THERMODYNAMICS OF ION EXCHANGE IN MORDENITE AND ITS GEOLOGICAL IMPLICATIONS

Mordenite, one of the most common zeolites, often occurs in hydrothermal and sedimentary environments. Mordenite exhibits extensive solid solution and ready ion exchange between extraframework cations, most notably Na⁺, K⁺, and Ca²⁺. A lot of analyses on the crystal chemistry of natural mordenite samples showed that alkali metal cations were more prevalent than Ca²⁺, and no mordenites with Ca²⁺ comprising greater than 60% of the extraframework charge are known. It appears that the compositional limits observed in nature are related to the ion exchange behavior of mordenite.

Ion exchange equilibria in mordenite and thermodynamic analyses were conducted in order to assess the controls on mordenite compositions in nature. The original sample used in this study was synthetic Na-mordenite powder (CBV 10A, Na_1.67Al_1.67Si_10.33O₂₄·7H₂O) obtained from Zeolyst International. The homoionic exchange experiments on Na-mordenite indicated that it could be fully exchanged by K⁺, but partially substituted by Ca²⁺. A series of binary ion exchange experiments including Na⁺, K⁺, and Ca²⁺ were performed using endmember Na- and K-mordenite at 25 °C. Ion exchange involving Ca²⁺ in mordenite is incomplete; maximum Ca²⁺mole fractions of ~ 0.5 were observed consistent with natural mordente sample compositions. Mordenite exhibits preference for K⁺ over Na⁺ and Ca²⁺, but the selectivity between Na⁺ and Ca²⁺ varies with the equivalent fraction of Ca in the aqueous phase. Thermodynamic fits to the binary isotherm data were used to develop models of mordenite compositions in equilibrium with aqueous solutions. These calculations suggest that the extraframework cation compositions of mordenites observed in marine settings are consistent with equilibration with seawater, whereas mordenites formed in alkaline lakes and meteoric hydrothermal systems exhibit different cation contents reflecting the fluid compositions in these environments.