or SULPHATE OF LIME (γύψος of the Greeks; Gypsum of Pliny and Vitruvius), is an abundant mineral, in some of its forms, in many countries. It is known in Britain and other places by the name of alabaster; but this designation is also applied to a granular carbonate of lime, seemingly deposited from water, which, by way of distinction, is named oriental alabaster. The ἀλάβαστρον of the ancient Greeks, however, and the alabastrum of the Latins, was certainly sometimes applied to designate our gypsum, as may be seen in Theophrastus and in Pliny.

Gypsum occurs crystallized and amorphous. The purest sort is crystallized, and yields on analysis, sulphuric acid 46, lime 33, water 21 per cent. The primitive form of its crystals is a right oblique prism, which is variously modified. Its specific gravity is from 2·20 to 2·40. It is so soft as to be easily scratched by the nail.

Gypsum occurs in all the geological formations, especially in the secondary, and is found crystallized, granular, fibrous, compact, and earthy. When crystallized, it is distinctly foliated; and when the folia are large, it is termed selenite, from its reflecting a moon-like lustre.

Crystallized gypsum chiefly occurs at Bex in Switzerland; at Salzburg in the Tyrol; in New Castille, especially between Tembleque and Aranjuez; in Bohemia, France, New York, Nova Scotia; at Shotover in Oxfordshire, and at Chatley in Essex.

Granular and compact gypsum are the kinds used in sculpture, and occur snow-white near Volterra in Tuscany, in Spain, the Hartz, and other parts of Germany; in France, and in the salt formation of Cheshire. Fibrous gypsum is found at Ilfeld in the Hartz, in New Castille, Aragon; and in England in Cheshire and Derbyshire. Earthy gypsum occurs abundantly in New Castille and Aragon in Spain, at Salzburg; in Norway, and in various parts of North America. It is found in great quantity in the environs of Paris, along with a compact gypsum, in both of which the bones of the palaeotherium, amynotherium, and other extinct mammals were discovered by Cuvier. The gypsum beds of Paris contain 17 per cent. of carbonate of lime; and this sort has been named Montmartre, from the locality where it occurs. This sort of gypsum forms the best plaster of Paris, as it resists the weather better than the purer gypsums.

There is another species of gypsum which contains no water of crystallization, and is therefore termed anhydrite. This mineral has a higher specific gravity than the common gypsum, and occasionally exhibits a fine pale blue colour. It occurs chiefly at Bex, at Sulz on the River Neckar, in Styria; at Bleyberg in Carinthia; in Upper Austria, and in Galicia.

Gypsum occurs in beds in gneiss and in mica-slate, and also in greywacke-slate; but its chief deposits are in the red clays of the secondary formation. It is found also in the tertiary formations; and even appears to be now forming, by the decomposition of iron-pyrites in contact with calcareous strata. It therefore must be considered as a member of all the great geological series of rocks.

Gypsum is important in an economical point of view. It is employed both in America and in Europe, as a top-dressing for meadow-lands with much advantage; but its principal use is for the formation of plaster of Paris. For this purpose it is burnt to expel its water of composition; and then when its powder is mixed with water, it forms a semi-liquid paste, that rapidly sets or dries by the absorption of the water; during which process a considerable heat is extricated, as was long ago remarked by Pliny. This extraction of heat is owing to the water giving out its latent heat on its condensation in the gypsum.

This property of rapid consolidation renders gypsum very available for taking casts of works of art, or objects of nature, as of sculptures, plants, and animals. It is much employed in architectural ornaments; and in Spain and France in the construction of vaults and floors. In smaller quantity it is employed in the glazing of porcelain; but its chief uses are for making casts, as a mortar, and in agriculture.