Covellite
Template:Short description Template:Infobox mineral
Covellite (also known as covelline) is a rare copper sulfide mineral with the formula CuS.<ref name="Mindat" /> This indigo blue mineral is commonly a secondary mineral in limited abundance and although it is not an important ore of copper itself, it is well known to mineral collectors.<ref name="Mindat" />
The mineral is generally found in zones of secondary enrichment (supergene) of copper sulfide deposits. Commonly found as coatings on chalcocite, chalcopyrite, bornite, enargite, pyrite, and other sulfides, it often occurs as pseudomorphic replacements of other minerals. The first records are from Mount Vesuvius, formally named in 1832 after Nicola Covelli.<ref name=Mindat/>
Composition
Covellite belongs to the binary copper sulfides group, which has the formula Template:Chem2 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 Template:Chem2 where x~ 1.1- 1.2 do exist, but they exhibit "superstructures", a modulation of the hexagonal ground plane of the structure spanning a number of adjacent unit cells.<ref>Template:Cite journal</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 states to the atoms that constitute covellite is deceptive.<ref name="Evans">Template:Cite journal</ref> The formula might seem to suggest the description Template:Chem2, Template:Chem2. In fact the atomic structure shows that copper and sulfur each adopt two different geometries. However photoelectron spectroscopy, magnetic, and electrical properties all indicate the absence of Template:Chem2 (d9) ions.<ref name="Evans" />Template:Page needed In contrast to the oxide CuO, the material is not a magnetic semiconductor but a metallic conductor with weak Pauli-paramagnetism.<ref name="SynthMinerals">Template:Cite book</ref>Template:Page needed Thus, the mineral is better described as consisting of Template:Chem2 and Template:Chem2 rather than Template:Chem2 and Template:Chem2. Compared to pyrite with a non-closed shell of Template:Chem2 pairing to form Template:Chem2, there are only 2/3 of the sulfur atoms held.<ref name="Evans" />Template:Page needed The other 1/3 remains unpaired and together with Cu atoms forms hexagonal layers reminiscent of the boron nitride (graphite structure).<ref name="Evans" />Template:Page needed Thus, a description Template:Chem2 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 Template:Chem2 with a mixed sulfur oxidation state −2 and −1/2 is more appropriate. Others have come up with variations, such as Template:Chem2.<ref>Template:Cite book</ref>Template:Page needed<ref name="Liang">Template:Cite journal</ref>
Structure
For a copper sulfide, covellite has a complicated lamellar structure, with alternating layers of CuS and Template:Chem2 with copper atoms of trigonal planar (uncommon) and tetrahedral coordination respectively. The layers are connected by S-S bonds (based on Van der Waals forces) known as Template:Chem2 dimers.<ref name="Liang" /> The Template:Chem2 layers only have one l/3 bond along the c-axis (perpendicular to layers), thus only one bond in that direction to create a perfect cleavage {0001}.<ref name="Evans" />Template:Page needed The conductivity is greater across layers due to the partially filled 3p orbitals, facilitating electron mobility.<ref name="Liang" />
Formation
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Naturally occurring
Covellite is commonly found as a secondary copper mineral in deposits. Covellite is known to form in weathering environments in surficial deposits where copper is the primary sulfide.<ref>Template:Cite journal</ref> As a primary mineral, the formation of covellite is restricted to hydrothermal conditions, thus rarely found as such in copper ore deposits or as a volcanic sublimate.<ref name="SynthMinerals" />Template:Page needed
Synthetic
Covellite's unique crystal structure is related to its complex oxidative formation conditions, as seen when attempting to synthesize covellite.<ref name="Sim2007">Template:Cite journal</ref> Its formation also depends on the state and history of the associated sulfides it was derived from. Experimental evidence shows ammonium metavanadate (Template:Chem2) to be a potentially important catalyst for covellite's solid state transformation from other copper sulfides.<ref name="Ghezelbash2005">Template:Cite journal</ref> Researchers discovered that covellite can also be produced in the lab under anaerobic conditions by sulfate reducing bacteria at a variety of temperatures. However, further research remains, because although the abundance of covellite may be high, the growth of its crystal size is actually inhibited by physical constraints of the bacteria.<ref name="GrampSasaki">Template:Cite journal</ref> It has been experimentally demonstrated that the presence of ammonium vanadates is important in the solid state transformation of other copper sulfides to covellite crystals.<ref name="Sim2007" />
