Zr is highly mobile in fluoride-bearing hydrothermal fluids: evidence for stabilityu of ZrF62- at high temperature

It is difficult to study fluoride complex stability by traditional solubility method, because most elements do not form solid fluorides, or they are easily hydrolyzed. To constrain the zirconium mobility in fluoride-bearing hydrothermal fluids, we developed a new method to study the stability of fluoride complexes of metals and metalloids. The method is based on increasing fluorite (CaF₂) solubility with an addition of zirconium in the system. Experimental study of fluorite solubility in acidic solutions in the system CaF₂-HCl-HF-H₂O gives a good agreement with the thermodynamic calculations. This new method is applied to study Zr fluoride complexes. Addition of zirconium in the solution increases solubility of fluorite; and slope of Ca versus Zr contents in the solution does not correspond to Zr(OH)₃F_(aq) nor Zr(OH)₂F_2(aq) complexes, but is in agreement with formation of ZrF₆^2- complex. The temperature dependence of ZrF₆^2- stability is estimated up to 255°C and it is extrapolated to higher temperature (up to 500°C). Thermodynamic calculations show that ZrF₆^2- is a key chemical complex responsible for zircon dissolution and re-crystallization at hydrothermal conditions (see Figure).

We also determined physical-chemical conditions for the maximal solubility of zircon and baddeleyite. The solubility of baddeleyite exceeds 1000 ppm at 0.1 mol/kg fluoride solution. Direct experiments on the solubility of zircon and baddeleyite are carried out at 150 to 400 °C and the predicted solubility values are obtained up to 500°C.