OXYGEN-ISOTOPE STUDIES OF ZEOLITES STILBITE, ANALCIME, HEULANDITE, AND CLINOPTILOLITE .2. KINETICS AND MECHANISMS OF ISOTOPIC EXCHANGE BETWEEN ZEOLITES AND WATER-VAPOR

Oxygen isotopic exchange experiments were conducted between water vapo r and the framework oxygen of zeolites stilbite, analcime, heulandite, and clinoptilolite. The results show that the minerals are extremely exchangeable at low temperatures. Measurable exchange was seen in a fe w hours for stilbite, heulandite, and clinoptilolite at a temperature as low as 100-degrees-C. Analcime is more stable at low temperatures ( below 250-degrees-C) than the other three zeolites, especially when it is fully hydrated. The overall rate of the exchange can be modelled n umerically by viewing the process as two sequential steps: self diffus ion of water vapor into the zeolite channels followed by isotope excha nge between the framework oxygen and the channel water. Fitting experi mental data into the mathematical model allows estimates of the effect ive diffusion coefficient and the rate constant of the exchange reacti on for a given mineral at a given temperature. Using the Arrhenius equ ation, the activation energies of self-diffusion are calculated to be 16 kcal/mol for stilbite and 20 kcal/mol for analcime. The activation energies of the exchange reaction for stilbite and analcime are 36 and 31 kcal/mol, respectively.