A natural or synthetic hydrated aluminosilicate with an open three-dimensional crystal structure, in which water molecules are held in cavities in the lattice. The water can be driven off by heating and the zeolite can then absorb other molecules of suitable size. Zeolites are used for separating mixtures by selective absorption – for this reason they are often called molecular sieves. They are also used in sorption pumps for vacuum systems and certain types (e.g. Permutit) are used in ion-exchange (e.g. water-softening).