THE AQUEOUS GEOCHEMISTRY OF ZR AND THE SOLUBILITY OF SOME ZR-BEARING MINERALS

Literature data on the thermodynamics of complexation of Zr with inorg anic species, at 25 degrees C, have been critically reviewed. The prep onderance of published complexation constants deal with F- and OH- ion s. Stability constants for the complexation reactions are relatively i ndependent of ionic strength and thus recomended values for each ligan d type are averages of the most reliable data. Complexation constants under elevated conditions (T less than or equal to 250 degrees C and P -v = P-H2O) have been predicted for various Zr complexes (F-,Cl-,SO42- and OH-) using Helgeson's electrostatic approach. Predominance diagra ms (calculated for simple systems with these constants) suggest that, over a wide range of pH conditions, Zr(OH)(4(aq)) will dominate the aq ueous geochemistry of Zr except under very high activities of competin g ligands (e.g., F-, SO42-). The solubilities of vlasovite [Na2ZrSi4O1 1] and weloganite [Sr3Na2Zr(CO3)(6).3H(2)O] have been measured in KCl solutions (0.5-1.0 M) at 50 degrees C. Weloganite dissolution is compl icated by the predictable precipitation of strontianite (SrCO3) wherea s vlasovite dissolves incongruently. Solubility products for the disso lution of weloganite and vlasovite are determined to be -28.96 +/- 0.1 4 and -20.40 +/- 1.18, respectively. Concentrations of Zr up to 10(-3) m were present in the experimental solutions; the presence of large a mounts of Zr in aqueous solutions support the possibility of extensive remobilization of Zr during hydrothermal mineralization.