Equilibrium Modeling of Clinoptilolite-Analcime Equilibria at Yucca Mountain, Nevada, USA

Abstract

Abstract--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 tufts 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 geochemical 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 Ge0-Calc PTA-SYSTEM using estimated thermodynamic data for measured chemical compositions of clinoptilolite and analcime at Yucca Mountain. Log[(aK+)2/aCa 2+] versus log[(aNa+)2/aCa 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 § concentrations in either clinoptilolite or the fluid phase, increased clinoptilolite K § concentration, increased temperature and decreased aqueous silica activity all stabilize analcime relative to clinoptilolite, assuming present-day Yucca Mountain water compositions. However, increased Ca 2 § concentrations in either clinoptilolite or the fluid phase, increased aqueous K § concentration and increased AI: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 ~ Even if temperatures rise significantly (for example, to 150 ~ not all clinoptilolite should alter to analcime. Perhaps more importantly, thermodynamic modeling suggests that some Yucca Mountain clinoptilolites, particularly those rich in Ca and A1, will remain stable at elevated temperatures, even with an aqueous silica activity at quartz saturation.

Distribution and chemistry of fracture-lining minerals at Yucca Mountain, Nevada

“…Lake Magadi sediments contain abundant trachytic glass that is altering primarily to erionite (Surdam and Eugster, 1976). Surdam and Eugster (1976) report water compositions for Lake Magadi that are high in alkalis and low in alkali-earths in accord with the calculations by Chipera et al (1995a). If the identification of kenyaite is indeed correct, geochemical conditions resulting in its formation at Yucca Mountain can be constrained to Na'/H+ ratios greater than 5 and silica activities approaching that of amorphous silica (Bricker, 1969).…”

Synthesis of zeolites A and P from 1:1 and HS from 2:1 clays

“…2:1 clay montmorillonite (Swy-2) was supplied by the Source Clay Minerals Repository Collection and characterized by the half cell structural formula [22][23][24]. First, the clay is purified and exchanged with NaCl 1N solution.…”

Willsher Group and geology of the Triassic Kaka Point coastal section, south‐east Otago, New Zealand

“…Fracture-filling veinlets of analcime have also formed without a mineral precursor. Nevertheless, the alteration of heulandite-clinoptilolite to analcime and quartz is a well-established low-temperature dehydration reaction accompanied by substitution of Na + for K + and Ca 2+ (Coombs 1993;Chipera & Bisch 1997). The reaction is dependent on temperature, total pressure, fluid pressure, and activities of SiO 2 and exchangeable cations.…”