The structure and stability of aqueous rare-earth elements in hydrothermal fluids: New results on neodymium(III) aqua and chloroaqua complexes in aqueous solutions to 500 °C and 520 MPa

X-ray absorption spectroscopy (XAS) measurements were made at the Nd L-3-edge on neodymium(III) aqua and chloroaqua complexes in low pH aqueous solutions from 25 to 500 degrees C and up to 520 MPa. Analysis of the extended X-ray absorption fine structure of the XAS spectra measured from a 0.07 m Nd/0.16 m HNO3 aqueous solution reveals a contraction of the Nd-O distance of the Nd3+ aqua ion at a uniform rate of similar to 0.013 angstrom/100 degrees C and a uniform reduction of the number of coordinated H2O molecules from 10.0 +/- 0.9 to 7.4 +/- 0.9 over the range from 25 to 500 degrees C and up to 370 MPa. The rate of reduction of the first-shell water molecules with temperature for Nd3+ (26%) is intermediate between the rate for the Gd3+ aqua ion (22% from 25 to 500 degrees C) and the rates for the Eu3+ (29% from 25 to 400 degrees C) and the Yb3+ aqua ions (42% from 25 to 500 degrees C) indicating an intermediate stability of the Nd3+ aqua ion consistent with the tetrad effect. Nd L-3-edge XAS measurements of 0.05 m NdCl3 aqueous solution at 25 to 500 degrees C and up to 520 MPa show that stepwise inner-sphere complexes most likely of the type Nd(H2O)(delta-n)Cl-n(+3-n) occur in the solution at elevated temperatures, where delta approximate to 9 at 150 degrees C decreasing to similar to 6 at 500 degrees C and the number of chloride ions (n) of the chloroaqua complexes increases uniformly with temperature from 1.2 +/- 0.2 to 2.0 +/- 0.2 in the solution upon increase of temperature from 150 to 500 degrees C. Conversely, the number of H2O ligands of Nd(H2O)(delta-n)Cl-n(+3-n) complexes is uniformly reduced with temperature from 7.5 +/- 0.8 to 3.7 +/- 0.3 in the aqueous solution, in the same temperature range. These data show greater stability of neodymium(III) than gadolinium(III) and ytterbium(III) chloride complexes in low pH aqueous solutions at elevated temperatures. Our data suggest a greater stability of aqueous light REE than that of heavy REE chloride complexes in low pH fluids at elevated temperatures consistent with REE analysis of fluids from deep-sea hydrothermal vents. (C) 2008 Elsevier B.V. All rights reserved.