ABSTRACT

We report here the first synchrotron radiation (SR)-based X-ray absorption fine structure (XAFS) spectra for Os at L_III-edge in geo- and cosmo-chemical materials. Samples were Negrillos meteorite, a II-A type iron meteorite, and a Pt ore specimen, aggregates of coarse grained Pt-Fe alloys, from the Chocó district, Colombia. X-ray absorption near-edge structure (XANES) spectra of Os are presented for Negrillos, which has an Os abundance of 65 ppm, as well as for the platinum-group mineral (PGM). The XANES spectra showed that Os in the iron meteorite and the PGM chiefly exist as a metallic species. The average interatomic distance of Os in the Pt ore from the Chocó district are calculated from the EXAFS spectra. The results suggest that the neighboring atoms of Os are not Pt. Instead, they are most likely Os and Ir. This is consistent with the common occurrence of fine lamellae of Os and iridosmine as exsolution products of Pt-Fe alloys. Furthermore, this confirms the limited solubility of Os in Pt-Fe alloys. EXAFS could thus be useful for identifying the host phases of Os in natural samples. This study has verified that the chemical state of Os in natural samples can be determined using XAFS. It is expected that further application of the XAFS technique to Os speciation in natural samples will help to elucidate Re/Os fractionation processes in the Re-Os isotopic system.