SMITHSONITE, a mineral species consisting of zinc carbo nate, and forming an important ore of zinc. It is rhombo hedral in crystallization and isomorphous with calcite and chaly bite. Distinct crystals are rare; they have the form of the primi tive cleavage rhombohedron. Botryoidal and stalactitic masses are more common, or again the mineral may be compact and granular or loose and earthy. The hardness is 4.5; specific gravity, The colour of the pure mineral is white; more often it is brownish, sometimes green or blue : a bright-yellow variety con taining cadmium has been found in Arkansas, and is known locally as "turkey-fat ore." The pure material contains 52% of zinc, but this is often partly replaced isomorphously by small amounts of iron and manganese, traces of calcium and magnesium, and sometimes by copper or cadmium.
Smithsonite is found in beds and veins in limestone rocks, and is often associated with galena and blende. It is a product of
alteration of blende, having been formed from this by the action of carbonated waters; or in many cases the zinc sulphide may have been first oxidized to sulphate, which in solution acted on the surrounding limestone, producing zinc carbonate. The latter mode of origin is suggested by the frequent occurrence of smith sonite pseudomorphous after calcite, that is, having the form of calcite crystals. It occurs in large amount in the province of Santander in Spain, in Missouri, and at several other places where zinc ores are mined. The best crystals are from Chessy near Lyons and Broken Hill in Northern Rhodesia. A translucent botryoidal smithsonite banded with blue and green is found at Laurion in Greece, and has sometimes been cut and polished for small orna ments. In Britain, smithsonite is usually called calamine.