Covellite

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Covellite
Category	Mineral
Chemical formula	copper(II) 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
Refractive index	opaque
Streak	lead gray to black
Specific gravity	4.6 - 4.8
Fusibility	2.5
Other Characteristics	micaceous cleavage

Covellite (also know as covelline) is a crystalline form of copper sulfide (CuS). In appearance, covellite is an enchanting indigo blue.

As described for the mineral pyrite, the assignment of formal oxidation states (or charges) to the atoms that constitute covellite is non-obvious. The formula might seem to suggest the description Cu²⁺, S^2-. 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⁺, S^2- and 2Cu⁺, S₂^2-.

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 Niccola Covelli (1790 - 1829).