Chalcocite

chalcocite

bornite

chalcopyrite

copper

Images

Formula: Cu2S
With oxidation states: Cu1+192(S2-)96 (CM 23.61-76)
Sulphide, copper-bearing 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
Common impurities: Fe
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.

Localities

The Two Mile and Three Mile deposits, Paddy's River, Paddys River District, Australian Capital Territory, Australia, are skarn deposits at the contact between granodiorite and volcanic rocks. Chalcocite occurs both as a primary sulphide and as a supergene mineral replacing chalcopyrite and sphalerite along fractures and grain boundaries. Primary chalcocite has been found as intergrowths with bornite (AJM 22.1.36).

At the Mount Kelly deposit, Gunpowder District, Queensland, Australia, the deposit has been mined for oxide and supergene copper ores, predominantly malachite, azurite and chrysocolla. The ores overlie primary zone mineralisation consisting of quartz-dolomite-sulphide veins hosted in dolomite-bearing siltstone and graphitic schist.
Chalcocite was found as a dull, blue-grey supergene mineral replacing chalcopyrite in the early stages of oxidation (AJM 22.1.19).

At the At the Mount Lyell mines, Queenstown district, West Coast municipality, Tasmania, Australia, chalcocite is common in many of the sulphide ore deposits, sometimes disseminated with bornite in schist, and sometimes as blocks weighing many kilograms (AJM 21.2.22).

At Tsumeb, Namibia, chalcocite was quite common, associated with native silver (R&M 93.6.542). Pseudomorphs of chalcocite after galena have been found here (KL p128).

At the Magma mine, Pioneer District, Pinal county, Arizona, USA, chalcocite has been found on a hematite matrix (R&M 95.1.83-84).

At the Leonard mine, Montana, USA, chalcocite pseudomorphs after covellite have been found (KL p127).

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)

skinnerite to chalcocite, antimony and sulphur
2Cu3SbS3 → 3Cu2S + 2Sb + 3/2S2
(CM 28.725-738)

skinnerite and sphalerite = Zn-tetrahedrite and chalcocite
4Cu3SbS3 + 2ZnS → Cu10Zn2Sb4S13 + Cu2S
(CM 28.725-738)

Zn-tetrahedrite to chalcocite, antimony, sphalerite and sulphur
Cu10Zn2Sb4S13 → 5Cu2S + 4Sb + 2ZnS + 3S2
(CM 28.725-738)

The diagram below is a Pourbaix diagram for Cu-Fe-S-H2O (IJNM 07(02).9.23). It shows the relationship between copper Cu, chalcopyrite CuFeS2, tenorite CuO, covellite CuS, cuprite Cu2O, chalcocite Cu2S, pyrite FeS2 and hematite Fe2O3.

Pourbaix Cu-Fe-S-H<sub>2</sub>O.jpg
























Back to Minerals