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Two of the most important variables that can be determined in any geologic study are the timing and conditions of formation. Although the discovery of radioactivity and isotopes at the beginning of this century revolutionized our understanding of the age and formation conditions of rocks and minerals, the isotope geochemistry of certain mineral groups such as the zeolites has remained elusive and poorly understood. To date fewer than 40 papers have been published on the isotope geochemistry of zeolites. This dearth of isotopic data is due to technical difficulties rather than lack of interest. Recently, however, great strides have been made in understanding the isotope geochemistry of zeolites as a result of a better understanding of their crystal structures, coupled with the increased popularity of isotope geochemistry and a rapid growth in the number of isotope laboratories. This chapter reviews the current status of our knowledge of isotopes in natural zeolites, including the methods used to prepare samples and measure isotopic abundances and ratios, some of the problems associated with these measurements, and some of the implications of the results obtained. Because the reader may not be familiar with isotope geochemistry, the fundamentals of the behavior of isotopes will also be reviewed briefly. It should be pointed out that a vast literature exists on the behavior and characteristics of isotopes in synthetic zeolites and on isotopic studies related to industrial applications (see e.g. Chang et al. 1995). This material is, however, outside the scope of this chapter and will only be mentioned when it has a direct bearing on the geochemistry of zeolites.
The earliest published report on isotopes in zeolites may have been that of Taylor and Urey (1938) who determined fractionation of stable Li, K, and N isotopes by an unspecified “zeolite” species. At Columbia University, Taylor and …