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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
Common impurities: Zn,Co,Ni
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).
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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