Editing Covellite (section)

==Composition==
Covellite belongs to the binary copper sulfides group, which has the formula Cu<sub>x</sub>S<sub>y</sub> and can have a wide-ranging copper/sulfur ratio, from 1:2 to 2:1 (Cu/S). However, this series is by no means continuous and the homogeneity range of covellite CuS is narrow. Materials rich in sulfur CuS<sub>x</sub> where x~ 1.1- 1.2 do exist, but they exhibit "[[Superstructure (condensed matter)|superstructures]]", a modulation of the hexagonal ground plane of the structure spanning a number of adjacent unit cells.<ref>{{Cite journal|last1=Putnis|first1=A.|last2=Grace|first2=J.|last3=Cameron|first3=W. E.|date=1977|title=Blaubleibender covellite and its relationship to normal covellite|journal=Contributions to Mineralogy and Petrology|volume=60|issue=2|pages=209–217|doi=10.1007/bf00372282|bibcode=1977CoMP...60..209P|s2cid=95661500|issn=0010-7999}}</ref> This indicates that several of covellite's special properties are the result of molecular structure at this level.

As described for [[copper monosulfide]], the assignment of formal [[Oxidation state|oxidation states]] to the atoms that constitute covellite is deceptive.<ref name=":8">{{Cite journal|last1=Evans|first1=Howard T.|last2=Konnert|first2=Judith A.|date=1976|title=Crystal structure refinement of covellite|journal=American Mineralogist|volume=61|pages=996–1000}}</ref> The formula might seem to suggest the description Cu<sup>2+</sup>, S<sup>2−</sup>. In fact the [[atomic structure]] shows that copper and sulfur each adopt two different geometries. However [[photoelectron spectroscopy]], [[Magnetism|magnetic]], and [[electrical]] properties all indicate the ''absence'' of Cu<sup>2+</sup> (d<sup>9</sup>) ions.<ref name=":8" /> In contrast to the oxide CuO, the material is not a magnetic [[semiconductor]] but a metallic conductor with weak [[Pauli paramagnetism|Pauli-paramagnetism]].<ref name=":9">{{Cite book|title=Synthesis, properties and mineralogy of important inorganic materials|last=Warner, Terence E.|date=2013|publisher=Wiley|isbn=9780470976234|oclc=865009780}}</ref> Thus, the mineral is better described as consisting of Cu<sup>+</sup> and S<sup>−</sup> rather than Cu<sup>2+</sup> and S<sup>2−</sup>. Compared to pyrite with a non-closed shell of S<sup>−</sup> pairing to form S<sub>2</sub><sup>2−</sup>, there are only 2/3 of the sulfur atoms held.<ref name=":8" /> The other 1/3 remains unpaired and together with Cu atoms forms hexagonal layers reminiscent of the boron nitride (graphite structure).<ref name=":8" /> Thus, a description Cu<sup>+</sup><sub>3</sub>S<sup>−</sup>S<sub>2</sub><sup>2−</sup> would seem appropriate with a delocalized hole in the [[valence band]] leading to metallic conductivity. Subsequent band structure calculations indicate however that the hole is more localized on the sulfur pairs than on the unpaired sulfur. This means that Cu<sup>+</sup><sub>3</sub>S<sup>2−</sup>S<sub>2</sub><sup>−</sup> with a mixed sulfur oxidation state −2 and −1/2 is more appropriate. Despite the extended formula of Cu<sup>+</sup><sub>3</sub>S<sup>2−</sup>S<sub>2</sub><sup>−</sup> from researchers in 1976 and 1993, others have come up with variations, such as Cu<sup>+</sup><sub>4</sub>Cu<sup>2+</sup><sub>2</sub>(S<sub>2</sub>)<sub>2</sub>S<sub>2</sub>.<ref>{{Cite book|title=The relationship between crystal structure, bonding and cell dimensions in the copper sulfides : supplementary unpublished material|last=Goble, Ronald J.|date=1985|oclc=45557917}}</ref><ref name=":3">{{Cite journal|last1=Liang|first1=W.|last2=Whangbo|first2=M.-H.|date=February 1993|title=Conductivity anisotropy and structural phase transition in Covellite CuS|journal=Solid State Communications|volume=85|issue=5|pages=405–408|doi=10.1016/0038-1098(93)90689-k|issn=0038-1098|bibcode=1993SSCom..85..405L}}</ref>