The solubility of the assemblage anorthite + andalusite + quartz and Ca
speciation were investigated in supercritical chloride solutions between
400 and 600 degrees C at 2 kb over a total chloride range of 0.005-5.6
m. Species interpretation is based on systematic regression of multiple
speciation schemes by a nonlinear, weighted least squares procedure.
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 the
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
determined at 600 degrees C to be -1.317 (+/-0.15).
The formation constants for Ca-chloride species determined in the
present study are in close agreement with those retrieved by Baumgartner
(1991) from the wollastonite + quartz solubility data of Popp and Frantz
(1979), which were obtained over a smaller range of total chloride
concentration. Prediction of the dissociation constant for the
portlandite hydrolysis reaction was made based on the present anorthite
solubility data and is in excellent agreement with the portlandite
solubility data of Walther (1986). Results of calculations on the
solubility of the assemblage plagioclase + muscovite + andalusite +
quartz indicate that the dominant Ca species are either Ca(OH)(+) or
CaCl3- depending on total chloride concentration. With the formation of
CaCl3-, increased solubility of calcium in saline fluids is predicted,
relative to Na and K.