Copper sulfide
Encyclopedia
Copper sulfides describe a family of chemical compound
s and minerals with the formula CuxSy. Both mineral
s and synthetic materials comprise these compounds. Some copper sulfides are economically important ore
s.
Prominent copper sulfide minerals include Cu2S (chalcocite
) and CuS (covellite
). In the mining industry, the minerals bornite
or chalcopyrite
, which consist of mixed copper-iron sulfides, are often referred to as "copper sulfides". In chemistry
, a "binary copper sulfide" is any binary chemical compound
of the elements copper and sulfur
. Whatever their source, copper sulfides vary widely in composition with 0.5 ≤ Cu/S ≤ 2, including numerous non-stoichiometric compound
s.
(CuS
) indicate that there are other metastable Cu-S phases still to be fully characterised.
Monosulfides, 1.6 ≤ Cu/S ≤ 2: their crystal structure
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 sites 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
s.
Mixed monosulfide and disulfide compounds of copper contain both monosulfide (S2−) as well as disulfide
(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 or metallic conductors.
At very high pressures, a copper disulfide, CuS2, can be synthesized. Its crystal structure is analogous to that of pyrite
, 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 composition can be obtained by changing the red-ox atmosphere of the synthetic environment.[6]
in character, and have a high degree of delocalization resulting in complicated electronic band structure
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
orbital. NMR studies on CuS show that there are two distinct species of copper atom, one with a more metallic nature than the other. and 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. The issue of the valence of copper in sulfides (as well as selenides an tellurides) continues to be revisited in the literature. A good example is a 2009 study of the ternary compound CuCo2S4 (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".
Chemical compound
A chemical compound is a pure chemical substance consisting of two or more different chemical elements that can be separated into simpler substances by chemical reactions. Chemical compounds have a unique and defined chemical structure; they consist of a fixed ratio of atoms that are held together...
s and minerals with the formula CuxSy. Both mineral
Mineral
A mineral is a naturally occurring solid chemical substance formed through biogeochemical processes, having characteristic chemical composition, highly ordered atomic structure, and specific physical properties. By comparison, a rock is an aggregate of minerals and/or mineraloids and does not...
s and synthetic materials comprise these compounds. Some copper sulfides are economically important ore
Ore
An ore is a type of rock that contains minerals with important elements including metals. The ores are extracted through mining; these are then refined to extract the valuable element....
s.
Prominent copper sulfide minerals include Cu2S (chalcocite
Chalcocite
Chalcocite, copper sulfide , is an important copper ore mineral. It is opaque, being colored dark-gray to black with a metallic luster. It has a hardness of 2½ - 3. It is a sulfide with an orthorhombic crystal system....
) and CuS (covellite
Covellite
Covellite is a rare copper sulfide mineral with the formula CuS. This indigo blue mineral is ubiquitous in copper ores, it is found in limited abundance and is not an important ore of copper itself, although it is well known to mineral collectors.The mineral is associated with chalcocite in zones...
). In the mining industry, the minerals bornite
Bornite
Bornite is a sulfide mineral with chemical composition Cu5FeS4 that crystallizes in the orthorhombic system .-Appearance:Bornite has a brown to copper-red color on fresh surfaces that tarnishes to various iridescent shades of blue to purple in places...
or chalcopyrite
Chalcopyrite
Chalcopyrite is a copper iron sulfide mineral that crystallizes in the tetragonal system. It has the chemical composition CuFeS2. It has a brassy to golden yellow color and a hardness of 3.5 to 4 on the Mohs scale. Its streak is diagnostic as green tinged black.On exposure to air, chalcopyrite...
, which consist of mixed copper-iron sulfides, are often referred to as "copper sulfides". In chemistry
Chemistry
Chemistry is the science of matter, especially its chemical reactions, but also its composition, structure and properties. Chemistry is concerned with atoms and their interactions with other atoms, and particularly with the properties of chemical bonds....
, a "binary copper sulfide" is any binary chemical compound
Chemical compound
A chemical compound is a pure chemical substance consisting of two or more different chemical elements that can be separated into simpler substances by chemical reactions. Chemical compounds have a unique and defined chemical structure; they consist of a fixed ratio of atoms that are held together...
of the elements copper and sulfur
Sulfur
Sulfur or sulphur is the chemical element with atomic number 16. In the periodic table it is represented by the symbol S. It is an abundant, multivalent non-metal. Under normal conditions, sulfur atoms form cyclic octatomic molecules with chemical formula S8. Elemental sulfur is a bright yellow...
. Whatever their source, copper sulfides vary widely in composition with 0.5 ≤ Cu/S ≤ 2, including numerous non-stoichiometric compound
Non-stoichiometric compound
Non-stoichiometric compounds are chemical compounds with an elemental composition that cannot be represented by a ratio of well-defined natural numbers, and therefore violate the law of definite proportions. Often, they are solids that contain crystallographic point defects, such as interstitial...
s.
Known copper sulfides
The naturally occurring mineral binary compounds of copper and sulfur are listed below. There are probably more yet to be discovered, for example investigations of "blaubleibender covellite" (blue remaining covellite) formed by natural leaching of covelliteCovellite
Covellite is a rare copper sulfide mineral with the formula CuS. This indigo blue mineral is ubiquitous in copper ores, it is found in limited abundance and is not an important ore of copper itself, although it is well known to mineral collectors.The mineral is associated with chalcocite in zones...
