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GYPSUM , a See also:common See also:mineral consisting of hydrous See also: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 (a common Teutonic word, cf. Ger. Mond, Du. maan, Dan. maane, &c., and cognate with such Indo-Germanic forms as Gr. µlip, Sans. ma's, Irish mi, &c.; Lat. uses luna, i.e. lucna, the shining one, lucere, to shine, for the moon, but preserves the word i
- MOON, SIR RICHARD, 1ST BARONET (1814-1899)
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, See also:marble-like gypsum is termed See also:alabaster (q.v.).
Gypsum crystallizes in the See also:monoclinic See also:system, the See also:habit of the crystals being usually either prismatic or See also:tabular; in the latter See also: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 See also: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 See also: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 See also: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 See also: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:finger-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:brewing See also:pale See also:ale, and the water used by the See also:Burton breweries is of this See also: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
- WHITE, ANDREW DICKSON (1832– )
- WHITE, GILBERT (1720–1793)
- WHITE, HENRY KIRKE (1785-1806)
- WHITE, HUGH LAWSON (1773-1840)
- WHITE, JOSEPH BLANCO (1775-1841)
- WHITE, RICHARD GRANT (1822-1885)
- WHITE, ROBERT (1645-1704)
- WHITE, SIR GEORGE STUART (1835– )
- WHITE, SIR THOMAS (1492-1567)
- WHITE, SIR WILLIAM ARTHUR (1824--1891)
- WHITE, SIR WILLIAM HENRY (1845– )
- WHITE, THOMAS (1628-1698)
- WHITE, THOMAS (c. 1550-1624)
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 See also: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 See also:England for See also:conversion into plaster of Paris, whence it is sometimes known as " plaster See also:- STONE
- STONE (0. Eng. shin; the word is common to Teutonic languages, cf. Ger. Stein, Du. steen, Dan. and Swed. sten; the root is also seen in Gr. aria, pebble)
- STONE, CHARLES POMEROY (1824-1887)
- STONE, EDWARD JAMES (1831-1897)
- STONE, FRANK (1800-1859)
- STONE, GEORGE (1708—1764)
- STONE, LUCY [BLACKWELL] (1818-1893)
- STONE, MARCUS (184o— )
- STONE, NICHOLAS (1586-1647)
stone," and since much is sent to the See also:Staffordshire See also:potteries for making moulds it is also termed " See also:potter's stone." The See also:chief workings are in the Keuper marls near See also: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 See also:Westmorland, and in Purbeck strata near See also:Battle in See also:Sussex.
Gypsum frequently occurs in association with See also:rock-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 See also: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 See also:hydrogen, on exposure to the air in the presence of See also:limestone, may yield gypsum by the formation of sulphuric See also: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 See also: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 See also: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:southern 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 See also:bent, occur in the Permian rocks of See also:Friedrichroda, near See also:Gotha, where there is a grotto called the Marienglashohle, See also: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
- HALL (generally known as SCHWABISCH-HALL, tc distinguish it from the small town of Hall in Tirol and Bad-Hall, a health resort in Upper Austria)
- HALL (O.E. heall, a common Teutonic word, cf. Ger. Halle)
- HALL, BASIL (1788-1844)
- HALL, CARL CHRISTIAN (1812–1888)
- HALL, CHARLES FRANCIS (1821-1871)
- HALL, CHRISTOPHER NEWMAN (1816—19oz)
- HALL, EDWARD (c. 1498-1547)
- HALL, FITZEDWARD (1825-1901)
- HALL, ISAAC HOLLISTER (1837-1896)
- HALL, JAMES (1793–1868)
- HALL, JAMES (1811–1898)
- HALL, JOSEPH (1574-1656)
- HALL, MARSHALL (1790-1857)
- HALL, ROBERT (1764-1831)
- HALL, SAMUEL CARTER (5800-5889)
- HALL, SIR JAMES (1761-1832)
- HALL, WILLIAM EDWARD (1835-1894)
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 See also: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 See also:Britain very bold crystals are yielded by the Kimeridge clay of Shotover See also:- HILL
- HILL (0. Eng. hyll; cf. Low Ger. hull, Mid. Dutch hul, allied to Lat. celsus, high, collis, hill, &c.)
- HILL, A
- HILL, AARON (1685-175o)
- HILL, AMBROSE POWELL
- HILL, DANIEL HARVEY (1821-1889)
- HILL, DAVID BENNETT (1843–1910)
- HILL, GEORGE BIRKBECK NORMAN (1835-1903)
- HILL, JAMES J
- HILL, JOHN (c. 1716-1775)
- HILL, MATTHEW DAVENPORT (1792-1872)
- HILL, OCTAVIA (1838– )
- HILL, ROWLAND (1744–1833)
- HILL, SIR ROWLAND (1795-1879)
Hill near See also:- OXFORD
- OXFORD, EARLS OF
- OXFORD, EDWARD DE VERE, 17TH EARL
- OXFORD, JOHN DE VERE, 13TH EARL OF (1443-1513)
- OXFORD, PROVISIONS OF
- OXFORD, ROBERT DE VERE, 9TH EARL OF (1362-1392)
- OXFORD, ROBERT HARLEY, 1ST
Oxford. See also:Twisted crystals and rosettes of gypsum found in the See also:Mammoth See also: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 See also: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.
End of Article: GYPSUM
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