Sulphates are compounds of Oxygen and Sulphur together with one or more metals. The Oxygen and Sulphur are combined as the sulfate ion, SO4 2-, in which the central Sulphur atom is surrounded by four Oxygen atoms located at the corners of a tetrahedron. The two negative charges are distributed evenly over the oxygen atoms.

Many sulphates are complex minerals. This happens because of the large number of ways in which metal ions can be placed amongst the sulphate groups in the crystal structure. Many of the minerals are strongly hydrated species, meaning that they are bound with water molecules in the crystal. Most are rare, although Gypsum and Anhydrite are quite common minerals. More than 150 species have been reported in total.

All minerals in the group are translucent to transparent. Many are light in colour. A few are very soluble in water, making them relatively rare minerals. None are hard, but most are fairly high in specific gravity becuse of the relatively dense metal packing in the crystal structure.

The Sulphates are divided into the following groups:

Gypsum Group - the most important members are Gypsum and Anhydrite, hydrated and anhydrous forms of Calcium Sulphate. Gypsum can be converted to Anhydrite by heating. The process is reversible and when mixed with water the Anhydrite reverts back to Gyspsum. In doing so, the ions recrystalise and become firmly cemented together, the basis of Plaster of Paris. Another member of the group is Epsomite, hydrated Magnesium Sulphate. It is one of several hydrated, soluble sulphates, many of which are rare due to their solubility.

Barite Group - members of the group include Barite, Celestite and Anglesite. In all three cases the metal ions are divalent (Ba2+, Sr2+ and Pb2+ respectively) and large. The size opens up the crystal structure and means that each metal is surrounded by ten oxygen atoms. Because all three metals are chemically similar the crystal structures of all three minerals are identical. It also means that theoretically a solid solution between the three minerals is possible as metal substitution takes place between them. This behaviour has been observed artificially in the laboratory, but natural occurrence is much more difficult to observe without complex analytical methods.