Clinoptilolite is the predominant zeolite in diagenetically altered volcanic rocks at Yucca Mountain, Nevada, having formed by posteruptive reactions of ground water with vitric tuffs in the pyroclastic deposits. Compositional variations of clinoptilolites in the fractured and zeolitized tuffs not presently in contact with ground water and the vulnerability of zeolites to burial diagenesis raise questions about the long-term stability of clinoptilolite. Equilibrium activity diagrams were calculated for clinoptilolite solid solutions in the seven-component system Ca-Na-K-Mg-Fe-Al-Si plus H 2 O, employing available thermodynamic data for related minerals, aqueous species, and water. The thermodynamic calculations show that ground water of the sodium-bicarbonate type, such as reference J-13 well water collected from fractured devitrified tuffs at the adjacent Nevada Nuclear Test Site, is approximately in equilibrium at 25 degrees C with calcite and several zeolites, including heulandite and calcic clinoptilolite. Mg-rich clinoptilolites are stabilized in ground water depleted in Ca (super 2+) . Decreasing Al (super 3+) activities result in the association of clinoptilolite with calcite and opal-CT observed in weathered zeolitized vitric tuffs at Yucca Mountain. The activity diagrams indicate that prolonged diagenetic reactions with ground water depleted in Al, enriched in Na or Ca, and heated by the thermal envelope surrounding buried nuclear waste may eliminate sorptive calcic clinoptilolites in fractured tuffs and underlying basal vitrophyre.--Modified journal abstract.