(CuS
Copper(II) sulfide
Copper monosulfide is a chemical compound of copper and sulfur. It occurs in nature as the dark indigo blue mineral covellite. It is a moderate conductor of electricity. A black colloidal precipitate of CuS is formed when hydrogen sulfide, H2S, is bubbled through solutions of Cu salts...
) indicate that there are other metastable Cu-S phases still to be fully characterised.
- CuS2, villamaninite or (Cu,Ni,Co,Fe)S2
- CuS, covelliteCovelliteCovellite is a rare copper sulfide mineral with the formula CuS. This indigo blue mineral is ubiquitous in copper ores, it is found in limited abundance and is not an important ore of copper itself, although it is well known to mineral collectors.The mineral is associated with chalcocite in zones...
, copper monosulfide - Cu9S8 (Cu1.12S), yarrowite
- Cu39S28 (Cu1.39S) spionkopite
- Cu8S5 (Cu1.6S), geeriteGeeriteGeerite is a copper sulfide mineral with the chemical formula Cu8S5. The mineral is named after the original collector, Adam Geer, of Utica, New York, USA.-Crystallography:...
- Cu7S4 (Cu1.75S), anilite
- Cu9S5 (Cu1.8S), digeniteDigeniteDigenite 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 forms. It has poor to indistinct cleavage and a brittle...
- Cu31S16 (Cu1.96S), djurleiteDjurleiteDjurleite is a copper sulfide mineral of secondary origin with formula Cu31S16 that crystallizes with monoclinic-prismatic symmetry. It is typically massive in form, but does at times develop thin tabular to prismatic crystals. It occurs with other supergene minerals such as chalcocite, covellite...
- Cu2S, chalcociteChalcociteChalcocite, copper sulfide , is an important copper ore mineral. It is opaque, being colored dark-gray to black with a metallic luster. It has a hardness of 2½ - 3. It is a sulfide with an orthorhombic crystal system....
Classes of copper sulfides
Copper sulfides can be classified into three groups:Monosulfides, 1.6 ≤ Cu/S ≤ 2: their crystal structure
Crystal structure
In mineralogy and crystallography, crystal structure is a unique arrangement of atoms or molecules in a crystalline liquid or solid. A crystal structure is composed of a pattern, a set of atoms arranged in a particular way, and a lattice exhibiting long-range order and symmetry...
s consist of isolated sulfide anions that are closely related to either hcp
Close-packing
In geometry, close-packing of equal spheres is a dense arrangement of congruent spheres in an infinite, regular arrangement . Carl Friedrich Gauss proved that the highest average density – that is, the greatest fraction of space occupied by spheres – that can be achieved by a regular lattice...
or fcc
Cubic crystal system
In crystallography, the cubic crystal system is a crystal system where the unit cell is in the shape of a cube. This is one of the most common and simplest shapes found in crystals and minerals....
lattices, without any direct S-S bonds. 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. Therefore, this group of copper sulfides shows ionic conductivity at slightly elevated temperatures. In addition, the majority of its members are semiconductor
Semiconductor
A semiconductor is a material with electrical conductivity due to electron flow intermediate in magnitude between that of a conductor and an insulator. This means a conductivity roughly in the range of 103 to 10−8 siemens per centimeter...
s.
Mixed monosulfide and disulfide compounds of copper contain both monosulfide (S2−) as well as disulfide
Disulfide
In chemistry, a disulfide usually refers to the structural unit composed of a linked pair of sulfur atoms. Disulfide usually refer to a chemical compound that contains a disulfide bond, such as diphenyl disulfide, C6H5S-SC6H5....
(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 or metallic conductors.
At very high pressures, a copper disulfide, CuS2, can be synthesized. Its crystal structure is analogous to that of pyrite
Pyrite
The mineral pyrite, or iron pyrite, is an iron sulfide with the formula FeS2. This mineral's metallic luster and pale-to-normal, brass-yellow hue have earned it the nickname fool's gold because of its resemblance to gold...
, 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 composition can be obtained by changing the red-ox atmosphere of the synthetic environment.[6]
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 rather than ionicIonic bond
An ionic bond is a type of chemical bond formed through an electrostatic attraction between two oppositely charged ions. Ionic bonds are formed between a cation, which is usually a metal, and an anion, which is usually a nonmetal. Pure ionic bonding cannot exist: all ionic compounds have some...
in character, and have a high degree of delocalization resulting in complicated electronic band structure
Electronic band structure
In solid-state physics, the electronic band structure of a solid describes those ranges of energy an electron is "forbidden" or "allowed" to have. Band structure derives from the diffraction of the quantum mechanical electron waves in a periodic crystal lattice with a specific crystal system and...
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
Antibonding
Antibonding is a type of chemical bonding. An antibonding orbital is a form of molecular orbital that is located outside the region of two distinct nuclei...
orbital. NMR studies on CuS show that there are two distinct species of copper atom, one with a more metallic nature than the other. and 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. The issue of the valence of copper in sulfides (as well as selenides an tellurides) continues to be revisited in the literature. A good example is a 2009 study of the ternary compound CuCo2S4 (a spinel mineral known as carrollite
Carrollite
Carrollite, CuCo2S4, is a sulfide of copper and cobalt, often with substantial substitution of nickel for the metal ions, and a member of the linnaeite group. It is named after the type locality in Carroll County, Maryland, USA, at the Patapsco mine, Finksburg, Sykesville.- Unit cell :Space group:...
) 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".