Ar. Felmy et al., THE SOLUBILITY OF (BA,SR)SO4 PRECIPITATES - THERMODYNAMIC-EQUILIBRIUMAND REACTION-PATH ANALYSIS, Geochimica et cosmochimica acta, 57(18), 1993, pp. 4345-4363
The solubility of (Ba,Sr)SO4 precipitates, varying in SrSO4 mole fract
ion from 0.05-0.90, was investigated at room temperature with an equil
ibration period extending to almost three years. The data show that on
or before 315 days of equilibration the precipitates reach a reversib
le equilibrium with the aqueous solution. The reversibility of this eq
uilibrium was verified both by the attainment of steady-state concentr
ations with time and by heating the samples to perturb the equilibrium
and then observing the slow return to the initial equilibrium state.
The dissolution of the (Ba,Sr)SO4 precipitates does not, in general, f
ollow limiting reaction paths as defined by the Lippmann solutus or st
oichiometric dissolution curves. In addition, activity coefficient cal
culations for the BaSO4 and SrSO4 components of the solid phase, using
either total bulk analysis or near-surface analysis of the component
mole fractions, do not satisfy the Gibbs-Duhem equation, demonstrating
that a single solid-solution phase does not control both the aqueous
Ba and Sr concentrations. Instead, our long-term equilibration data ca
n be explained by the unavoidable formation of small amounts of barite
and substitution of Sr into a solid-solution phase with the BaSO4 com
ponent of the solid-solution phase never reaching thermodynamic equili
brium with the aqueous phase.