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Copper sulfides describe a family of
chemical compounds A chemical compound is a chemical substance composed of many identical molecules (or molecular entities) containing atoms from more than one chemical element held together by chemical bonds. A molecule consisting of atoms of only one element ...
and minerals with the formula CuxSy. Both
mineral In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid chemical compound with a fairly well-defined chemical composition and a specific crystal structure that occurs naturally in pure form.John P. Rafferty, ed. (2 ...
s and synthetic materials comprise these compounds. Some
copper Copper is a chemical element with the symbol Cu (from la, cuprum) and atomic number 29. It is a soft, malleable, and ductile metal with very high thermal and electrical conductivity. A freshly exposed surface of pure copper has a pink ...
sulfides are economically important ores. Prominent copper sulfide minerals include Cu2S (
chalcocite Chalcocite (), copper(I) sulfide (Cu2S), is an important copper ore mineral. It is opaque and dark gray to black, with a metallic luster. It has a hardness of 2.5–3 on the Mohs scale. It is a sulfide with a monoclinic crystal system. ...
) and CuS ( covellite). In the mining industry, the minerals
bornite Bornite, also known as peacock ore, is a sulfide mineral with chemical composition Cu5 Fe S4 that crystallizes in the orthorhombic system (pseudo-cubic). Appearance Bornite has a brown to copper-red color on fresh surfaces that tarnishes to v ...
or
chalcopyrite Chalcopyrite ( ) is a copper iron sulfide mineral and the most abundant copper ore mineral. It has the chemical formula CuFeS2 and crystallizes in the tetragonal system. It has a brassy to golden yellow color and a hardness of 3.5 to 4 on the Mo ...
, which consist of mixed copper-iron sulfides, are often referred to as "copper sulfides". In
chemistry Chemistry is the scientific study of the properties and behavior of matter. It is a natural science that covers the elements that make up matter to the compounds made of atoms, molecules and ions: their composition, structure, proper ...
, a "binary copper sulfide" is any binary
chemical compound A chemical compound is a chemical substance composed of many identical molecules (or molecular entities) containing atoms from more than one chemical element held together by chemical bonds. A molecule consisting of atoms of only one element ...
of the elements copper and
sulfur Sulfur (or sulphur in British English) is a chemical element with the symbol S and atomic number 16. It is abundant, multivalent and nonmetallic. Under normal conditions, sulfur atoms form cyclic octatomic molecules with a chemical formul ...
. Whatever their source, copper sulfides vary widely in composition with 0.5 ≤ Cu/S ≤ 2, including numerous
non-stoichiometric compound In chemistry, non-stoichiometric compounds are chemical compounds, almost always solid inorganic compounds, having elemental composition whose proportions cannot be represented by a ratio of small natural numbers (i.e. an empirical formula); m ...
s.


Known copper sulfides

The naturally occurring mineral binary compounds of copper and sulfur are listed below. Investigations of covellite ( CuS) indicate that there are other metastable Cu-S phases still to be fully characterised. * CuS2, villamaniniteWells A.F. (1984) ''Structural Inorganic Chemistry'' 5th edition Oxford Science Publications or * CuS, covellite, copper monosulfide * Cu9S8 (Cu1.12S), yarrowiteCopper sulfides from Alberta; yarrowite Cu9S8 and spionkopite Cu39S28 R. J. Goble, The Canadian Mineralogist; (1980); 18; 4; 511-518 * Cu39S28 (Cu1.39S) spionkopite * Cu8S5 (Cu1.6S),
geerite Geerite is a copper sulfide mineral with the chemical formula Cu8S5. The mineral is named after the original collector, Adam Geer, of Utica, New York, US. Crystallography Geerite is in the crystal class . This means that the crystal could be ...
* Cu7S4 (Cu1.75S), anilite * Cu9S5 (Cu1.8S),
digenite Digenite is a copper sulfide mineral with formula: Cu9S5. Digenite is a black to dark blue opaque mineral that crystallizes with a trigonal - hexagonal scalenohedral structure. In habit it is usually massive, but does often show pseudo-cubic f ...
* Cu58S32 (Cu1.8S), roxbyite *Cu31S16 (Cu1.96S), djurleite *Cu2S,
chalcocite Chalcocite (), copper(I) sulfide (Cu2S), is an important copper ore mineral. It is opaque and dark gray to black, with a metallic luster. It has a hardness of 2.5–3 on the Mohs scale. It is a sulfide with a monoclinic crystal system. ...


