Covellite

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).

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)., 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 plain 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.