Computer graphics representation of a member of the zeolite group of minerals, designated ZSM-5. Zeolite minerals containing a silica-alumina crystalline structure are used as catalysts in the catalytic cracking process during the refining of petroleum. Zeolite are sometimes called molecular sieves". Cracking involves the fragmenting of large molecules in the initial charge into smaller, lower-boiling, stable hydrocarbon molecules which leave the reactor vessel as volatile fractions such as gasoline, kerosene and gas oils."