CRYSTAL-CHEMISTRY OF THE ZEOLITES ERIONITE AND OFFRETITE

Many known occurrences of the zeolites erionite and offretite have bee n characterized by electron probe microanalysis, X-ray powder diffract ion, and optical microscopy. For the first time, a substantial amount of experimentally consistent and homogeneous chemical and crystallogra phic data have been evaluated for these natural zeolites. Systematic a nalysis of the data, performed by statistical multivariate analysis, l eads to the following conclusions: (1) the two zeolites have well-defi ned compositional fields in the chemical space describing the extrafra mework cation content, best illustrated in a Mg-Ca(+Na)-K(+Sr+Ba) diag ram; (2) no discrimination is possible on the basis of the framework S i/Al ratio because of the extensive compositional overlap between the two species, however the SI-AI content in the framework tetrahedra is the major control on the unit-cell volume dimensions, particularly in erionite; (3) the crystal chemistry of the Mg cations is a major facto r in controlling the crystallization of the mineral species; (4) catio n compositions at the boundary of the recognized compositional fields might be due to chemical averaging of two-phase intergrowths, although these mixed-phase occurrences are much less common than previously th ought; (5) the sign of optical elongation is not a distinctive charact er of the two phases, it is related to the Si/Al ratio in the framewor k tetrahedra of each zeolite type and cannot be used for identificatio n purposes; (6) the zeolite mineral species epitaxially overgrown on l evyne in all cases is identified as erionite; in a few cases offretite was found to be overgrown on chabazite; (7) erionite samples epitaxia lly overgrown on levyne are substantially more Al-rich and Mg-poor tha n the erionite samples associated with other zeolites.