The structural role of Ag in Galena (PbS). A XANES study

A number of Ag-bearing galena samples (PbS) has been investigated by Ag K-edge XAS in order to determine the Ag structural role within the host galena structure. Despite the very low absorber concentration and the high absorbance of the PbS matrix, XANES spectra were successfully collected for samples with Ag content down to 500 ppm. Theoretical spectra have been calculated with the CONTINUUM code in order to discriminate the contributions from Ag located in the two possible sites in the galena structure: the Pb site (located at Wyckoff position 4b, octahedral S coordination, site symmetry m3m), and an interstitial site ( located at Wyckoff position 8c, tetrahedral S plus tetrahedral Pb coordination, site symmetry (4) under bar 3m). Experimental spectrum of diaphorite (a Ag-bearing mineral phase forming nano- to micro inclusions in Ag-rich galena) have been used for comparison. The XANES experimental spectra show two different types of Ag environment: a) the samples with high Ag content display spectra similar to that of diaphorite, as evidenced by the theoretical spectra, suggesting that, when present in considerable amount, most of the Ag partitions into the diaphorite inclusions rather than entering the galena structure; b) trace level Ag-bearing samples display spectra very different from that of diaphorite. In view of the absence of micro- and nano- inclusions in the latter samples (confirmed by electron microscopy), these spectra are taken as representative of the Ag environment in the host galena structure. In these samples the comparison between experimental and theoretical spectra shows that the spectrum can be interpreted as the sum of the two contributions from Ag located in the 8c and 4b positions in the galena structure, in approximately the same proportions. Therefore, the data suggest that in this type of galena sample, half of the Ag occupies the vacant Pb site whereas the remaining part is located in the interstitial 4b site.