In many geological settings chlorine is the most common ligand of importance for the mobility of metals. Modelling of the transport of metals in these fluids requires knowledge of the structure and coordination of aqueous complexes as well as thermodynamic data of metal-ligand stability constants. EXAFS spectra provides direct evidence of the radial distribution of atoms around a metal centre (Brown, 1992; Binsted et al., 1990). Previous EXAFS studies of geochemical importance include aqueous solutions of AuCl3, Zn-NaHS, CdCl2, AgNO3, AgClO4 and SrCl2 (Farges et al., 1993; Helz et al., 1993; Seward et al., 1993; Seward et al., 1994). This study provides in situ evidence for complexes present in chloride-bearing solutions of yttrium, tin, and antimony
to 340°C. The X-ray adsorption fine structure adsorption spectroscopy (EXAFS) of 0.1 M yttrium-chloride solutions, 0.01-0.5M tin-chloride solutions, and 0.01-1M antimony-chloride solutions were measured at the Daresbury Synchrotron Laboratory, U.K. at temperatures from 25 to 340°C using transmission and fluorescence detection. NaCl, KCl, and/or HCl were added to these solutions to assess the effect of chloride concentration.
Analysis of the EXAFS spectra indicate that aqueous yttrium is present as Y3+ for all temperatures and solution compositions investigated. This aqueous species is surrounded by an inner shell of 8 H2O. Y-chloride complex formation is negligible even at NaCl concentrations as high as 1 molal. The lack of substantial chloride complexing in high temperature NaCl/HCl bearing solutions strongly implies that aqueous Y-Cl complexes, and by analogy heavy REE-Cl complexes, play a negligible role in the transport and solubility of these metals in crustal processes at sub-
critical temperatures. Analysis of EXAFS spectra of dilute tin-chloride solutions (0.01M) suggests that tin has an average of 0.5 Cl in its inner shell. In 0.5M Cl solutions SnCl+ predominates and in concentrated Cl solutions (>2M) SnCl3- dominates. In contrast analysis of EXAFS spectra of
antimony-chloride solutions indicate that aqueous antimony is present as an oxychloride species of the form of SbOCln. These results confirm experimental solubility and thermodynamic modelling results which suggested that Cl is a major transporting agent of Sn (Wilson and Eugster, 1990; Jackson and Helgeson, 1985) and Sb (Wood et al., 1987) in hot, acidic solutions.
Binsted, N., Campbell, J.W., Gurmann, S.J. & Stephenson, P.C., SERC Daresbury Laboratory EXCURV90 Program. (Daresbury Laboratory, Warrington, UK, 1990).
Brown, R.G., Spectro. Eur. 4, 10-20 (1992).
Farges, F., Sharps, J.A. & Brown, G.E., Geochim. Cosmochim. Acta 57, 1243-1252 (1993).
Helz, G.R., Charnock, J.M., Vaughan, D.J. & Garner, C.D., Geochim. Cosmochim. Acta 57, 15-25 (1993).
Jackson, K.J. & Helgeson, H.C., Geochim. Cosmochim. Acta 49, 1-22 (1985).
Seward, T.M., Henderson, C.M., Charnock, J.M. & Dobson, B.R., In Proc. 4th Internat. Symp. on Hydrothermal Reactions (Cuney, M. & Cathelineau, eds), 231-233 (1993).
Seward, T.M., Henderson, C.M., Charnock, J.M. & Dobson, B.R., Min. Mag. 58A, 819-820 (1994).
Seward, T.M., Henderson, C.M., Charnock, J.M. & Dobson, B.R., In Water-Rock Interaction-9 (Kharaka, Y.K. & Chudaev, O.V., eds.) 43-46 (Balkema, Rotterdam, 1995).
Wilson, G.A. & Eugster, H.P., In Fluid-mineral interactions (Spencer, R.J. & Chou, I-Ming, eds.). The Geochem. Soc. Spec. Pub. 2 (1990).
Wood, S.A., Crerar, D.A. & Borcsik, M.P., Econ. Geol. 82, 1864-1887 (1987).