Mineral equilibria in the six-component seawater system,Na-K-Mg-Ca-SO4-Cl-H2O at 25°C. II: Compositions of the saturated solutions

Using the solubility model developed by Harvie and Weare (1980), the stable mineral-solution assemblages for the six-component system Na-K-Mg-Ca-SO₄-Cl-H₂O and its constituent 5-, 4- and 3-component systems at 25°C have been defined. Invariant point maps have been constructed showing the connections by univariant lines. The solubility volumes for all 20 minerals considered are also illustrated. Of the 37 invariant points, only 3 have solutions which are Ca-rich; the remaining 34 can be plotted in the reciprocal system Na-K-Mg-SO₄-Cl, which is similar to the seawater system, except that the restriction of halite saturation has been removed. Application of these results and implications for the evolution of major brine types are briefly discussed.     

Permeation of hydrogen through platinum: A re-evaluation of the data of Chou et al

The calculations of Chou et al. are revised to include effects that were neglected in the original work. The measured data are associated with permeability rather than diffusivity. The hydrogen concentration in the platinum membrane is assumed to be proportional to the square root of the hydrogen fugacity in the gas phase in conformity with experimental measurements at lower pressures. An approximate solution to the resulting mass transport equations is derived which simplifies the data analysis. The calculated permeabilities agree within experimental uncertainty with those determined by time lag experiments.     

Mineral equilibria in a six-component seawater system,Na-K-Mg-Ca-SO4-Cl-H2O,at 25°C

Phase relations in the 6-component system Na-K-Mg-Ca-SO₄-Cl-H₂O have been calculated for halite saturation, 25°C and 1 atm pressure. Using a Jänecke projection with the apices Ca-Mg-K₂-SO₄, 27 stable invariant points have been located which are connected by 69 univariant curves. Polyhalite is the only quaternary solid, but anhydrite occupies the bulk of the interior tetrahedral space. Consequently, 24 of the invariant points lie very close to the Ca-free base, Mg-K₂-SO₄. The remaining three points involve tachyhydrite and/or antarcticite. All points but two (20,27) represent peritectic conditions. Metastable equilibria have been calculated for the Ca-free system and yield relations corresponding to the solar diagram.Seawater lies in the subspace anhydrite-halite-carnallite-kieserite-bischofite (point 20) and its evaporation has been discussed for conditions of equilibrium and fractional crystallization. After gypsum is converted to anhydrite, halite precipitates. The next phase, under equilibrium conditions, is glauberite, crystallizing at the expense of anhydrite. Continued evaporation leads to glauberite resorption and eventual replacement by polyhalite. Then follow the magnesium sulfates epsomite, hexahydrite and kieserite, which are joined by carnallite. Polyhalite is replaced by anhydrite and bischoflte is added at the final invariant condition. Kainite does not appear as a primary phase under equilibrium conditions, but it is an important phase during fractional crystallization, where Ca-phases are not allowed to back-react with the brine.Up to the appearance of glauberite, thickness ratios of halite: anhydrite couplets (equilibrium or fractionation) can vary from 0 to 7, the relative amount of halite increasing with more intense evaporation. During evaporation, the activity of H₂O decreases from 0.98 (seawater) to 0.34 (final invariant brine). The data provided can be used to evaluate the effects of mineral precipitation, evaporation and brine mixing for a wide variety of natural brines.     

The prediction of mineral solubilities in natural waters: The Na-K-Mg-Ca-H-Cl-SO4-OH-HCO3-CO3-CO2-H2O system to high ionic strengths at 25°C

The mineral solubility model of Harvie and Weare (1980) is extended to the eight component system, Na-K-Mg-Ca-H-Cl-SO₄-OH-HCO₃-CO₃-CO₂-H₂O at 25°C to high concentrations. The model is based on the semi-empirical equations of Pitzer (1973) and co-workers for the thermodynamics of aqueous electrolyte solutions. The model is parameterized using many of the available isopiestic, electromotive force, and solubility data available for many of the subsystems. The predictive abilities of the model are demonstrated by comparison to experimental data in systems more complex than those used in parameterization. The essential features of a chemical model for aqueous electrolyte solutions and the relationship between pH and the equilibrium properties of a solution are discussed.     

The prediction of mineral solubilities in natural waters: the NaKMgCaClSO4H2O system from zero to high concentration at 25° C

A chemical model of the seawater system, NaKMgCaClSO₄H₂O, is developed for predicting mineral solubilities in brines from zero to high ionic strengths. The calculated solubilities are shown to be in agreement with the experimental data from gypsum saturation (I < 0.06 m) to bischofite saturation (I > 20 m). The model utilizes activity coefficient expressions recently developed by Pitzer and co-workers and an algorithm for rapidly identifying the coexisting phases and their composition at equilibrium. The activity coefficient expressions are parameterized using binary and ternary system solubility and osmotic data. The results indicate that a free energy model defined by binary and ternary system data will accurately predict solubilities in more complex systems. The algorithm for solving the general chemical equilibrium problem is briefly discussed. The method can be used to model systems with an arbitrary number of possible non-ideal solution phases. The iterative procedure is guaranteed to converge and has been found to be efficient and easy to implement.Calculated phase diagrams associated with the seawater system are compared to experimental data. Our calculations are within experimental accuracy whereas the prediction of other seawater models are in substantial disagreement with the data even at low concentration. The calculation of evaporation sequences is also briefly discussed and qualitatively compared to field data. The mineral assemblages predicted by this method are in substantially better agreement with core samples than the sequences predicted by phase diagram methods (Braitsch, 1971), which do not explicitly include the Ca component.