Covellite

Covellite
Covellite from the Leonard Mine, Butte, Montana
General
Category	Sulfide mineral
Formula (repeating unit)	copper sulfide:CuS
Strunz classification	2.CA.05a
Dana classification	02.08.12.01
Crystal system	Hexagonal
Crystal class	Dihexagonal dipyramidal (6/mmm) H–M Symbol (6/m 2/m 2/m)
Space group	P63/mmc
Unit cell	a = 3.7938 Å, c = 16.341 Å; Z = 6
Identification
Color	Indigo-blue or darker, commonly highly iridescent, brass-yellow to deep red
Crystal habit	Thin platy hexagonal crystals and rosettes also massive to granular.
Cleavage	Perfect on {0001}
Tenacity	Flexible
Mohs scale hardness	1.5 - 2
Luster	Submetallic, inclining to resinous to dull
Streak	Lead gray
Diaphaneity	Opaque
Specific gravity	4.6 - 4.8
Optical properties	Uniaxial (+)
Refractive index	nω = 1.450 nε = 2.620
Pleochroism	Marked, deep blue to pale blue
Fusibility	2.5
Other characteristics	Micaceous cleavage
References

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 Nicola Covelli (1790–1829).

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)., However this series is by no means a continuous one and the width of the homogeneity range of covellite CuS is narrow. Materials rich in sulfur CuS_x 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. Even though the peculiar monovalent nature of copper in sulfides has been known since the 1980s there are many places in the later literature, even in standard texts, where the divalent copper idea pops up again.

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. 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⁻). As happens in the pyrite structure the non-closed shell S⁻ undergoes pairing to form S₂²⁻, but interestingly this only holds for 2/3 of the sulfur atoms. 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. Later bandstructure calculations however indicate 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. There is a precedence for that in the (synthetic) pyrite CuS₂ that should also be seen mostly as Cu⁺S₂⁻ producing a Pauli-paramagnetic metallic conductor rather than a semiconductor with strong magnetic properties. (For references see: copper monosulfide).