Silicic vitric tuffs of the Alaçatı (Çeşme) area, west of Izmir (Turkey), are partly altered to authigenic zeolites and clay minerals. Mordenite and clinoptilolite-heulandite accompanied by smectite are the main alteration products of the tuffs. Scanning electron microscope examination indicates that mordenite forms mainly from a gel-like precursor and to a lesser degree from volcanic glass. Mordenite and clinoptilolite-heulandite have formed by the hydrolysis and dissolution of silicic vitric tuffs by thermal waters which circulated through the fracture zone into porous vitric tuffs. These thermal waters, which resulted from mixing of thermal (Na-Cl type) and groundwaters, provided the extra Na and Ca that are necessary for the formation of mordenite. Various percentages of calcite are present in the altered tuffs. The enrichment of calcite in the altered tuffs indicates that thermal waters supply Ca, sourced from dissolution of Triassic carbonate rocks.

The alteration minerals in the Alaçatı (Çeşme) area do not seem to be laterally zoned. The occurrence of the zeolites seems to be related to inferred fracture zones trending NE—SW. These fracture zones are important because they provide porosity and permeability in the tuffs for zeolite formation. The large glass content and permeability of tuffs accompanied by the favorable hydrochemical condition resulted in zeolitization of the Alaçatı (Çeşme) area.

The zeolites and coexisting minerals of the silicic vitric tuffs in the Alaçatı (Çeşme) area, west of İzmir (Turkey), were studied. Mordenite is the most abundant zeolite in tuffs of the Alaçatı area and usually coexists with clinoptilolite-heulandite, smectite and calcite. Opal-CT was identified by means of its crystal morphology and EDX spectrum. Scanning electron microscopy (SEM) revealed the relative age relationships between the zeolites and coexisting minerals, namely mordenite, clinoptilolite-heulandite, smectite, calcite, and, in addition, opal-CT. Smectite consistently crystallized earlier than any of the zeolites, and it occasionally coats the outer walls of some of the vitric material. The zeolites are commonly located on the smectite, although some mordenites were observed to be in direct contact with glass shards that lacked a smectite coating. Clinoptilolite-heulandite formed after smectite and before mordenite. Opal-CT is seen to postdate both smectite and needle-shaped mordenite. Calcite was probably the latest mineral to crystallize in the Alaçatı tuffs. The zeolites in the tuffs of the Alaçatı area formed by dissolution of silicic vitric tuffs by Na- and Ca-rich thermal waters which passed through the fracture zone. The appearance of zeolites together with smectite along this zone may be attributed to a semi-open system which subdivided into smaller closed systems. Small changes in the pH and chemical composition of the thermal waters during alteration produced the corrosion effects observed by SEM. Small amounts of clinoptilolite-heulandite were corroded prior to crystallization of coexisting mordenite. The different compositions of the thermal waters were probably inherited from water that resulted from mixing of thermal and groundwaters.