Chalcocite

chalcocite

bornite

chalcopyrite

copper

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Formula: Cu₂S
With oxidation states: Cu¹⁺₁₉₂(S^2-)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 Saint-Pierre-de-Broughton, Les Appalaches RCM, Chaudière-Appalaches, Quebec, Canada, chalcocite occurs in the talc-carbonate rocks in solid sulphide veins and masses intergrown with the more common bornite and chalcopyrite. It also occurs in massive quartz veins and lenses, as irregular black metallic patches in bornite and associated with chalcopyrite (R&M 85.6.502).

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 M'Passa Mine, Mindouli District, Pool Department, Republic of the Congo, chalcocite crystals occur with associated pyrite (Dr Marco Tam Shing Yau, The Mineralogy Society of Hong Kong Newsletter 19.8).

At Geevor Mine, Pendeen, St Just, Cornwall, England, UK, chalcocite has been found as rare, pseudohexagonal prismatic crystals, rather than the more common massive material (AESS).

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 Copper Falls Mine, Copper Falls, Keweenaw county, Michigan, USA, mineralisation occurs primarily in hydrothermal veins cutting preexisting lavas and as amygdules in the Ashbed flow.
Chalcocite is very rare at the Copper Falls mine. A single significant specimen, with long, striated crystals to 2 cm, is in the A E Seaman Mineral Museum (MinRec 54.1.105).

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.


























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