The solubility of fluorite as a function of ionic strength and solution composition at 25°C and 1 atm total pressure

Abstract

The stoichiometric solubility of fluorite was measured in a variety of aqueous solutions, including artificial seawater at 25°C and 1 atm total pressure in order to determine the effect of ionic strength and the potential co-precipitation of seawater constituents on the equilibrium ion concentration product. The ion concentration products (ICP=[Ca 2+][F −] 2) measured after 32 weeks of equilibration converged to constant values. Extrapolation of the results in pure water and simple salt solutions (i.e., Na–Cl) to infinite dilution using the general specific ion interaction theory (SIT) or model estimates of the single ion activity coefficients of F − and Ca 2+, yield an average K sp° of 3.08±0.08×10 −11 (or −log K sp°=10.51±0.01). Application of speciation models to estimate the CaF 2 ion activity coefficient product (i.e., (γ Ca 2+ )(γ F − ) 2) in a high ionic strength Na–Mg–Ca–Cl solution ( I t=0.6951 m) failed to reproduce the values calculated from the ratio of the thermodynamic to the stoichiometric solubility constants (i.e., K sp°/ K sp*) measured in this study. The discrepancy is attributed to the formation of a Mg-bearing CaF 2 solid solution in the Na–Mg–Ca–Cl solution. Despite the close agreement between estimated and measured CaF 2 activity coefficient products in artificial seawater, a more complex (i.e., containing Mg but also Sr and possibly other seawater constituents) CaF 2 solid solution must also be precipitated on the surface of the fluorite in this solution.