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Formula: Fe(SO4).7H2O
Hydrated normal sulphate, melanterite group
Varieties
Pisanite is a copper-bearing variety of melanterite
Crystal System: Monoclinic
Specific gravity: 1.89 measured, 1.893 calculated
Hardness: 2
Streak: White
Colour: Colourless to white or green, also greenish-blue to blue with increased copper content, colourless to pale green in
transmitted light. Usually a yellowish-white after exposure to air and moisture.
Solubility: Readily soluble in water
Common impurities: Cu,Mg
Environments:
Melanterite is a secondary mineral formed by the oxidation of
pyrite, marcasite
and other iron sulphides due to the action of surface waters. It is often found in mines as a post-mining
formation on mine walls. Other sulphates such as halotrichite,
chalcanthite and rozenite are characteristic
accompanying minerals, others include epsomite and
pickeringite.
Melanterite may dehydrate to siderotil or rozenite
(Mindat, HOM).
Localities
At the Sunny Corner Mine, Sunny Corner, Roxburgh county, New South Wales, Australia, melanterite has been found associated
with chalcanthite, siderotil and minor amounts
of römerite
(AJM 17.1.49).
At the Brownley Hill mine, Nenthead, Alston Moor, Eden, Cumbria, England, UK, melanterite occurs as masses of curved capillary
crystals commonly associated with jarosite and gypsum.
It typically occurs in shale, and appears to have formed by post-mining oxidation of
pyrite
(Minrec 31.3.247).
At the Parys Mountain mines, Amlwch, Isle of Anglesey, Wales, UK, the vivid blue-green copper-bearing melanterite variety
pisanite is abundant, but when exposed to the air it rapidly decays to a white powder
(Minrec 42.4.379).
At the San Rafael Mine, Quartz Mountain camp, Lodi Mining District, Lodi Hills, Nye county, Nevada, USA, masses to 1 cm in thickness
of slightly fibrous melanterite occur as a postmining efflorescence, and somewhat less commonly in areas of heavy
epsomite formation upon underlying primary
sulphide vein minerals
(R&M 85.6.519).
Alteration
pyrite (primary) O and
H2O to
secondary melanterite and sulphuric acid.
FeS2 + 7O + 8H2O → FeSO4.7H2O + H2SO4
Melanterite indicates the presence of sulphuric acid, and it should be handled with care.
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