Melanterite

minerals

siderotil

halotrichite

chalcanthite

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Formula: Fe(SO4).7H2O
Hydrated normal sulphate, melanterite group

Varieties

Pisanite is a copper-bearing variety of melanterite

Specific gravity: 1.89
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:

Hydrothermal 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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