Coordination sites of metals in tetrahedrite minerals determined by EXAFS

Abstract

Synchrotron radiation has been used to collect Cu, Ag, Fe, Cd, and Sb K-edge EXAFS data from a series of naturally occurring and synthetic tetrahedrite, (Cu, Ag) 10(Cu, Fe, Zn, Cd) 2Sb 4S 13, minerals. Analysis of the spectra has provided direct evidence about the coordination environment of the various metals, and revealed which sites in the structure each will preferentially occupy. The results complement crystallographic data by giving information about each element in sites occupied by more than one cation, whereas crystallography can only reveal the average geometry of each site. Data from the copper and silver EXAFS have demonstrated that the silver goes into trigonal rather than tetrahedral coordination sites, and in the case of one silver-rich sample a close silver-antimony contact has been identified which provides evidence about the nature of a compression of the structure which is found in this sample. In synthetic samples the iron mainly occupies tetrahedral sites, but in natural tetrahedrites of low silver content, can also go into the trigonal site. The cadmium is tetrahedrally coordinated, and the antimony has threefold tetrahedral geometry with a vacant site, although in the silver-rich sample the antimony-silver interaction is also observed. The results have shown the value of multi-element EXAFS spectroscopy in the investigation of a chemically and structurally complex mineralogical system.