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Cu L3 X-ray absorption spectroscopy and the electronic structure of minerals: Spectral variations in non-stoichiometric bornites, Cu₅FeS₄

¹ Department of Earth Sciences and Williamson Research Centre, University of Manchester, Manchester, M13 9PL, U.K.
² Magnetic Spectroscopy, Daresbury Laboratory, Warrington WA4 4AD, U.K.
³ X-ray Absorption Spectroscopy, Daresbury Laboratory, Warrington WA4 4AD, U.K.
⁴ WMC Mount Keith Operations, P.O. Box 238, Welshpool Delivery Centre, W.A. 6986, Australia

Correspondence: ^* E-mail: richard.pattrick{at}man.ac.uk

Synthetic bornites with compositional variations within 2 at% of the stoichiometric formula of Cu₅FeS₄ show dramatic changes in their Cu L_2,3 absorption spectra, determined using synchrotron radiation. Many of the bornites show L₃ spectra with a strong peak at 932.6 eV and a broader region of intensity in the region 933 eV to 940 eV with specific features at 935.6 eV and 938 eV. However, the intensity of the peak at 932.6 eV varies greatly, and there is a clear correlation between the loss of intensity of this peak and the gain of intensity of the peak at 935.6 eV. EPMA reveals that this change in relative intensities is directly correlated to the excess of valence electrons present in the bornites, calculated the using formal valences of Cu⁺, Fe³⁺ and S^2– [The Cu L_3,2 spectra are typical of Cu(I) sulfides and Fe L₃ edge spectra reveal the Fe to be present as Fe³⁺]. A simple cluster model can explain this transfer in intensity among the three different peaks. The three peaks in the bornite XAS 2p spectrum can be assigned (in order of increasing energy) to the final states cd¹⁰Ls², cd¹⁰s, and cd⁹s²; the transition to an empty s state is possible for all three configurations, d¹⁰Ls cd¹⁰Ls², d¹⁰ cd¹⁰s, and d⁹s cd⁹s². When the amount of excess electrons increases, the holes are filled and the weight of the d¹⁰Ls and d⁹s configurations will decrease, whereas that of the d¹⁰ configuration will increase, exactly as observed in the spectra. This study demonstrates the value of metal L-edge spectroscopy, and Cu L_2,3 spectra in particular, in investigations of minerals with variations in stoichiometry and chemistry; the information derived on electronic structure can be used to explain bonding and other variations in physical and chemical properties of the minerals.