypsum with fibrous structure, called satin spar, is used in jewelry and for other ornaments, but it is soft and easily marred. Plaster of Paris, a fine white powder, is produced by heating gypsum to expel the water. If this powder is moistened and then allowed to dry, it becomes hard, or sets. Its major use is in the manufacture of gypsum lath and wall board, and for casts and molds. It is widely used for staff, the material of which temporary exposition buildings are made. Uncalcined gypsum is added to Portland cement as a retarder.
a mineral, hydrous calcium sulfate CaSO4 · 2H2O; when pure it contains 32.56 percent CaO, 46.51 percent SO3, and 20.93 percent H2O.
Gypsum crystallizes in the monoclinic system. The crystal lattice structure of gypsum is of the layer type. Double layers consist of tetrahedra (SO4) joined through calcium. Crystals are tabular, acicular (single or twinned, so-called swallow-tailed), needle-shaped, and fibrous. Gypsum is found mainly as continuous granular (alabaster) and fibrous (selenite) masses and as various crystal groups (such as gypsum rosettes). Pure gypsum is colorless and transparent; if impurities are present, it can be gray, yellowish, pinkish, brown, or other colors. The mineralogical-scale hardness is 1.5, density 2,300 kg/m3, and solubility 2.05 g per liter at 20° C (maximum between 32° and 41° C). It is precipitated from aqueous solutions rich in sulfates (in the drying up of marine lagoons and salt lakes). Gypsum is precipitated at relatively low salinity; when the salinity is raised, anhydrous calcium sulfate, or anhydrite, starts developing instead, followed by salts. Because of this effect, gypsum is often found with anhydrite, more rarely with halite and other salts. Many deposits were formed through hydration of anhydrite. The main gypsum formations are of the sedimentary type and are widely distributed in deposits of varying age. In the USSR the largest deposits occur in the Donbas and in Moscow, Kuibyshev, and Perm oblasts, as well as in the Caucasus and Middle Asia. Gypsum is widely used to make cementing materials, gypsum concrete, and articles made from gypsum and gypsum concrete; as sundry-purpose stone (selenite) and lining or facing stone; in making paints, enamels, and glazes; for gypsuming of the soil; and in medicine and optics.
Gypsum is used as the starting material in solutions for making hollow molds from a sculptured original, these molds serving for casting copies from bronze, porcelain, or other materials or from gypsum (details of modeled architectural decoration). Gypsum enters into the composition of ganch (a gypsum-clay mixture) and stucco, and stands up well to assembling and coloring.
REFERENCEBudnikov, P. P. Gips, ego issledovanie i primenenie, 3rd ed. Moscow-Leningrad, 1943.
V. P. PETROV