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Covellite: Article contributions

I will continue working upon the applications section in the article, with new subheadings for lithium batteries, solar electric devices, and ammonium gas sensors (and potentially more). Currently there is only a sentence in the superconductors sub-section in the Application section which states that superconductivity research is being used into it but not more. Within this, synthesized nanostructures are critical, so subheadings in the Synthesis section will be added, such as nanoplatelets and nanocrystals, to provide some transition into applications.

Currently, the information on covellite's usages is limited, with only a small paragraph on superconductors (though this can also be expanded upon, such as covellite being a p-type semiconductor) and with an increase in peer-reviewed scientific articles of covellite usage in diverse applications over the past decade, this portion should be updated to match ongoing research. Readers will then be able to make connections between covellite and nanostructures and their use in everyday objects. However, I will need to be careful in ensuring the synthesis processes don't overlap with applications, many nanostructures which require different process for different uses.

I will also be adding/updating sources for the composition and characteristics sections (the later using less reliable sources).

Covellite (also known as covelline) 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 of secondary enrichment (supergene) of copper sulfide deposits. Commonly found with and as coatings on chalcocite, chalcopyrite, bornite, enargite, pyrite, and other sulfides, it often occurs as pseudomorphic replacements after other minerals. Despite the very rare occurrence as a volcanic sublimate, the initial description was at Mount Vesuvius by Italian mineralogist Nicola Covelli (1790–1829).

Composition
Covellite belongs to the binary copper sulfides group, which has the formula CuxSy 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 CuSx 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. This indicates that several of covellite's special properties are the result of molecular structure at this level.

As described for copper monosulfides like pyrite, the assignment of formal oxidation states to the atoms that constitute covellite is deceptive. The formula might seem to suggest the description Cu2+, S2−. 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 Cu2+ (d9) ions. In contrast to the oxide CuO, the material is not a magnetic semiconductor but a metallic conductor with weak Pauli-paramagnetism. Thus, the mineral is better described as consisting of Cu+ and S− rather than Cu2+ and S2−. Compared to pyrite with a non-closed shell of S− pairing to form S22−, there are only 2/3 of the sulfur atoms held. The other 1/3 remains unpaired and together with Cu atoms forms hexagonal layers reminiscent of the boron nitride (graphite structure). Thus, a description Cu+3S−S22− would seem appropriate with a delocalized hole in the valence band leading to metallic conductivity. Subsequent bandstructure calculations indicate however that the hole is more localized on the sulfur pairs than on the unpaired sulfur. This means that Cu+3S2−S2− with a mixed sulfur oxidation state -2 and -1/2 is more appropriate. Despite the extended formula of Cu+3S2−S2− from researchers in 1976, others have come up with variations, such as Cu+4Cu2+2(S2)2S2.

Structure
For a copper sulfide, covellite has a complicated lamellar structure, with alternating layers of CuS and Cu2S2 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 S2 dimers. The Cu2S2 layers only has one l/3 bond along the c-axis (perpendicular to layers), thus only one bond in that direction to create a perfect cleavage {0001}. The conductivity is greater across layers due to the partially filled 3p orbitals, facilitating electron mobility.

Geologic 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. 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 1,150 m found in Silver Bow Country, Montana. 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. An unusual occurrence of covellite was found replacing organic debris in the red beds of New Mexico.

Nicolla 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. His studies of its lava led to the discovery of several unknown minerals including covellite.

Synthesis
Covellite's unique crystal structure is related to its complex oxidative formation conditions, as seen when attempting to synthesize covellite. Its formation also depends on the state and history of the associated sulfides it was derived from. Experimental evidence shows ammonium metavanadate (NH4VO3) to be a potentially important catalyst for covellite's solid state transformation from other copper sulfides. There are multiple methods of direct reactions between copper and sulfur together to form covellite though the majority have required temperatures greater than 150°C or else the product will only form on the surface of copper particles. The exception to this is corrosion reaction in water in a 60°C system. Alternative methods include solvothermal (with acetic acid), hydrothermal, and γ-radiation. 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. 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.

Nanocrystals
The variety of stoichiometric structures, from Cu1.1S to Cu2S, have led to specific band gaps, which in turn, have been critical for the diverse applications in nanocrystals.

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. Advantages include limited toxicity and low costs. Covellite’s high electrical conductivity (10−3 S cm−1) and a high theoretical capacity (560 mAh g−1) with flat discharge curves when cycled versus Li+/Li has been determined to play critical roles for capacity. The variety of methods of formations is also a factor of the low costs. However, issues with cycle stability and kinetics have been limiting covellite's progress into mainstream lithium batteries until future developments in its research.

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. 2 nm thick, with one unit cell and two copper atoms layers, and diameters around 100 nm 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 4 nm width and greater than 30 nm diameter have been experimentally synthesized with less cost and energy. Conversely, localized surface plasmon resonances observed in covellite nanoparticles have recently been linked to the stoichiometry-dependent bandgap key for nanocrystals. Thus, future chemical sensing devices, electronics, and others are being explored with the use of nanostructures with covellite CuS.