GYPSUM, a common mineral consisting of hydrous calcium sulphate, named from the Gr. a word used by Theophrastus to denote not only the raw mineral but also the product of its calcination, which was employed in ancient times, as it still is, as a plaster. When crystallized, gypsum is often called selenite, the creXsjvirris of Dioscorides, so named from o EX vri, "the moon," probably in allusion to the soft moon-like reflection of light from some of its faces, or, according to a legend, because it is found at night when the moon is on the increase. The granular, marble-like gypsum is termed alabaster (q.v.).
Gypsum crystallizes in the monoclinic system, the habit of the crystals being usually either prismatic or tabular; in the latter case the broad planes are parallel to the faces of the clinopinacoid. The crystals may become lenticular by curvature of certain faces. Twins are common, forming in some cases arrow-headed and swallow-tailed crystals. Cleavage is perfect parallel to the clino pinacoid, yielding thin plates, often diamond-shaped, with pearly lustre; these flakes are usually flexible, but may be brittle, as in the gypsum of Montmartre. Two other cleavages are recognized, but they are imperfect. Crystals of gypsum, when occurring in clay, may enclose much muddy matter; in other cases a large proportion of sand may be mechanically entangled in the crystals without serious disturbance of form; whilst certain crystals oc casionally enclose cavities with liquid and an air-bubble. A fibrous variety occurs in veins, often running through marls, with the fibres disposed at right angles to the direction of the vein. Such gypsum when cut and polished has a pearly opalescence, or satiny sheen, whence it is called satin-spar (q.v.).
Gypsum is so soft as to be scratched even by the finger-nail (H=1•5 to 2) . Its specific gravity is about 2.3. The mineral is slightly soluble in water, one part of gypsum being soluble, accord ing to G. K. Cameron, in 372 parts of pure water at 26° C. Waters percolating through gypseous strata, like the Keuper marls, dissolve the calcium sulphate and thus become permanently hard or "selenitic." Such water has special value for brewing pale ale, and the water used by the Burton breweries is of this char acter; hence the artificial dissolving of gypsum in water for brew ing purposes is known as "burtonization." Deposits of gypsum are formed in boilers using selenitic water.
Pure gypsum is colourless or white, but it is of ten tinted, especially in the alabaster variety, grey, yellow or pink. By expo sure to strong heat all the water may be expelled, and the sub stance then has the composition of anhydrite (q.v.). When the calcination, however, is conducted at such a temperature that only about 75% of the water is lost, it yields a white pulverulent sub stance, known as "plaster of Paris," which may readily be caused to recombine with water, forming a hard cement. The gypsum quarries of Montmartre, in the north of Paris, were worked in Tertiary strata, rich in fossils. Gypsum is largely quarried in England for conversion into plaster of Paris, whence it is some times known as "plaster stone," and since much is sent to the Staf fordshire potteries for making moulds it is also termed "potter's stone." The chief workings are in the Keuper marls near Newark in Nottinghamshire, Fauld in Staffordshire and Chellaston in Derbyshire. It is also worked in Permian beds in Cumberland and Westmorland, and in Purbeck strata near Battle in Sussex. The principal American workings are in New York, California, Iowa, Kansas, Texas, Ohio, Oklahoma, Michigan, Wyoming, Nova Scotia and New Brunswick.
Gypsum frequently occurs in association with rock-salt, having been deposited in shallow basins of salt water. Much of the calcium in sea-water exists as sulphate ; and on evaporation of a drop of sea-water under the microscope this sulphate is deposited as acicular crystals of gypsum. In volcanic districts gypsum is produced by the action of sulphuric acid, resulting from the oxida tion of sulphurous vapours, on lime-bearing minerals, like labra dorite and augite, in the volcanic rocks : hence gypsum is common around solfataras. Again, by the oxidation of iron-pyrites and the action of the resulting sulphuric acid on limestone or on shells, gypsum may be formed ; whence its origin in most clays. Gyp sum is also formed in some cases by the hydration of anhydrite, the change being accompanied by an increase of volume to the extent of about 6o%. Conversely gypsum may, under certain conditions, be dehydrated or reduced to anhydrite.
Some of the largest known crystals of selenite have been found in southern Utah, where they occur in huge geodes, or crystallined cavities, in deposits from the old salt-lakes. Fine crystals, some times curiously bent, occur in the Permian rocks of Friedrichroda, near Gotha, where there is a grotto called the Marien-glashohle, close to Rheinhardsbrunn. Many of the best localities for selenite are in the New Red Sandstone formation (Trias and Permian), notably the salt-mines of Hall and Hallein, near Salzburg, and of Bex in Switzerland. Excellent crystals, usually of a brownish colour arranged in groups, are often found in the brine-chambers and the launders used in salt-works. Selenite also occurs in fine crystals in the sulphur-bearing marls of Girgenti and other Sicilian localities ; whilst in Britain very bold crystals are yielded by the Kimeridge clay of Shotover Hill near Oxford. Twisted crystals and rosettes of gypsum found in the Mammoth Cave, Kentucky, have been called "oulopholites" (oi5Xos, "woolly"; 4 wXs6r, "cave") .
In addition to the use of gypsum in the making of Keene's cement, plaster of Paris, fireproof building blocks, wall and parti tion material, foundry cores, the mineral finds application as an agricultural agent in dressing land, and it has also been used in the manufacture of porcelain and glass. Formerly it was employed, in the form of thin cleavage-plates, for glazing windows, and seems to have been, with mica, called lapis specularis. It is still known in Germany as Marienglas and Fraueneis. Delicate cleav age-plates of gypsum are used in microscopic petrography for the determination of certain optical constants in the rock-forming minerals. (R. H. RA.)