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sulphates Group of non-silicate minerals in which the SO42-radical is in combination with a number of metal cations. Examples include barite (BaSO4), celestite (SrSO4), anglesite (PbSO4), and anhydrite (CaSO4). Gypsum (CaSO4.2H2O) is the most common of a number of hydrated sulphates which also occur. Sulphates are normally colourless or white, soft (hardness about 3), massive or earthy, but tabular when crystalline. They are low-temperature minerals and occur as gangue minerals in hydrothermal veins and as chemical precipitates and evaporites.