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.

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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