A natural or synthetic material used to selectively separate substances from mixtures such as water from alcohol. They have an open three-dimensional crystal structure that naturally contains water molecules. Synthetic zeolites include hydrated aluminosilicate. The water can be released by heating and the zeolite can be used to absorb other substances of an appropriate molecular size. Literally meaning ‘boiling stone’, they are often called molecular sieves. They are also used in sorption pumps for vacuum systems, separating organic compounds such as high-octane petroleum for lead-free petrol, and in ion exchange for applications such as in water softening. Zeolites are one of the most diverse and industrially useful mineral groups.