Equilibrium in the clinoptilolite-H2O system

A thermodynamic formulation for the sorption of H2O in clinoptilolite has been obtained from analysis of equilibrium data collected by thermogravimetry on near end-member Ca-, Na-, and K-exchanged natural clinoptilolite (Fish Creek Mountains, Nevada). Temperature and pressure of the experiments ranged from 25 to 250 degrees C and 0.2 to 35 mbar H2O vapor pressure. Equilibrium of three clinoptilolite species was successfully formulated with the following expression for the Gibbs free energy of hydration as a function of temperature and pressure: Delta mu(Hy)/T = Delta mu(Hy)(0)/T-0 + <Delta(H)over bar>(0)(Hy)>(1/T - 1/T-0) - 3R[ln(T/T-0) + (T-0/T - 1)] + R ln[theta/(1 - theta)P] + W-1/T theta + W-2/T theta(2) where R is the gas constant, P is the vapor pressure of H2O, W-1 and W-2 are the excess mixing parameters, and theta is the ratio H2O/(maximum H2O) with maximum water contents for the K, Na, and Ca end-members of 13.49, 15.68, and 16.25 wt%, respectively. The molar Gibbs free energy of hydration for calcium, sodium, and potassium clinoptilolite is -36.13 +/- 3.02, -29.68 +/- 3.77, and -25.53 +/- 1.37 kJ/mol H2O, respectively. The molar enthalpy of hydration for these phases is -76.92 +/- 2.88, -74.19 +/- 3.46, and -67.78 +/- 1.25 kJ/mol H2O. 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. Rock units with abundant clinoptilolite (or by analogy other hydrous phases) would remain significantly cooler than units with anhydrous minerals and would evolve a substantial volume of water.