The aqueous geochemistry of Zr and the solubility of some Zr-bearing minerals

Literature data on the thermodynamics of complexation of Zr with inorganic species, at 25°C, have been critically reviewed. The preponderance of published complexation constants deal with F⁻ and OH⁻ ions. Stability constants for the complexation reactions are relatively independent of ionic strength and thus recomended values for each ligand type are averages of the most reliable data. Complexation constants under elevated conditions (T 250°C andP_v = P_H2O) have been predicted for various Zr complexes (F⁻, Cl⁻, SO₄²⁻ and OH⁻) using Helgeson's electrostatic approach. Predominance diagrams (calculated for simple systems with these constants) suggest that, over a wide range of pH conditions, Zr(OH)_4(aq) will dominate the aqueous geochemistry of Zr except under very high activities of competing ligands (e.g., F⁻, SO₄²⁻).The solubilities of vlasovite Na₂ZrSi₄O₁₁] and weloganite Sr₃Na₂Zr(CO₃)₆·3H₂O have been measured in KCI solutions (0.5–1.0 M) at 50°C. Weloganite dissolution is complicated by the predictable precipitation of strontianite (SrCO₃) whereas vlasovite dissolves incongruently. Solubility products for the dissolution of welonganite and vlasovite are determined to be −28.96±0.14 and −20.40±1.18, respectively. Concentrations of Zr up to 10⁻³ m were present in the experimental solutions; the presence of large amounts of Zr in aqueous solutions support the possibility of extensive remobilization of Zr during hydrothermal mineralization.