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 Lin Ma Hang mine, North District, New Territories, Hong Kong, China, the lead-zinc deposit is a hydrothermal deposit which lies along a fault zone within altered acid volcanic rocks, consisting mainly of chlorite, biotite, sericite and actinolite, with scattered quartz. (Hong Kong Minerals (1991). Peng, C J. Hong Kong Urban Council)
The mineralisation consists of a series of fissure vein deposits varying from a few mm to several metres on width. The initial vein filling was coarse milky quartz. this was followed by an intrusion of fine-grained quartz carrying the metallic minerals, galena, pyrite, sphalerite and chalcopyrite, in order of abundance (Geological Society of Hong Kong Newsletter, 9.4.3-27).
Malachite and azurite are occasionally found in close association with chalcopyrite (Hong Kong Minerals (1991). Peng, C J. Hong Kong Urban Council).
The Ma On Shan Mine, Ma On Shan, Sha Tin District, New Territories, Hong Kong, China, is an abandoned iron mine, with both underground and open cast workings. The iron ores contain magnetite as the ore mineral and occur predominantly as masses of all sizes enclosed in a large skarn body formed by contact metasomatism of dolomitic limestone at the margins of a granite intrusion. In parts of the underground workings magnetite is also found in marble in contact with the granite. The skarn rocks consist mainly of tremolite, actinolite, diopside and garnet. Malachite and azurite are occasionally found in close association with chalcopyrite, formed by the action of carbonated water on the chalcopyrite (Hong Kong Minerals (1991). Peng, C J. Hong Kong Urban Council)
The Lin Fa Shan deposit, Tsuen Wan District, New Territories, Hong Kong, China, is located in a remote area of the Tai Mo Shan Country Park, on a steep west facing slope of Lin Fa Shan, just above the abandoned village of Sheung Tong. The surrounding hillsides are covered with shallow excavations, representing past searches for wolframite, the natural ore of tungsten. The abandoned workings are extremely dangerous with unsupported tunnels, open shafts and no maintenance since their closures in 1957; the workings should not be entered (http://industrialhistoryhk.org/lin-shan).
Malachite and azurite are occasionally found in close association with chalcopyrite formed by the action of carbonated waters on the chalcopyrite (Hong Kong Minerals (1991). Peng, C J. Hong Kong Urban Council).
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 Balliway Rigg, Caldbeck, Allerdale, Cumbria, England, UK, a 4 cm specimen of iron stained quartz matrix with a rich cover, front and back, of fibrous malachite and minor chrysocolla has been found (AESS).
At Red Gill Mine, Roughton Gill, Caldbeck, Allerdale, Cumbria, England, UK, a specimen was found with vugs containing a combination of malachite, cerussite and brochantite crystals with light blue chrysocolla (AESS).
At Roughton Gill Mine, Roughton Gill, Caldbeck, Allerdale, Cumbria, England, UK, microscopic green, vitreous, acicular crystals of malachite occur on and in quartz (AESS).
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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