Malachite

copper

cuprite

Formula: Cu2(CO3)(OH)2
Anhydrous carbonate containing hydroxyl, copper mineral
Specific gravity: 3.6 to 4.05
Hardness: 3½ to 4
Streak: Green
Colour: Green
Solubility: Readily soluble in hydrochloric, sulphuric and nitric acid
Environments:

Carbonatites
Hydrothermal environments

Malachite is the most abundant secondary copper mineral, found in the oxidation zones of high temperature hydrothermal copper deposits, often in limestone, associated with azurite, cuprite, native copper, and iron oxides.
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).

At Tsumeb, Namibia, malachite occurs as pseudomorphs after azurite and, rarely, after cuprite (R&M 93.6.545).

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 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).

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, someperched 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 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 (Min Mag (1986) 50:41-47).

Common impurities: Zn,Co,Ni

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