Covellite



Covellite (also known as covelline) is a rare copper sulfide mineral with the formula CuS. This indigo blue mineral was the first discovered natural superconductor. Because of it limited abundance, it is not an important copper ore, although it is well known to mineral collectors.

As described for the mineral pyrite, the assignment of formal oxidation states (or charges) 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. Thus, the mineral is probably best described as consisting of both Cu2+, S2- and 2Cu+, S22-.

The mineral is associated with chalcosite 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. Very rare occurrence as a volcanic sublimate at Mount Vesuvius the site of its discovery or first description by Nicola Covelli (1790 - 1829).

Composition
Covellite (CuS) 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). Although the fundamental structure of covellite is CuS, research into its "superstructure" (large numbers of adjoined of CuS molecules) indicates that several of covellite's special properties are the result of molecular structure at this level. Both Cu+ and Cu2+ cations are included as part of the minerals "superstructure", resulting in an elemental composition of 2Cu+, Cu+2 and S-2

Formation
Covelite is commonly found as secondary copper mineral in deposits, more rarely as a primary mineral in copper deposits, and very rare as volcanic sublimate. Covellite is known to form in weathering environments near the surface in deposits where copper is the primary sulfide. As a primary mineral, the formation of covellite is restricted to hydrothermal conditions.

Covellite's unique crystal structure are related to the conditions of its formation. The covellite (CuxSy) framework of CuS3 planes which are isolated by electronegatively charged CuS2 planes. This distinct framework is produced during oxidative formation conditions. Attempts to synthesize covellite indicate it undergoes a complex oxidation process, and that its formation 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.

Geologic occurrence
Covellite's occurrence is widespread and some of the highest quality specimens have been found in the United States. In Silver Bow County, Montana, covellite has been found in veins at depths of 1,150 meters, as the primary mineral. Covellite formed as clusters in these veins reaching one meter across in Leonard mines, 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). Locally, findings of covellite have been discovered in salt domes and at the McCellan copper mine in Foard County, Texas. The finest mineral specimens of covellite come from Rio Grande County, Colorado. Beautiful colors of deep blue covellite and bright metallic pyrite forms hexagonal plates 2cm across. An unusual occurrence of covellite was found replacing organic debris in the red beds of New Mexico. Another interesting occurrence of covellite is in Maska, Lebanon. Covellite is one of two major ore minerals that have been mined where the major component is mineral ice. A high-grade ore outcrop weathered and debris settle into adjacent glaciers producing 200,000 tons of ore.

Applications
Covellite was the first discovered natural superconductor The framework of CuS3 /CuS2 allow for an electron excess that facilitate superconduction during particular states, with exceptionally low thermal loss. Material science is now aware of several of covellites favorable properties and several researchers are intent on sythesizing covellite. Gramp et al. (2006) 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