GYPSUM , a
See also:mineral consisting of hydrous calcium sulphate, named from the Gr. yb 'os, a word used by
See also:Theophrastus to denote not only the raw mineral but also the
See also:pro-duct of its calcination, which was employed in
See also:ancient times, as it still is, as a
See also:plaster . When crystallized, gypsum is often called selenite, the cre rIYLT c of Dioscorides, so named from aeX$v1i, " the
See also:moon," probably in allusion to the soft moon-like reflection of
See also:light from some of its faces, or, according to a
See also:legend,. because it is found at
See also:night when the moon is on the increase . The granular, marble-like gypsum is termed alabaster (q.v.) . Gypsum crystallizes in the
See also:system, the
See also:habit of the crystals being usually either prismatic or
See also: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-
See also: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
See also: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
See also:clay, may enclose much muddy
See also:matter; in other cases a large proportion of sand may be mechanically entangled in the crystals without serious disturbance of
See also:form; whilst certain crystals occasionally enclose cavities with liquid and an air-bubble .
Gypsum not infrequently becomes fibrous . This variety occurs in
See also:veins, often
See also:running through gypseous marls, with the
See also: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
See also:nail (H= 5 to 2) . Its specific gravity is about 2.3 . The mineral is slightly soluble in
See also:water, one
See also:part of gypsum being soluble, according to G . K .
See also:Cameron, in 372 parts of pure water at 26' C .
See also:Waters percolating through gypseous strata, like the
See also:Keuper marls, dissolve the calcium sulphate and thus become permanently hard or " selenitic." Such water has
See also:special value for
See also:ale, and the water used by the
See also: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
See also:white, but it is often tinted, especially in the alabaster variety,
See also:grey, yellow or
See also:pink . Gypsum crystallizes with two molecules of water, equal to about 21 % by
See also:weight, and consequently has the
See also:formula CaSO4•2H20 . By exposure to strong
See also:heat all the water may be expelled, and the substance then has the composition of
See also: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
See also:Paris," which may readily be caused to recombine with water, forming a hard
See also:cement . The gypsum quarries of Montmartre, in the
See also:north of Paris, were worked in
See also:Tertiary strata,
See also:rich in fossils . Gypsum is largely quarried in England for conversion into plaster of Paris, whence it is sometimes known as " plaster
See also:stone," and since much is sent to the
See also:potteries for making moulds it is also termed "
See also:potter's stone." The chief workings are in the Keuper marls near Newark in
See also:Nottinghamshire, Fauld in Staffordshire and Chellaston in
See also:Derbyshire . It is also worked in
See also:Permian beds in
See also:Cumberland and Westmorland, and in Purbeck strata near
See also:Battle in
See also:Sussex . Gypsum frequently occurs in association with
See also:salt, having been deposited in shallow basins of salt water . Much of the calcium in
See also: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
See also: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
See also:action of sulphuric acid, resulting from the oxidation of sulphurous vapours, on lime-bearing minerals, like
See also:labradorite and
See also:augite, in the volcanic rocks: hence gypsum is common around solfataras . Again, by the oxidation of iron-
See also:pyrites and the action of the resulting sulphuric acid on limestone or on shells, gypsum may be formed; whence its origin in most
See also:clays . Gypsum is also formed in some cases by the hydration of anhydrite, the
See also:change being accompanied by an increase of
See also: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
See also:Utah, where they occur in huge geodes, or crystal-lined cavities, in deposits from the old salt-lakes .
See also:Fine crystals, sometimes curiously bent, occur in the Permian rocks of
See also:Friedrichroda, near
See also:Gotha, where there is a grotto called the Marienglashohle, close to Rheinhardsbrunn . Many of the best localities for selenite are in the New Red
See also:Sandstone formation (Trias and Permian), notably the salt-mines of
See also:Hall and Hallein, near
See also:Salzburg, and of Bex in
See also:Switzerland . Excellent crystals, usually of a brownish
See also:colour arranged in groups, are often found in the brine-
See also:chambers and the launders used in salt-
See also:works . Selenite also occurs in fine crystals in the
See also:sulphur-bearing marls of
See also:Girgenti and other Sicilian localities; whilst in Britain very bold crystals are yielded by the Kimeridge clay of Shotover
See also:Hill near
See also:Oxford .
See also:Twisted crystals and rosettes of gypsum found in the
See also:Mammoth Cave,
See also: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
See also:agent in dressing
See also:land, and it has also been used in the manufacture of
See also:porcelain and
See also:glass . Formerly it was employed, in the form of thin cleavage-plates, for
See also:glazing windows, and seems to have been, with
See also: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
See also:optical constants in the rock-forming minerals . (F . W .
GYROSCOPE AND GYROSTAT
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