Activity diagrams for clinoptilolite: Susceptibility of this zeolite to further diagenetic reactions
- Massachusetts Institute of Technology, Cambridge (USA)
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{sub 2}O, employing available thermodynamic data for related minerals, aqueous species, and water. Stability fields are portrayed graphically, assuming the presence of potassium feldspar, saponite, and hematite, and using ranges of activities for SiO{sub 2} and Al{sup 3+} defined by the saturation limits for several silica polymorphs, gibbsite, kaolinite, and pyrophyllite. 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 is approximately in equilibrium at 25C with calcite and several zeolites, including heulandite and calcic clinoptilolite. Mg-rich clinoptilolites are stabilized in ground water depleted in Ca{sup 2+}. 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.
- OSTI ID:
- 137974
- Journal Information:
- American Mineralogist, Journal Name: American Mineralogist Journal Issue: 5-6 Vol. 75; ISSN AMMIAY; ISSN 0003-004X
- Country of Publication:
- United States
- Language:
- English
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Related Subjects
58 GEOSCIENCES
ALUMINIUM
CALCITE
CALCIUM
CLINOPTILOLITE
DIAGENESIS
FELDSPARS
GEOCHEMISTRY
GIBBSITE
GROUND WATER
HEMATITE
IRON
KAOLINITE
MAGNESIUM
MATERIAL SUBSTITUTION
NEVADA
NEVADA TEST SITE
PHASE DIAGRAMS
POTASSIUM
QUARTZ
RADIOACTIVE WASTE DISPOSAL
SILICON
SODIUM
SOLID SOLUTIONS
STABILITY
TEMPERATURE DEPENDENCE
THERMODYNAMIC PROPERTIES
TUFF
YUCCA MOUNTAIN
Yucca Mountain Project
ZEOLITES