Solubilities have been measured of Na- and K-exchanged clinoptilolite from Castle Creek, Idaho (Si/Al=4.50) and
K-exchanged clinoptilolite from Mud Hills, California (Si/Al=5.14) at 90° to 265°C and Pv=PH2O. Reversible solubility measurements in aqueous solutions of high-purity zeolite samples are a direct and accurate means of deriving mineral thermodynamic properties, primarily because of an improved understanding of Al aqueous speciation (Castet et al., 1993; Bourcier et al., 1993; Wesolowski and Palmer, 1994) and because such experiments ensure that a constant and maximum state of hydration is maintained. The measurements yield DG°f (T) for zeolites containing variable cation contents and Si/Al ratios and they provide a basis for relating measurements of dissolution and precipitation reaction rates to departures from (metastable) equilibrium conditions. Thermochemical and kinetic data of zeolites with variable compositions are required to understand the formation and evolution during water-rock interactions of zeolite mineral assemblages in volcanic and sedimentary rocks.
The DG°f values for Castle Creek Na- and K-clinoptilolite extrapolated to 25°C are -6263 ±15 kJ/mol and -6119 ±18 kJ/mol based on 12 oxygens per unit cell, respectively. For Mud Hills K-clinoptilolite, with a higher Si/Al, DG°f extrapolated to 25°C is -5940 ±15 kJ/mol. The values are within 0.46% of the estimated values using the polyhedral model of Chermak and Rimstidt (1989). Variations in the standard molal Gibbs free energies are largely influenced by the number of water molecules involved in the dissolution reaction and, consequently on the measured water content of the zeolite. K-clinoptilolites generally contain less water than Na and Ca varieties (Boles, 1972). Thus, Na- and K-clinoptilolites cannot strictly be treated as end-members of a binary system. When calculated on an anhydrous basis, however, Castle Creek K-clinoptilolite has a DG°f that is 51 kJ/mol and 23 kJ/mol smaller compared to the Na-exchanged and the high Si/Al variety, respectively, relationships consistent with
the observed propensity for clinoptilolite to incorporate K over Na in cation exchange experiments (Palaban, 1994). Water molecules in clinoptilolite are exchangeable and held in the structure with various energies dependent on the cation content (Bish, 1988). We are currently extending solubility experiments to Ca and Mg clinoptilolites.
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