Occurrence
Covellite's occurrence is widespread around the world, with a significant number of localities in Central Europe, China, Australia, Western United States, and Argentina.<ref name="Mindat" /> Many are found close to orogenic belts, where orographic precipitation often plays a role in weathering. An example of primary mineral formation is in hydrothermal veins at depths of Template:Convert found in Silver Bow County, Montana.<ref name="Mindat" /> As a secondary mineral, covellite also forms as descending surface water in the supergene enrichment zone oxidizes and redeposits covellite on hypogene sulfides (pyrite and chalcopyrite) at the same locality.<ref name="Mindat" /> An unusual occurrence of covellite was found replacing organic debris in the red beds of New Mexico.<ref>Template:Cite web</ref>
Nicola Covelli (1790-1829), the discoverer of the mineral, was a professor of botany and chemistry though was interested in geology and volcanology, particularly Mount Vesuvius' eruptions.<ref name="Mindat" /> His studies of its lava led to the discovery of several unknown minerals including covellite.Template:Cn
Applications
Superconductors
Covellite is a superconductor with a critical temperature is 1.6 K.<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref> The framework of Template:Chem2 / Template:Chem2 allow for an electron excess that facilitate superconduction during particular states, with exceptionally low thermal loss. Material science is now aware of several of covellite's favorable properties and several researchers are intent on synthesizing covellite.<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref> Uses of covellite CuS superconductivity research can be seen in lithium battery cathodes, ammonia gas sensors, and solar electric devices with metal chalcogenide thin films.<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref><ref>Template:Cite journal</ref>
Lithium ion batteries
Research into alternate cathode material for lithium batteries often examines the complex variations in stoichiometry and tetrahedron layered structure of copper sulfides.<ref>Template:Cite journal</ref> Advantages include limited toxicity and low costs.<ref name=":02">Template:Cite journal</ref> The high electrical conductivity of covellite (Template:Val) and a high theoretical capacity (Template:Val) with flat discharge curves when cycled versus Li+/Li have been determined to play critical roles for capacity.<ref name=":02" /> The variety of methods of formation is also a factor of the low costs. However, issues with cycle stability and kinetics have been limiting the progress of utilizing covellite in mainstream lithium batteries until advances in research occur.<ref name=":02" />
Nanostructures
The electron mobility and free hole density characteristics of covellite makes it an attractive choice for nanoplatelets and nanocrystals because they provide the structures the ability to vary in size. However, this ability can be limited by the plate-like structure all copper sulfides possess. Its anisotropic electrical conductivity has been experimentally proven to be greater within layers (i.e. perpendicular to c-axis). Researchers have shown that covellite nanoplatelets of approx. Template:Val thick, with one unit cell and two copper atom layers, and diameters around Template:Val are ideal dimensions for electrocatalysts in oxygen reduction reactions (ORR). The basal planes experience preferential oxygen adsorption and larger surface area facilitates electron transfer. In contrast, with ambient conditions, nanoplatelets of dimensions of Template:Val width and greater than Template:Val diameter have been experimentally synthesized with less cost and energy. Localized surface plasmon resonances observed in covellite nanoparticles have recently been linked to the stoichiometry-dependent band gap key for nanocrystals. Thus, future chemical sensing devices, electronics, and other instruments which utilize nanostructures of covellite CuS are being explored.<ref name="LiuACS">Template:Cite journal</ref><ref name=":2">Template:Cite journal</ref>