EXPERIMENTAL-DETERMINATION OF ANORTHITE SOLUBILITY AND CALCIUM SPECIATION IN SUPERCRITICAL CHLORIDE SOLUTIONS AT 2 KB FROM 400-DEGREES-C TO600-DEGREES-C

The solubility of the assemblage anorthite + andalusite + quartz and C a speciation were investigated in supercritical chloride solutions bet ween 400 and 600 degrees C at 2 kb over a total chloride range of 0.00 5-5.6 m. Species interpretation is based on systematic regression of m ultiple speciation schemes by a nonlinear, weighted least squares proc edure. Best fits to the data were obtained by including the Ca species CaCl3- and Ca(OH)(+), in addition to the species Ca2+, CaCl+, CaCl20, H+, OH-, Cl-, H2O Al(OH)(3)(0), H4SiO40, and HCl0. The logarithm of t he equilibrium constant for the reaction CaAl2Si2O8 + 2HCl = CaCl20 Al2SiO5 + SiO2 + H2O is given by: log K-3 = -17.13 (+/-0.41) + 16483 ( +/-301)/T(K). For the reaction, Ca(OH)(+) = Ca2+ + OH-: log K-18 = -35 .39 (+/-0.41) + 22558 (+/-283)/T(K) was obtained. The logarithm of the dissociation constant for the reaction CaCl3- = CaCl2 +/- Cl- was det ermined at 600 degrees C to be -1.317 (+/-0.15). The formation constan ts for Ca-chloride species determined in the present study are in clos e agreement with those retrieved by Baumgartner (1991) from the wollas tonite + quartz solubility data of Popp and Frantz (1979), which were obtained over a smaller range of total chloride concentration. Predict ion of the dissociation constant for the portlandite hydrolysis reacti on was made based on the present anorthite solubility data and is in e xcellent agreement with the portlandite solubility data of Walther (19 86). Results of calculations on the solubility of the assemblage plagi oclase + muscovite + andalusite + quartz indicate that the dominant Ca species are either Ca(OH)(+) or CaCl3- depending on total chloride co ncentration. With the formation of CaCl3-, increased solubility of cal cium in saline fluids is predicted, relative to Na and K.