It is frequently found as pseudomorphs after azurite, or as alteration pseudomorphs after cuprite. It occurs Less frequently as pseudomorphs after atacamite, brochantite, chalcopyrite, tetrahedrite, chalcophyllite, gypsum, libethenite, calcite, sphalerite, cerussite, and pyrite. It is found rarely altered to azurite or cuprite (Mindat).
Malachite is a relatively high pH (alkaline) mineral, and brochantite converts to malachite as the pH increases. If the carbonate content of the environment increases, then the boundary where malachite is more stable than brochantite moves to a lower pH (more acid) environment (JRS 18.13).
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. malachite is a secondary carbonate occurring as crusts and fibres on magnetite and actinolite (AJM 22.1.42).
At the Mount Kelly deposit, Gunpowder District, Queensland, Australia, the copper ores overlie primary zone mineralisation consisting of quartz-dolomite-sulphide veins hosted in siltstone and schist. Malachite is the most common secondary copper mineral in the deposit. Overgrowths and coatings of malachite were observed on azurite, chrysocolla, romanèchite and hematite (AJM 22.1.23).
At the Shangulowe mine, Kambove district, Democratic Republic of Congo, malachite pseudomorphs after baryte have been found (KL p180).
At Chessy-les-Mines, Villefranche, Rhône, Auvergne-Rhône-Alpes, France, malachite pseudomorphs after azurite have been found (R&M 95.3.275).
At Dzezkazgan, Kazakstan, malachite has been found with iodargyrite (FM 42699).
At Tsumeb, Namibia, malachite occurs as pseudomorphs after azurite and, rarely, after cuprite (R&M 93.6.545). Also rosasite pseudomorphs after malachite after azurite with cerussite have been found (KL p181).
At Alderley Edge, Cheshire, England, UK, supergene azurite and malachite are common (RES pps 49-50), and cuproasbolane has been found associated with malachite (RES p53).
At the Snelston mine, near Ashbourne, Derbyshire, England, UK, malachite occurs on sandstone (RES p140).
At Croft Quarry, Croft, Blaby, Leicestershire, England, UK, very small spherules and encrustations of malachite are associated with oxidised chalcopyrite; the presence of such green oxidation products can be useful in distinguishing chalcopyrite from marcasite. (JRS 20.20-21).
At Lane's Hill quarry, Stoney Stanton, Blaby, Leicestershire, England, UK, malachite occurred intimately associated with djurleite in a large vein of Fe-bearing dolomite. Present usually as thin films and encrustations, it occasionally occurred as minute single crystals, up to 1.1 mm in length (JRS 20.21).
At Bardon Hill quarry, Coalville, Leicestershire, England, UK, malachite occurs with azurite on dacite (RES p193).
At Newhurst quarry, Shepshed, Leicestershire, England, UK, malachite has been found with minor baryte, replacing earlier chalcopyrite and bornite (RES p199).
At Breedon quarry, Breedon on the Hill, Leicestershire, England, UK, malachite has been found with calcite (RES p203).
At the Eardiston mine, near West Felton, Shropshire, England, UK, malachite occurs on sandstone (RES p291).
At Llynclys quarry, near Oswestry, Shropshire, England, UK, malachite occurs with chalcopyrite, goethite and dolomite (RES p294, 295).
At Judkins quarry, Nuneaton, Warwickshire, England, UK, malachite is associated with calcite (RES p324).
At Bisbee, Cochise county, Arizona, USA, fine pseudomorphs of malachite after azurite on limonitic matrix have been found in many mines, including the Campbell, Cole, Sacramento and Junction mines (R&M 94.2.167, KL p179).
At the Live Oak Pit of the Inspiration mine, Gila county, Arizona, USA, coatings of chalcedony over chrysocolla form over malachite replacements of azurite. Also many specimens of malachite replacing azurite, some perched on chrysocolla, have come from this locality (R&M 94.2.162).
At the Ray mine, Pinal county, Arizona, USA, malachite pseudomorphs after gypsum have been found (R&M 94.2.165).
At the Bagdad mine, Yavapai county, Arizona, USA, rare pseudomorphs of malachite after azurite have been found (R&M 94.2.164).
At the Piedmont mine, Yavapai county, Arizona, USA, extremely rare fine specimens of centimetre sized pseudomorphs of malachite after azurite have been found, coated with a crust of quartz (R&M 94.2.167-168). At the Apex mine, Jarvis Peak, Beaver Dam mountains, Washington county, Utah, USA, an azurite and malachite pseudomorph after gypsum has been found (KL p177).
At the Kabwe mine, Central Province, Zambia, malachite is an extremely rare secondary copper mineral, but it has been found as a coating on, and partly replacing, cerussite. Also in silicified dolomite, with a mammillary habit, with malachite at the core, passing through zinc-rich malachite to rosasite in the outer layer (R&M 94.2.130).
Alteration
azurite and H2O to malachite and CO2
2Cu3(CO3)2(OH)2 + H2O → 3Cu2(CO3(OH)2 + CO2
Azurite is unstable under atmospheric conditions, and slowly converts to the more stable malachite according to the above reaction. This instability is evidenced by the existence of many pseudomorphs of malachite after azurite; pseudomorphs of azurite after malachite are extremely rare (MM 50.41-47).
duftite (s) and H2CO3 (aq) to cerussite (s), malachite (s), H2AsO4- (aq) and H+ (aq)
2PbCuAsO4(OH) + 3H2CO3 ⇌ 2PbCO3 + Cu2CO3(OH)2 + 2H2AsO4- + 2H+
(MM 52.688)
The Activity-pH diagram below was calculated at 298.2 K for some carbonates and copper arsenates for constant activity (roughly equivalent to concentration) of H2AsO4- in solution, over a range of values of pH and of H2CO3 activity (MM 52.687).
The mineral formulae are:
azurite: Cu3(CO3)2(OH)2
malachite: Cu2(CO3)(OH)2
olivenite: Cu2(AsO4)(OH)
cornubite: Cu5(AsO4)2(OH)4
clinoclase: Cu3(AsO4)(OH)3
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