Copper

native element

malachite

Formula: Cu native element
Specific gravity: 8.93
Hardness: 2½ to 3
Streak: Copper red
Colour: Copper red
Solubility: Slightly soluble in hydrochloric acid; moderately soluble in sulphuric acid; readily soluble in nitric acid
Environments:

Volcanic igneous environments
Hydrothermal environments
Basaltic cavities

Small amounts of native copper have been found at many localities in the oxidation zones of copper deposits, associated with cuprite, malachite and azurite. These minerals may be carried in solution down to the enriched zone, where secondary copper may be redeposited. Deposits of native copper are also found in cavities in basalt lavas, resulting from the reaction of hydrothermal solutions with iron oxide minerals, and the richest source of copper in the world is the basalt lava flows of the Keweenaw peninsula, USA.

At the Bardon Hill quarry, Coalville, Leicestershire, England, UK, native copper is found associated with cuprite and altering to malachite (RES p194).

At the New Cliffe quarry, Stanton under Barton, Leicestershire, England, UK, native copper is found associated with cuprite and altering to malachite (RES p194).

At the Old Dominion mine, Gila county, Arizona, USA, copper pseudomorphs after cuprite have been found (R&M 94.2.169).

At the Kabwe mine, Central Province, Zambia, small amounts of native copper have been found associated with malachite, cuprite and chalcocite (R&M 94.2.124).

Alteration

chalcocite and oxygen to native copper and sulphate ions
Cu2S(s) + 2O2(g) → Cu(s) + Cu2+(aq) + SO42-(aq)
If acidic copper sulphate solutions pass through the oxidation zone to below the water table, conditions usually change to reducing and the dissolved copper ions react with sulphide ions (S2-) to form copper sulphides such as chalcocite. If the water table falls, allowing the chalcocite to be exposed to the oxidation zone, then native copper can form according to the above reaction (JRS 18.14).

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