Copiapite - Encyclopedia

    Class : Sulfates, chromates, molybdates
    Subclass : Hydrated sulfates
    Crystal system : Triclinic
    Chemistry : Fe5(SO4)6(OH)2 20H2O
    Rarity : Quite common


Copiapite is a sulfate of ferrous iron (Fe2+) and ferric iron (Fe3+). It is a fairly widespread secondary mineral, formed by alteration of iron sulfides, mainly in arid climates. It is part of a classic mineralogical procession to which other iron sulfates belong (melanterite, jarosite, coquimbite, botryogen, etc...). These sulfates, like it, result from precipitation from "natural" sulfuric acid, formed by reaction of surface water with iron sulfides ; by changing environmental conditions (temperature, pH, redox potential) they are easily hydrolyzed into ferric hydroxides (goethite and "limonites"). Copiapite takes its name from where it was discovered : Copiapo (Chile). It appears in the form of tabular crystals, often very modified, in loosely compact aggregates or in encrustations of sulfur yellow to golden yellow and orange color, becoming greenish-yellow to olive green in massive varieties.

Main photo : Copiapite and chalcanthite from Coronel Manuel Rodríguez mine, Antofagasta, Chili © Tony Peterson

Copiapite in the World

Among the many occurrences where copiapite is reported, the finest samples come from the large copper porphyries of Alcaparrosa (Copiapo Province, Chile) where it was first described, and from Quetena (Chuquicamata, Chile). It is also known on the Island of Elba (Italy), in Germany (Rammelsberg), as well as in several American mines in California and Nevada.

Right photo : Copiapite from Clara Mine, Germany © Stephan Wolfsried

Copiapite in France

In France, copiapite is known in the burning coal mines of Commentry (Allier) and Ricamarie (Loire), where it still comes from the late oxidation of iron sulfides mixed with coal debris. It is reported as a product of alteration of pyrite in numerous mines such as St-Laurent-le-Minier (Gard) or the Chizeuil mine (Saône-et-Loire).

Twinning

A contact twin with double axis [101] and composition plane {010} is known but not very common.

Fakes and treatments

No fake identified for this mineral species.



Hardness : 2.5 to 3
Density : 2.1
Fracture : Micaceous to irregular
Streak : White


TP : Transparent to opaque
RI : 1.506 to 1.600
Birefringence : 0.069
Optical character : Biaxial +
Pleochroism : None
Fluorescence : None


Solubility : Water


Magnetism : None
Radioactivity : None

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