An experimental study of the solubility of baddeleyite (ZrO2) in fluoride-bearing solutions at elevated temperature

The solubility of baddeleyite (ZrO2) and the speciation of zirconium have been investigated in HF-bearing aqueous solutions at temperatures up to 400 degrees C and pressures up to 700 bar. The data obtained suggest that in HF-bearing solutions zirconium is transported mainly in the form of the hydroxyfluoride species ZrF(OH)(3)degrees and ZrF2(OH)(2)degrees. Formation constants determined for these species (Zr4+ + nF(-) + mOH(-) = ZrFn(OH)(m)degrees) range from 43.7 at 100 degrees C to 46.41 at 400 degrees C for ZrF(OH)(3)degrees, and from 37.25 at 100 degrees C to 43.88 at 400 degrees C for ZrF2(OH)(2)degrees. Although the solubility of ZrO2 is retrograde with respect to temperature, the measured concentrations of Zr are orders of magnitude higher than those predicted from theoretical extrapolations based on simple fluoride species (ZrF3+-ZrF62-). Model calculations performed for zircon show that zirconium can be transported by aqueous fluids in concentrations sufficient to account for the concentration of this metal at conditions commonly encountered in fluoride-rich natural hydrothermal systems. (C) 2011 Elsevier Ltd. All rights reserved.