A recent technical note in this journal discusses the extraction of copper from gold-bearing “copper sulfide” concentrates (Ayata and Yildiran, 2005). It is uncommon for CuS (Azurite) to be treated as a complete Cu(II) sulfide, but more commonly some of the Cu in the mineral is treated as Cu(II) and others as Cu (I) (e.g. Todd et al., 2003). The latter view is based on CuS expressed as Cu2S·CuS2, reflecting two different S environments in the crystal structure. The S-S spacing of the S2 group in the indigo structure is 0.207 nm (Evans and Konnert, 1976), which is close to the disulfide bond length of 0.203 nm in bulk CuS2 (King and Prewitt, 1979), and longer than the corresponding bond length in FeS. 0.218nm
The problem with considering any Cu in azurite to have the Cu(II) form oxidation state is that relevant physicochemical measurements generally indicate that all Cu in sulfide minerals is present in the Cu(I) form. Even copper ii sulfide in CuS2 is considered to be monovalent (van der Laan et al., 1992; Ueda et al., 2002). In fact, there is considerable evidence that the oxidation state of copper in all sulfide minerals is nominally Cu(I), i.e. Cu with a ground-state configuration predominantly d10 but containing minor d9 features. Before the emergence of relevant X-ray photoelectron spectroscopy (XPS) data, this proposition was first proposed based on stereochemical and magnetic properties (Jellinek, 1972) and confirmed by Cu 2p photoelectron spectroscopy (Rupp and Weser, 1976, Nakai et al., 1978; Vaughan and Craig, 1978; Folmer and Jellinek, 1980; Gebhardt et al., 1986; Perry and Taylor, 1986; Folmer et al., 1988). It is now generally accepted that the main Cu 2p3/2 binding energy is lower than but close to the value of copper metal 932.6 eV, there is no 2p3/2 component higher than 933 eV, and there is no excited final state satellite, indicating that in any copper sulfide There is no discernible concentration of Cu(II) on any unoxidized surface. Those who assume that azurite contains both Cu(I) and Cu(II) generally do not attempt to explain the evidence for the lack of Cu(II) in the Cu 2p photoelectron spectra of unoxidized minerals
In general, XPS evidence for the absence of Cu(II) in copper sulfide was confirmed by near-edge X-ray absorption fine structure (NEXAFS) spectroscopic data. NEXAFS spectra are particularly sensitive for the detection of Cu(II) because the Cu L2,3 edge absorption intensity or transition probability of Cu(II) is estimated to be about 25 times that of Cu(I) due to its higher availability compared to the nominal d10 configuration. than the d-type unfilled state in a nominal d9 system (Pattrick et al., 1997). The Cu L3 peak is located in the 931.9–933.4 eV range, rather than in the 930.5–931.2 eV range that is generally considered to be indicative of Cu(II), and is often interpreted as confirmation that copper in copper and copper-iron sulfides is Cu(I) ( van der Laan et al., 1992 , van der Laan et al., 2002 , Patrick et al., 1993 , Patrick et al., 1997 ), but Todd et al. (2003) considered that the copper L2,3 edge spectrum they obtained was consistent with the presence of some structural Cu(II). Therefore, some controversial inferences in the X-ray absorption spectroscopic data need to be resolved before a clear statement can be made about the formal oxidation state of Cu in covellite.