Equilibrium in the Clinoptilolite-H2O System
by J. William Carey and David L. Bish
American Mineralogist Vol 81: 952-962
Abstract
A thermodynamic formulation of equilibrium in the clinoptilolite-H2O system has been obtained from analysis of equilibrium data. The data were collected by thermogravimetry on Ca-, Na-, and K-exchanged natural clinoptilolite (Fish Creek Mountains, Nevada). Temperatures and pressures of the experiments ranged from 25 to 250 oC and 0.2 to 35 mbar water-vapor pressure. Equilibrium in all three clinoptilolite species was successfully formulated with the following expression for the partial molar Gibbs free energy of hydration:
in which theta is the ratio H2O/(maximum H2O), P is the vapor pressure of H2O, and W1 and W2are excess mixing parameters. The energetic preference for H2O in clinoptilolite increases in the order K, Na, and Ca with a difference between K and Ca of 40% in the Gibbs free energy of hydration and 14% in the enthalpy of hydration.
The thermodynamic formulation is applied to the occurrence of clinoptilolite at Yucca Mountain, Nevada, where the proposed emplacement of nuclear waste would lead to heating of clinoptilolite-bearing tuffs. The effect of dehydration processes on the thermal and hydrologic evolution of Yucca Mountain is illustrated by calculations on equilibrium water content, contributions to groundwater saturation, and depression of temperature evolution as a function of thermal history. Rock units with abundant clinoptilolite (or by analogy other hydrous phases) will remain significantly cooler than units with anhydrous mineralogy and will evolve substantial water/rock volumes of pore water.
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