Covellite

copper

chalcopyrite

Formula: CuS
Sulphide, copper mineral
Specific gravity: 4.68
Hardness: 1½ to 2
Streak: Blue-black
Colour: Blue-black
Solubility: Slightly soluble in hydrochloric acid and sulphuric acid; moderately soluble in nitric acid
Environments:

Metamorphic environments
Hydrothermal environments
Volcanic sublimates (very rarely)

Covellite is not an abundant mineral; it is usually found as a secondary copper mineral in copper deposits, more rarely as a primary mineral, and only very rarely as a volcanic sublimate. It is found in the enrichment zone of most copper deposits, usually as a coating, associated with other copper minerals, principally chalcocite, chalcopyrite, bornite and enargite, and is derived from them by alteration.
Supergene covellite is formed by the process
Cu2+ + HS- → CuS + H+
Covellite requires an acid environment for stability.

At Herrensegen, Schapbach, Schartzwald, Germany, covellite occurs with chalcopyrite (FM 2318).

Alteration

Covellite may occur as an alteration product of chalcopyrite (AJM 18.2.26).

Oxidation of pyrite forms ferrous (divalent) sulphate and sulphuric acid:
pyrite + oxygen + water → ferrous sulphate + sulphuric acid
FeS2 + 7O + H2O → FeSO4 + H2SO4
The ferrous (divalent) sulphate readily oxidizes to ferric (trivalent) sulphate and ferric hydroxide:
ferrous sulphate + oxygen + water → ferric sulphate + ferric hydroxide
6FeSO4 + 3O + 3H2O → 2Fe2(SO4)3 + 2Fe(OH)3
Ferric sulphate is a strong oxidizing agent and attacks both chalcocite and covellite.

chalcocite and ferric sulphate to copper sulphate, ferrous sulphate and covellite
Cu2S + Fe2(SO4)3 → CuSO4 + 2FeSO4 + CuS
(AMU b3-3.7)

covellite and ferric sulphate to ferrous sulphate, copper sulphate and sulphur
CuS + Fe2(SO4)3 → 2FeSO4 + CuSO4 + S
Covellite is further oxidised according to the above reaction to form sulphur (AMU b3-3.7).

sphalerite to covellite: Because covellite is less soluble than sphalerite, supergene covellite may form below the zone of oxidation when dissolved copper ions Cu2+ replace zinc ions Zn2+ from sphalerite
Cu2+ + sphalerite → covellite + Zn2+
Cu2+ + ZnS → CuS + Zn2+
(KB p527)

Common impurities: Fe,Se,Ag,Pb

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