DETERMINATION OF THE LOCAL-STRUCTURE AND SPECIATION OF ZINC IN INDIVIDUAL HYPERSALINE FLUID INCLUSIONS BY MICRO-XAFS

X-ray absorption fine structure (XAFS) spectra were obtained for the f irst time from metal complexes in individual fluid inclusions in quart z using the synchrotron X-ray microprobe (X26A) at the National Synchr otron Light Source (NSLS), Brookhaven National Laboratory. Analysis of Zn K-edge absorption spectra obtained from hypersaline fluid inclusio ns in quartz indicate that the dominant complex of zinc in these inclu sions at room temperature is ZnCl42-, having a Zn-Cl bond length of 2. 33 Angstrom, coordination number of 4.2 atoms and a mean-square relati ve displacement of the atoms of 0.004 Angstrom(2). XAFS spectra obtain ed from zinc chloride test solutions having a Cl/Zn ratio of 8 closely match the spectra derived from the inclusions. The presence of ZnCl42 - is consistent with results of previous spectroscopic studies of zinc chloride solutions having high Cl/Zn ratios and with solubility studi es of zinc compounds in chloride solutions. Our results demonstrate th e potential of micro-XAFS as a tool for direct measurement of the stru cture of aqueous metallic species in fluid inclusions. The sensitivity of the method will improve as brighter and more focussed X-ray beams become available.