Yucca Mountain, Nevada, is being investigated to determine its suitability to host a potential high-level radioactive waste respository. An important reason for its choice as a potential repository site was the presence of thick zeolite-rich horizons in the altered volcanic tuffs that compose the mountain. Clinoptilolite is the most abundant zeolite at Yucca Mountain and may be important in radionuclide retardation and in determining hydrologic properties. Therefore, it is necessary to understand the geo-chemical conditions affecting its long-term stability. For example, it has been suggested that long-term, repository-induced heating of the rocks at Yucca Mountain may lead to the transformation of clinoptilolite to analcime, thereby significantly affecting the hydrologic properties and retardation capabilities of the rock. Thermodynamic modeling of clinoptilolite-analcime equilibria was conducted with the program GeO-Calc PTA-SYSTEM using estimated thermodynamic data for measured chemical compositions of clinoptilolite and analcime at Yucca Mountain. Log[aK (super +) ) 2 /aCa (super 2+) ] versus log[(aNa (super +) ) 2 /aCa (super 2+) ] diagrams were calculated to model the conditions under which clinoptilolite may transform to analcime. Temperature, relative cation abundances and silica activity are all important factors in determining clinoptilolite-analcime equilibria. Increased Na (super +) concentrations in either clinoptilolite or the fluid phase, increased clinoptilolite K (super +) concentration, increased temperature and decreased aqueous silica activity all stabilize analcime relative to clinoptilolite, assuming present-day Yucca Mountain water compositions. However, increased Ca (super 2+) concentrations in either clinoptilolite or the fluid phase, increased aqueous K (super +) concentration and increased Al:Si ratios in clinoptilolite (heulandite) all stabilize clinoptilolite with respect to analcime. Assuming well J-13 water as the analog chemistry for Yucca Mountain water, clinoptilolite should remain stable with respect to analcime if temperatures in the clinoptilolite-bearing horizons do not significantly exceed 100 degrees C. Even if temperatures rise significantly (for example, to 150 degrees C), not all clinoptilolite should alter to analcime. Perhaps more importantly, thermodynamic modeling suggests that some Yucca Mountain clinoptilolites, particularly those rich in Ca and Al, will remain stable at elevated temperatures, even with an aqueous silica activity at quartz saturation.

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