Zeolites are used in numerous agricultural, commercial, and environmental applications (e.g. as soil conditioners and fertilizers, and as adsorbents for ammonia, heavy metals, nuclear and organic wastes). It is important to understand their stability to insure their persistence and effectiveness in these applications. An analysis of the occurrence of natural zeolites provides basic data on conditions favorable for zeolite stability and formation. All such environments have neutral to alkaline waters and, with few exceptions, are associated with low-temperature (<300°C) alteration of highly reactive volcanic rocks containing natural glasses. The principal exceptions are certain deep sedimentary sequences, sometimes associated with petroleum maturation where volcanic ash falls are absent, and deep-sea sediments where biogenic opal may be the highly reactive phase. Since initial recognition of the zeolite facies in diagenetic alteration (Coombs et al. 1959), investigators have struggled to develop a quantitative basis for defining zeolite stability fields by development of both the thermodynamic relations and kinetic factors governing zeolite formation. An additional motivation for study in the United States and elsewhere is the occurrence (and use) of zeolites at potential sites for disposing of high-level radioactive waste in underground repositories. Regulatory requirements for a high-level radioactive waste repository, reflecting public concerns over the integrity of subsurface disposal facilities, require unprecedented quantitative predictions of the physical-chemical response of zeolites to heat generated during radioactive decay and to waste-rock interactions over a time period exceeding tens of thousands of years. In light of such requirements, investigators are taking a new look at the thermodynamic and kinetic factors affecting zeolitization. The geochemical conditions that result in zeolite formation have been outlined in numerous studies (e.g. Hay 1966, 1978; Iijima 1978, Surdam and Sheppard 1978, Sheppard and Hay, this volume; Hay and Sheppard, this volume). Aqueous silica activity, cation concentrations, and …