Indium(III) chloride complexing and solvation in hydrothermal solutions to 350°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 saturated 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(ClO4)(H2O)(5)(2+), was detected in which the In3+-chlorine (perchlorate) distance varied from 3.14 to 3.12 Angstrom with increasing temperature to 250 degrees 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) complexes are present in which the In3+-chloride and -oxygen (water) distances 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 2.37 Angstrom. In concentrated 1.00 m InCl3 (no added HCl) solutions, indium-indium interactions (4.10 Angstrom) were also noted together with enhanced In3+-oxygen distance contraction (i.e., 2.18 to 2.10 Angstrom) in the temperature 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 OH- ligand. The data demonstrate the lack of contraction of the octahedral hydration shell of the aquated In3+ ion up to 300 degrees C as well as the predominance of the tetrahedrally coordinated tetrachloroindium(III) species in chloride solutions at 350 degrees C at the saturated vapour pressure. Indium chloride and hydroxy chloride complexes will play an important role in indium transport by hydrothermal fluids in the earth's crust. (C) 2000 Elsevier Science B.V. All rights reserved.