An experimental study of the solubility and speciation of neodymium (III) fluoride in F-bearing aqueous solutions

The solubility of neodymiurn (III) fluoride was investigated at temperatures of 150, 200 and 250 degrees C, saturated water vapor pressure, and a total fluoride concentration (HFaqo + F-) ranging from 2.0 x 10(-3) to 0.23 mol/l. The results of the experiments show that Nd3+ and NdF2+ are the dominant species in solution at the temperatures investigated and were used to derive formation constants for NdF2+ and a solubility product for NdF3. The solubility product of NdF3 (log K-sp = log a(ND3+) + 3 log a(F-)) is -24.4 +/- 0.2, -22.8 +/- 0.1, and -21.5 +/- 0.2 at 250, 200 and 150 degrees C, respectively, and the formation constant of NdF2+ (log beta = log a(Nd2+) - log a(Nd3+) - log a(F-)) is 6.8 +/- 0.1, 6.2 +/- 0.1, and 5.5 +/- 0.2 at 250, 200 and 150 degrees C, respectively. The results of this study show that published theoretical predictions significantly overestimate the stability of NdF2+ and the solubility of NdF3. The potential impact of the results on natural systems was evaluated for a hypothetical fluid with a composition similar to that responsible for REE mineralization in the Capitan pluton, New Mexico. In contrast to results obtained using the theoretical predictions of Haas [Haas J. R., Shock E. L., and Sassani D. C. (1995) Rare earth elements in hydrothermal systems: estimates of standard partial molal thermodynamic properties of aqueous complexes of the rare earth elements at high pressures and temperatures. Geochim. Cosmochim. Acta 59, 4329-4350.], which indicate that NdF2+ is the dominant species in solution, calculations employing the data presented in this paper and previously published experimental data for chloride and sulfate species [Migdisov A. A., and Williams-Jones A. E. (2002) A spectrophotometric study of neodymium(III) complexation in chloride solutions. Geochim. Cosmochim. Acta 66, 4311-4323; Migdisov A. A., Reukov V. V., and Williams-Jones A. E. (2006) A spectrophotometric study of neodymium(III) complexation in sulfate solutions at elevated temperatures. Geochim. Cosmochim. Acta 70, 983-992.] show that neodymium chloride species predominate and that neodymium fluoride species are relatively unimportant. This suggests that accepted models for REE deposits that invoke fluoride complexation as the method of hydrothermal REE transport may need to be re-evaluated. (C) 2007 Elsevier Ltd. All rights reserved.