Phase Behavior of Aqueous Na-K-Mg-Ca-CI-NO3 Mixtures: Isopiestic Measurements and Thermodynamic Modeling

A comprehensive model has been established for calculating thermodynamic properties of multicomponent aqueous systems containing the Na{sup +}, K{sup +}, Mg{sup 2+}, Ca{sup 2+}, Cl{sup -}, and NO{sub 3}{sup -} ions. The thermodynamic framework is based on a previously developed model for mixed-solvent electrolyte solutions. The framework has been designed to reproduce the properties of salt solutions at temperatures ranging from the freezing point to 300 C and concentrations ranging from infinite dilution to the fused salt limit. The model has been parameterized using a combination of an extensive literature database and new isopiestic measurements for thirteen salt mixtures at 140 C. The measurements have been performed using Oak Ridge National Laboratory's (ORNL) previously designed gravimetric isopiestic apparatus, which makes it possible to detect solid phase precipitation. Water activities are reported for mixtures with a fixed ratio of salts as a function of the total apparent salt mole fraction. The isopiestic measurements reported here simultaneously reflect two fundamental properties of the system, i.e., the activity of water as a function of solution concentration and the occurrence of solid-liquid transitions. The thermodynamic model accurately reproduces the new isopiestic data as well as literature data for binary, ternary and higher-order subsystems. Because of its high accuracy in calculating vapor-liquid and solid-liquid equilibria, the model is suitable for studying deliquescence behavior of multicomponent salt systems

Kinetics of Quenching of Hydrous Feldspathic Melts: Quantification Using Synthetic Fluid Inclusions

microthermometric analysis of fluid inclusions preserved during the isobaric quenching of H-2O-saturated, vesicular silicate melts provides a method for the determination of the glass transition temperature of hydrous silicate melts at high pressure. The method is based on the principle that the contraction of inclusion cavities during quenching is rate-limited by the volume relaxation of the melt. Viscous relaxation of the melt ceases during cooling at the glass transition temperature. Bulk densities of the fluid inclusions whose volumes are frozen at the glass transition preserve a record of the trapping event, i.e., the glass transition temperature. Liquid-vapor homogenization temperatures [T(H(L-V)] of the trapped inclusions are measured using a microscope heating-stage assembly. Bulk densities of H2O present in the inclusions at T(H(L-V)) and P(saturation) are determined from literature values as are the P-T trajectories of the corresponding isochores. The intersection of an isochore with the experimental pressure during the quench yields the glass transition temperature for that particular glass composition and quench rate. The method has been applied to seven compositions on the join albite-orthoclase. H2O-saturated melts along this join have been rapidly and isobarically quenched at 2000 bars. The total solubilities of H2O range from 5.12 to 6.03 +/- 0.15 wt%. The glass transition temperatures of the H2O-saturated melts range from 525 to 412-degrees-C. The compositional dependence of the glass transition is strongly nonlinear. Melts of intermediate composition exhibit a significantly lower glass transition than either end-member. The deviation from additivity reaches a maximum of 70-degrees-C at Ab50Or50 (molar basis). The information on T(g) can be combined with data for the properties of the quenched glasses to obtain liquid properties at hydrothermal conditions, for example, the viscosity and the thermal expansivity of the wet melts. The quantification of trapping temperatures for fluid inclusions in silicate melts also has potential applications in the study of the kinetics of melt degassing