Covellite

Covellite can be identified by its hardness of 1.5 - 2 on the Mohs scale, Indigo-blue or darker color, Sub-metallic luster, Hexagonal crystal system. Look for these key characteristics when examining specimens.

Covellite typically appears in Indigo-blue or darker, inclining towards blue-black, often iridescent with purplish, deep red, brassy-yellow reflections. Color can vary depending on impurities and formation conditions.

Covellite has a hardness of 1.5 - 2 on the Mohs scale. This makes it a soft mineral that can be scratched easily.

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 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 (pyrite and chalcopyrite) at the same locality. An unusual occurrence of covellite was found replacing organic debris in the red beds of New Mexico. Nicola 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.

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 Cu, S. 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 Cu (d) 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 Cu and S. Compared to pyrite with a non-closed shell of S pairing to form S2, 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 Cu3SS2 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 Cu3SS2 with a mixed sulfur oxidation state -2 and -1/2 is more appropriate. Despite the extended formula of Cu3SS2 from researchers in 1976 and 1993, others have come up with variations, such as Cu4Cu2(S2)2S2.