The copper ions are distributed in a complicated manner over interstitial sites with both trigonal as well as distorted tetrahedral coordination and are rather mobile.
Their crystal structures usually consist of alternating hexagonal layers of monosulfide and disulfide anions with Cu cations in trigonal and tetrahedral interstices.
Although many textbooks (e.g.[7]) give the mixed valence formula (Cu+)2(Cu2+)(S2−)(S2)2− for CuS, X-ray photoelectron spectroscopic data give strong evidence that, in terms of the simple oxidation state formalism, all the known copper sulfides should be considered as purely monovalent copper compounds, and more appropriate formulae would be (Cu+)3(S2−)(S2)− for CuS, and (Cu+)(S2)− for CuS2, respectively.
[13] This apparent discrepancy with the X-ray photo-electron spectrum data simply highlights the problem that NMR has in assigning oxidation states in a mixed-valence compound.
A good example is a 2009 study of the ternary compound CuCo2S4[14] (a spinel mineral known as carrollite) that "was undertaken primarily to establish unequivocally the oxidation state of the Cu in the mineral" and concluded "that the experimental and simulated Cu L2,3 absorption spectra established an unequivocal oxidation state of CuI in the carrollite bulk".