Chalcocite

copper

pyrite

Formula: Cu2S
Sulphide, copper mineral
Specific gravity: 5.7 to 5.8
Hardness: 2½ to 3
Streak: Blackish to dark grey
Colour: Dark lead grey to blackish
Solubility: Moderately soluble in nitric acid
Environments:

Metamorphic environments
Hydrothermal environments

Chalcocite may occur as a primary mineral in veins with bornite, chalcopyrite, enargite and pyrite, but its principal occurrence is as a secondary, supergene mineral in enriched zones of mesothermal (moderate temperature) and hypothermal (high temperature) sulphide deposits. Under surface conditions the primary copper sulphides are oxidised; the soluble sulphates so formed move downwards, reacting with the primary minerals to form chalcocite, enriching the ore in copper. The water table is the lower limit of the zone of oxidation and a chalcocite shelf may form there. The formation of chalcocite from Cu2+ is a reduction reaction requiring the presence of some reducing agent.
2Cu2+ + HS- + 2e- → Cu2S + H+
Chalcocite has a stability range at any pH (acid or alkaline) in a relatively reducing environment.

At Tsumeb, Namibia, chalcocite was quite common, associated with native silver (R&M 93.6.542).

Alteration

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

chalcocite to covellite
Ferric sulfate is a strong oxidizing agent; covellite is formed from chalcocite by the reaction below.
chalcocite and ferric sulphate to copper sulphate, ferrous sulphate and covellite
Cu2S + Fe2(SO4)3 → CuSO4 + 2FeSO4 + CuS
(AMU b3-3.7)

chalcocite to cuprite
If chalcocite is exposed to the oxidation zone, then conditions for the formation of cuprite and native copper can occur readily.
chalcocite + oxygen + water → cuprite + sulphuric acid
Cu2S(solid) + 2O2(gaseous) + H2O(liquid) → Cu2O(solid) + H2SO4(aqueous)
(JRS 18.14)

chalcocite to native copper
chalcocite + oxygen → copper + cupric sulphate
Cu2S(solid) + 2O2(gaseous) → Cu(solid) + Cu2+SO4(aqueous)
(JRS 18.14)

chalcopyrite and chalcocite to bornite
CuFe3+S2 + 2Cu2S = Cu5FeS4

pyrite to chalcocite: Because chalcocite is less soluble than pyrite, supergene chalcocite may form below the zone of oxidation when dissolved copper ions Cu2+ replace ferrous ions Fe2+ from pyrite.
Cu2+ + pyrite + H2O to chalcocite + Fe2+ + (SO4)2- + H+
14Cu2+ + 5FeS2 + 12H2O → 7Cu2S + 5Fe2+ + 3(SO4)2- +24H+
(KB p527)

Common impurities: Fe

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