Indium(III) chloride complexing and solvation in hydrothermal solutions to350 degrees C: an EXAFS study

X-ray absorption fine structure (EXAFS) measurements on indium(III) nitrate and perchlorate solutions from 25 degrees C to 350 degrees C at the satura ted vapour pressure indicate that the In3+-oxygen (water) distance remains constant at 2.14 (+/-0.01) Angstrom with an octahedral configuration. In 1. 00 m HClO4 solutions, the formation of the In3+-perchlorate ion pair, In(Cl O4)(H2O)(5)(2+), was detected in which the In3+-chlorine (perchlorate) dist ance varied from 3.14 to 3.12 Angstrom with increasing temperature to 250 d egrees C. In 0.10 m HCl solutions at 25 degrees C, octahedrally coordinated InCln(H2O)(6-n)(3-n) (0 less than or equal to n less than or equal to 4) c omplexes are present in which the In3+-chloride and -oxygen (water) distanc es are 2.40 and 2.13 Angstrom, respectively. With increasing temperature to 300 degrees C, the complex geometry becomes predominantly tetrahedral with the formation of InCl4- having an In3+-chloride distance of similar to 2.3 7 Angstrom. In concentrated 1.00 m InCl3 (no added HCl) solutions, indium-i ndium interactions (4.10 Angstrom) were also noted together with enhanced I n3+-oxygen distance contraction (i.e., 2.18 to 2.10 Angstrom) in the temper ature range from 25 degrees C to 300 degrees C due to the loss of two water molecules from the first shell environment, leaving a more tightly bound O H- ligand. The data demonstrate the lack of contraction of the octahedral hydration sh ell of the aquated In3+ ion up to 300 degrees C as well as the predominance of the tetrahedrally coordinated tetrachloroindium(III) species in chlorid e solutions at 350 degrees C at the saturated vapour pressure. Indium chlor ide and hydroxy chloride complexes will play an important role in indium tr ansport by hydrothermal fluids in the earth's crust. (C) 2000 Elsevier Scie nce B.V. All rights reserved.