Classes of copper sulfides

Copper sulfides can be classified into three groups: Monosulfides, 1.6 ≤ Cu/S ≤ 2: their
crystal structure In crystallography, crystal structure is a description of the ordered arrangement of atoms, ions or molecules in a crystalline material. Ordered structures occur from the intrinsic nature of the constituent particles to form symmetric pattern ...
s consist of isolated sulfide anions that are closely related to either hcp or fcc lattices, without any direct S-S bonds. The copper ions are distributed in a complicated manner over
interstitial site In crystallography, interstitial sites, holes or voids are the empty space that exists between the packing of atoms (spheres) in the crystal structure. The holes are easy to see if you try to pack circles together; no matter how close you get ...
s with both trigonal as well as distorted tetrahedral coordination and are rather mobile. Therefore, this group of copper sulfides shows ionic conductivity at slightly elevated temperatures. In addition, the majority of its members are
semiconductor A semiconductor is a material which has an electrical conductivity value falling between that of a conductor, such as copper, and an insulator, such as glass. Its resistivity falls as its temperature rises; metals behave in the opposite way ...
s. Mixed monosulfide and disulfide compounds of copper contain both monosulfide (S2−) as well as
disulfide In biochemistry, a disulfide (or disulphide in British English) refers to a functional group with the structure . The linkage is also called an SS-bond or sometimes a disulfide bridge and is usually derived by the coupling of two thiol groups. In ...
(S2)n− anions. Their crystal structures usually consist of alternating hexagonal layers of monosulfide and disulfide anions with Cu cations in trigonal and tetrahedral interstices. CuS, for example, can be written as Cu3(S2)S. Several nonstoichiometric compounds with Cu:S ratios between 1.0 and 1.4 also contain both monosulfide as well as disulfide ions. Depending on their composition, these copper sulfides are either
semiconductors A semiconductor is a material which has an electrical conductivity value falling between that of a conductor, such as copper, and an insulator, such as glass. Its resistivity falls as its temperature rises; metals behave in the opposite way. ...
or metallic conductors. At very high pressures, a copper disulfide, CuS2, can be synthesized. Its crystal structure is analogous to that of
pyrite The mineral pyrite (), or iron pyrite, also known as fool's gold, is an iron sulfide with the chemical formula Iron, FeSulfur, S2 (iron (II) disulfide). Pyrite is the most abundant sulfide mineral. Pyrite's metallic Luster (mineralogy), lust ...
, with all sulfur atoms occurring as S-S units. Copper disulfide is a metallic conductor due to the incomplete occupancy of the sulfur p band. Different stoichiometric compositions can be obtained by changing the redox atmosphere of the synthetic environment.


Oxidation states of copper and sulfur

The bonding in copper sulfides cannot be correctly described in terms of a simple oxidation state formalism because the Cu-S bonds are somewhat
covalent A covalent bond is a chemical bond that involves the sharing of electrons to form electron pairs between atoms. These electron pairs are known as shared pairs or bonding pairs. The stable balance of attractive and repulsive forces between atom ...
rather than ionic in character, and have a high degree of
delocalization In chemistry, delocalized electrons are electrons in a molecule, ion or solid metal that are not associated with a single atom or a covalent bond.IUPAC Gold Boo''delocalization''/ref> The term delocalization is general and can have slightly dif ...
resulting in complicated
electronic band structure In solid-state physics, the electronic band structure (or simply band structure) of a solid describes the range of energy levels that electrons may have within it, as well as the ranges of energy that they may not have (called '' band gaps'' or ...
s. Although many textbooks (e.g.) 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. Further evidence that the assignment of the so-called "valence hole" should be to the S2 units in these two formulae is the length of the S-S bonds, which are significantly shorter in CuS (0.207 nm) and CuS2 (0.203 nm) than in the "classical" disulfide Fe2+(S2)2− (0.218 nm). This bond length difference has been ascribed to the higher bond order in (S-S) compared to (S-S)2− due to electrons being removed from a π*
antibonding In chemical bonding theory, an antibonding orbital is a type of molecular orbital that weakens the chemical bond between two atoms and helps to raise the energy of the molecule relative to the separated atoms. Such an orbital has one or more no ...
orbital. NMR studies on CuS show that there are two distinct species of copper atom, one with a more metallic nature than the other. 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 Inner sphere electron transfer (IS ET) or bonded electron transfer is a redox chemical reaction that proceeds via a covalent linkage—a strong electronic interaction—between the oxidant and the reductant reactants. In inner sphere electron tran ...
compound. The issue of the valence of copper in sulfides (as well as selenides and tellurides) continues to be revisited in the literature. A good example is a 2009 study of the ternary compound CuCo2S4Electronic environments in carrollite, CuCo2S4, determined by soft X-ray photoelectron and absorption spectroscopy
Alan N. Buckley, William M. Skinner, Sarah L. Harmer, Allan Pring and Liang-Jen Fan
Geochimica et Cosmochimica Acta Volume 73, Issue 15, 1 August 2009, Pages 4452-4467
(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".


See also

* Copper(I) sulfide * Copper(II) sulfide


References

*J.C.W. Folmer ''Holes in the valence band of copper chalcogenides'' Thesis 1981 Groningen State University (Neth). {{reflist


External links


Copper sulfides mineral information and data
Sulfide minerals Sulfides Copper compounds