OXYGEN-ISOTOPE STUDIES OF ZEOLITES STILBITE, ANALCIME, HEULANDITE, AND CLINOPTILOLITE .3. OXYGEN-ISOTOPE FRACTIONATION BETWEEN STILBITE ANDWATER OR WATER-VAPOR

The oxygen isotope fractionation between the aluminosilicate framework of stilbite and water vapor (or bulk water) was studied between 125 a nd 300-degrees-C. The fractionation factor for exchange between the ch annel water of stilbite and water vapor was determined at room tempera ture. At this temperature, the channel water of stilbite is enriched i n O-18 by 6 parts per thousand relative to water vapor (and hence is d epleted in O-18 by 3 parts per thousand relative to the liquid water). Reliable values of the fractionation between framework oxygen and wat er vapor were obtained only at 220-degrees-C and higher temperatures. At these temperatures, the fractionation is described by the equation: 1000 In alpha(stilbite-water) = -2.4 + 2.7 X 10(6)/T2. The measured f ractionation factors are similar to the fractionation factors in the a lbite-water system extrapolated from values measured at higher tempera tures (greater-than-or-equal-to 350-degrees-C). This observation is co nsistent with the fact that the stilbite has the same Al/Si ratio as a lbite.