Mossbauer study of bornite and chemical bonding in Fe-bearing sulphides

The Mossbauer spectra of a nearly stoichiometric natural bornite, Cu5FeS4, specimen were reinvestigated between 295 and 4.2 K. There is no difference between the Ne,l temperature T (N) as determined by the Mossbauer effect or by the susceptibility measurements (T (N) = 67.5 K). No additional paramagnetic doublet can be observed in the low-temperature MS spectra. The valence state of Fe is Fe(3-x)+ caused by a partial electron transfer from the Cu+ ions to the Fe3+ ions which increases the shielding of the s-electrons by the d-electron density and by this increases the isomer shift to a value intermediate between tetrahedral high-spin Fe2+ and tetrahedral high-spin Fe3+.