Zeolite equilibria in the system CaAI2Si208‐ NaAISi3O8‐ SiO2‐ H2O

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Abstract
On the basis of experimentally determined stabilities of common zeolites, including analcime, stilbite, heulandite, yugawaralite, laumontite and wairakite, T ‐ XQ,phase relations of these zeolites and plagioclase in the presence of excess quartz and H2O were constructed for the system CaAl2Si208 ‐ NaAlSi3O8 ‐ SiO2 ‐ H2O at Pfluid = 1000, 500, and 300 bars. Extensive solid solutions exist for wairakite‐analcime and for heulandite‐clinoptilolite series, whereas very limited Na substitution occurs in laumontite, stilbite, and yugawaralite. Such compositional variability results in co‐existence of two zeolites in natural parageneses. Progressive changes both in assemblages and in compositions of co‐existing zeolites with increasing temperatures are schematically shown with many compositional loops. Ca zeolites vary in composition, depending on the mineral assemblage. Progressive changes in zeolite assemblages are sensitive to bulk‐rock composition. Some zeolites such as yugawaralite, stilbite, and laumontite may be restricted only to high Ca/Na rocks. For rocks with a low Ca/Na ratio, zeolite parageneses (+ quartz) with increasing temperature, are analcime + heulandite ? heulandite + albite (or laumontite + analcime) ? laumontite + albite ? albite + wairakitess. ? wairakites.s. + plagioclase. In low SiO2 metabasites, however, the progressive change could be stilbite + analcime ? laumontite + albite ? wairakitess,+ plagioclase. Wairakite (or calcian analcime) + plagioclase occurs and heulandite is not common. These paragenetic sequences are consistent with the observed zeolite assemblages in hydrothermal, burial, and ocean‐floor metamorphism described in the literature.