Abstract:
The formation constants of neodymium complexes in chloride solutions have been determined spectrophotometrically at temperatures of 25 to 250°C and a pressure of 50 bars. The simple ion, Nd3+, is dominant at 25°C, whereas NdCl2+ and NdCl2+ are the dominant species at elevated temperatures. Equilibrium constants were calculated for the following reactions:Nd3+ + Cl− = NdCl2+ β1,Nd3+ + 2 · Cl− = NdCl+2 β2.The values of β1 were found to be identical within experimental error to the values reported by Gammons et al. (1996) but substantially different from those proposed by Stepanchikova and Kolonin (1999). The values of β2 obtained in this study agree relatively well with those of Gammons et al. (1996); differences are greatest at intermediate temperature and reach a maximum of one half an order of magnitude at 200°C.Theoretical estimates of β1 and β2 by Haas et al. (1995) using the revised Helgeson-Kirkham-Flowers (HKF) equation of state predict lower stability of NdCl2+ and NdCl2+ at temperatures above 150°C than determined in this study. A new fit to the HKF equation of state is therefore proposed, which yields values for β1 and β2 similar to those obtained experimentally.Using the formation constants reported in this study, we predict that typical seafloor hydrothermal vent fluids will contain a maximum concentration of Nd of ∼2 ppb. This value is several orders of magnitude lower than would be required to explain the levels of Nd mobility commonly reported for seafloor hydrothermal systems and suggests that other ligands may be more important than Cl in transporting rare earth elements in the Earth’s crust.
