Group of non-silicate minerals in which the SO₄^2- 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.