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Formula: Cu2O
Oxide
Varieties
Chalcotrichite is an acicular variety of cuprite
Properties of cuprite
Specific gravity: 6.15
Hardness: 3½ to 4
Streak: Brownish red
Colour: Red
Solubility: Moderately soluble in hydrochloric acid; slightly soluble in sulphuric and nitric acid
Environments:
Cuprite is a secondary mineral found in the oxidation
portions of high temperature copper deposits, associated with
limonite and secondary
copper minerals such as
native copper,
malachite,
azurite and
chrysocolla.
It may be found as an oxidation product coating native copper, or as an alteration
product of
chalcopyrite.
Localities
At the Mount Kelly deposit, Gunpowder District, Queensland, Australia, the deposit has been mined for oxide and
supergene
copper ores, predominantly malachite,
azurite and chrysocolla. The ores
overlie primary zone mineralisation consisting of
quartz-dolomite-sulphide veins hosted
in dolomite-bearing siltstone
and graphitic
schist.
Cuprite crystals to 2 mm occur, some pseudomorphing
chalcopyrite and some encapsulated in
calcite. The crystals commonly occur in small cavities and lining fractures
in siliceous goethite-hematite
rich gossan. The acicular variety chalcotrichite has also been
found here
(AJM 22.1.21).
At the M'sesa mine, Kambove District, Haut-Katanga, DR Congo, shattuckite
pseudomorphs after cuprite have been found
(KL p231).
At the Mashamba West Mine, Kolwezi mining district, Lualaba, DR Congo, shattuckite
pseudomorphs after cuprite occur
(R&M 87.4.304-336).
At Tsumeb, Namibia, cuprite occurs in dolomite ore, and
pseudomorphs of
malachite after cuprite have been recorded
(R&M 93.6.542).
At the Rubtsovsk mine, Rubtsovsky District, Altai Krai, Russia, copper
pseudomorphs after cuprite have been found
(R&M 95.3.275).
At the Bardon Hill quarry, Coalville, Leicestershire, England, UK, cuprite occurs with native copper
altering to chrysocolla and malachite
(RES p193).
At New Cliffe Hill quarry, Stanton under Bardon, Leicestershire, England, UK, cuprite occurs with
native copper and malachite
(RES p197).
At the Cotopaxi Mine, Colorado, USA, cuprite crystals are found rimmed with green
malachite.
(R&M 84.6.547).
The Central Mine, Central, Keweenaw county, Michigan, USA, initially targeted a series of sub-parallel mineralised
fissure veins where the most copper-rich portion of the vein was close to the
base of the main greenstone flow.
Cuprite from the Central mine occasionally forms thin veinlets and masses in rich parts of the mineralised
fissure veins, but is mainly known as the source of fine deep red patinas sometimes seen on
copper specimens, particularly from early finds in the 19th century
(MinRec 54.1.53-81).
At the Copper Falls Mine, Copper Falls, Keweenaw county, Michigan, USA, mineralisation occurs primarily in hydrothermal veins
cutting preexisting lavas and as amygdules in the Ashbed flow.
Cuprite occurs as oxide coatings on native copper, imparting a distinctive reddish
patina. Native copper specimens coloured reddish by cuprite are among the most
desirable of all specimens from this mine. In addition, specimens have been found of the acicular
“chalcotrichite” variety with needles measuring as much as 1 cm in length
(MinRec 54.1.107).
The Cliff Mine, Phoenix, Keweenaw county, Michigan, USA, is situated at the base of a roughly 70-metre
basalt cliff. A curious feature of the impressive thickness of the
greenstone flow here is that it contains zones of “pegmatoid”: areas
where
slow cooling in the core of the lava flow allowed for large feldspar crystals
exceeding 1 cm to grow. Such features are normally only observed in intrusive igneous rocks and are almost unheard of
in basalt flows.
The Cliff mine primarily exploited rich copper mineralisation in the Cliff
fissure (vein). Although mineralised with copper to some extent along its
entire length, the part of the vein just below the greenstone flow
carried the richest copper mineralisation by far. A significant amount of the
copper recovered at the Cliff mine came from amygdaloids in the tops of 13
basalt flows which were cut by the Cliff vein. The discovery and mining
of this vein proved that the veins were the source of the large masses of float
copper that were already well known, and proved that the
primary ore mineral in the district was native
copper, not sulphides, as had been suspected earlier.
Although cuprite is most common as a red oxide coating on copper, the
Cliff mine also produces occasional dark red microcrystals to 2 mm showing octahedrons, cubes and the very rare gyroid
(MinRec 54.1.25-49).
At the Chino mine, New Mexico, USA, cuprite occurs overgrown with
goethite.
(R&M 84.6.492-500).
Alteration
chalcocite, oxygen and water to cuprite and sulphuric acid
2S(s) + 2O2(g) + H2O(l) → Cu2O(s) + H2SO4(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 to form copper sulphides such as
chalcocite. If the water table falls, allowing the
chalcocite to be exposed to the oxidation zone, then cuprite may be
formed according to the above equation
(JRS 18.14).
The diagram below is a Pourbaix diagram for copper (GSJ).
It shows the relationship between copper Cu, cuprite Cu2O
and spertiniite Cu(OH)2.
The diagram below is a Pourbaix diagram for Cu-Fe-S-H2O
(IJNM 07(02).9.23).
It shows the relationship between copper Cu,
chalcopyrite CuFeS2,
tenorite CuO,
covellite CuS,
cuprite Cu2O,
chalcocite Cu2S,
pyrite FeS2 and
hematite Fe2O3.
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