Zeolites

a group of alumina silicates of sodium, potassium, calcium and barium that contain loosely held water that can be removed by heating and regained by exposure to water. However, the cavities created by the loss of water can be occupied by other molecules of a similar size. Zeolites have thus found uses in ION EXCHANGE and as adsorbents. Zeolites containing small amounts of platinum or palladium are used as CATALYSTS in the CRACKING of HYDROCARBONS. There are both natural and synthetic zeolites.

Taken from Dictionary of Science