WITHERITE  , a

mineral consisting of barium carbonate (BaCO3), crystallizing in the orthorhombic system . The crystals are invariably twinned together in groups of three, giving rise to pseudo-hexagonal forms somewhat resembling bipyramidal crystals of quartz, the faces are usually rough and striated horizontally . The colour is dull white or sometimes greyish, the hardness is 32 and the specific gravity 4.3 . The mineral is named after W . Withering, who in 1784 recognized it to be chemically distinct from barytes . It occurs in veins of lead ore at Hexham in Northumberland, Alston in Cumberland, Anglezark, near Chorley in Lancashire, and a few other localities . Witherite is readily altered to barium sulphate by the action of water containing calcium sulphate in solution, and crystals are therefore frequently encrusted with barytes . It is the chief source of barium salts, and is mined in considerable amounts in Northumberland . It is used for the preparation of rat poison, in the manufacture of glass and porcelain, and formerly for refining sugar . (L . J .