GYPSUM  , a

common mineral consisting of hydrous calcium sulphate, named from the Gr. yb 'os, a word used by Theophrastus to denote not only the raw mineral but also the pro-duct of its calcination, which was employed in ancient times, as it still is, as a plaster . When crystallized, gypsum is often called selenite, the cre rIYLT c of Dioscorides, so named from aeX$v1i, " 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 . In the characteristic type represented in fig . 1, f represents the prism, 1 the hemi-pyramid and P the clinopinacoid . Twins are common, as in fig . 2, forming in some cases arrow-headed and swallow-tailed crystals . Cleavage is perfect parallel to the clinopinacoid, 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 occasionally enclose cavities with liquid and an air-bubble .

Gypsum not infrequently becomes fibrous . This variety occurs in

veins, often running through gypseous 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= 5 to 2) . Its specific gravity is about 2.3 . The mineral is slightly soluble in water, one part of gypsum being soluble, according 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 character; hence the artificial dissolving of gypsum in water for brewing purposes is known as " burtonization." Deposits of gypsum are formed in boilers using selenitic water . Pure gypsum is colourless or white, but it is often tinted, especially in the alabaster variety, grey, yellow or pink . Gypsum crystallizes with two molecules of water, equal to about 21 % by weight, and consequently has the formula CaSO4•2H20 . By exposure to strong heat all the water may be expelled, and the substance 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 substance, 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 sometimes known as " plaster stone," and since much is sent to the Staffordshire 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 . 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 salt-lagoons the deposition of the gypsum is probably effected in most cases by means of micro-organisms . Waters containing sulphuretted hydrogen, on exposure to the air in the presence of limestone, may yield gypsum by the formation of sulphuric acid and its interaction with the calcium carbonate . In volcanic districts gypsum is produced by the action of sulphuric acid, resulting from the oxidation of sulphurous vapours, on lime-bearing minerals, like labradorite 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 . Gypsum 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 crystal-lined cavities, in deposits from the old salt-lakes . Fine crystals, sometimes curiously bent, occur in the Permian rocks of Friedrichroda, near Gotha, where there is a grotto called the Marienglashohle, 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 " (ovXos, " woolly "; qSwXe6s, " cave ") . In addition to the use of gypsum in cement-making, 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 cleavage-plates of gypsum are used in microscopic petrography for the determination of certain optical constants in the rock-forming minerals . (F . W .