Covellite
Image:Cp--Covellite.jpg|thumb|Covellite replacing and embaying chalcopyrite, polished section from Horn Silver Mine, San Francisco Mining District, Utah. Enlarged to 210 diameters. |alt=
Covellite is a rare copper sulfide mineral with the formula CuS. 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.
The mineral is generally found in zones of secondary enrichment 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.
Composition
Covellite belongs to the binary copper sulfides group, which has the formula and can have a wide-ranging copper/sulfur ratio, from 1:2 to 2:1. However, this series is by no means continuous and the homogeneity range of covellite CuS is narrow. Materials rich in sulfur 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 monosulfide, the assignment of formal oxidation states to the atoms that constitute covellite is deceptive. The formula might seem to suggest the description,. 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 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 and rather than and. Compared to pyrite with a non-closed shell of pairing to form, 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. Thus, a description 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 with a mixed sulfur oxidation state −2 and −1/2 is more appropriate. Others have come up with variations, such as.
Structure
For a copper sulfide, covellite has a complicated lamellar structure, with alternating layers of CuS and with copper atoms of trigonal planar and tetrahedral coordination respectively. The layers are connected by S-S bonds known as dimers. The layers only have one l/3 bond along the c-axis, thus only one bond in that direction to create a perfect cleavage. The conductivity is greater across layers due to the partially filled 3p orbitals, facilitating electron mobility.Formation
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. 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.Synthetic
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 to be a potentially important catalyst for covellite's solid state transformation from other copper sulfides. 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.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 found in Silver Bow County, Montana. As a secondary mineral, covellite also forms as descending surface water in the supergene enrichment zone oxidizes and redeposits covellite on hypogene sulfides at the same locality. An unusual occurrence of covellite was found replacing organic debris in the red beds of New Mexico.Nicola Covelli, 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.