a process in which feldspathic rocks are replaced by zeolites, occurring under conditions of relatively low temperatures and pressures by the action of hydrothermal solutions. Zeolitization is one of the forms of the metamorphism of rocks under conditions of zeolite facies. Basic effusive rocks are most frequently zeolitized, either fully or partially. Crystals of zeolites and other hydrothermal minerals, such as calcite, apophyllite, and analcime, form in the cavities of such rocks.
Zeolitized rocks are developed extensively in regions of ancient volcanic activity; modern zeolitization processes are observed in volcanic regions at sites where thermal waters seep out, for example at Paratunka and Pauzhetka on Kamchatka in the USSR and at various places in New Zealand and Iceland. Deposits of Iceland spar are associated with rocks that have been zeolitized. Individual layers of volcanic ash that have undergone zeolitization and are composed entirely of zeolites are used in industry as a cement additive and a valuable adsorbent.