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 H20, employing available thermodynamic data for related minerals, aqueous species, and water. Stability fields are portrayed graphically on plots of log(aNa+/aH+) versus log(aCa2+/(aH+)2), assuming the presence of potassium feldspar, saponite, and hematite, and using ranges of activities for SiO2 and Al3+ defined by the saturation limits for several silica polymorphs, gibbsite, kaolinite, and pyrophyllite. Formation of clinoptilolite is favored by higher SiO2 activities than allowed for by the presence of quartz, thus accounting for the coexistence of clinoptilolite with opal-CT in zeolitized vitric tuffs. The clinoptilolite stability field broadens with increasing atomic substitution of Ca for Na, and K for Ca, reaches a maximum for intermediate activities of dissolved Al, and decreases with increasing temperature. 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 °C with calcite and several zeolites, including heulandite and calcic clinoptilolite. Mg-rich clinoptilolites are stabilized in ground water depleted in Ca2+. Decreasing Al3+ 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.