covellite

Covellite
General
Category	Sulfide mineral
Chemical formula	copper sulfide:CuS
Identification
Color	Indigo blue (tarnished to purple or black)
Crystal habit	thin platy hexagonal crystals also massive to granular.
Crystal system	Hexagonal 6/m2/m2/m
Cleavage	Perfect in one direction
Fracture	flakey
Mohs Scale hardness	1.5 - 2
Luster	metallic
Streak	lead gray to black
Specific gravity	4.6 - 4.8
Refractive index	opaque
Fusibility	2.5
Other characteristics	micaceous cleavage

Covellite (gray) replacing and embaying chalcopyrite (light), polished section from Horn Silver Mine, San Francisco Mining District, Utah. Enlarged 210 diameters

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^[1]. Because of its 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 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. 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 Cu_xS_y and can have a wide-ranging copper/sulfur ratio from 1:2 to 2:1 (Cu/S) ^[2]. 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^[3]. 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 [2]}

Formation

Covellite 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 is related to the conditions of its formation. The covellite (Cu_xS_y) framework of CuS₃ planes which are isolated by negatively charged CuS₂ planes. This distinct framework is produced during oxidative formation conditions. Attempts to synthesize covellite ^[4][5] 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 ^[4] shows ammonium metavanadate (NH₄VO₃) to be a potentially important catalyst for covellite's solid state transformation from other copper sulfides.

Geologic occurrence

Covellite from the Black Forrest, Germany

Covellite's occurrence is widespread 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.^[6] 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^{[citation needed]} and at the McCellan copper mine in Foard County, Texas.^[7] An unusual occurrence of covellite was found replacing organic debris in the red beds of New Mexico.^[8]

Applications

Covellite was the first discovered natural superconductor^[1] 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 covellite's favorable properties and several researchers are intent on synthesizing covellite ^[9][10]. 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 ^[11]. 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 ^[4]

See also

• Copper monosulfide
• List of minerals
• List of minerals named after people

References

*Dana's Manual of Mineralogy ISBN 0-471-03288-3

• Mineralgalleries.com
• Webmineral
• Mindat.org