Jarosite - Encyclopedia

    Class : Sulfates, chromates, molybdates
    Subclass : Anhydrous sulfates
    Crystal system : Trigonal
    Chemistry : KFe3(SO4)2(OH)6
    Rarity : Common

Jarosite is a secondary mineral very common in the oxidation zone of deposits containing iron, in an arid climate. With other iron sulfates (copiapite, voltaite, römerite, amarantite, botryogen, fibroferrite, etc...), it belongs to a classic mineralogical procession resulting like it from precipitation from concentrated acid solutions (natural sulfuric acid from of the reaction of surface water with iron sulfides). By changing environmental conditions (temperature, pH, redox potential, etc...) these sulfates are easily hydrolyzed into ferric hydroxides (goethite and limonite). Jarosite, sulfates, and iron hydroxides are particularly common in gossans of pyrite clusters and pyrite-rich hydrothermal veins. It is also found in certain industrial slags. Its name derives from its place of discovery : Barranco Jaroso (Spain). Jarosite occurs quite often in encrustations, in compact granular, concretionary or earthy masses, rarely in flattened and very small rhombohedral crystals (3 mm maximum). It is ocher-yellow to dark brown in color, rarely gray-yellow.

Main photo : Jarosite from Grand Central Mine, Arizona, USA © Stephan Wolfsried

Jarosite from Mina Santa Barbara, Almeria, Spain © Christian Rewitzer
Jarosite from María Josefa Mine, Almeria, Spain © Christian Rewitzer
Jarosite from Jaroso Ravine Ravine, Almeria, Spain © Juan Miguel Segura
Jarosite from Echassières, Allier, France © Michael Förch

Jarosite in the World

The best collector's items come from the type locality of Barranco Jaroso near Almeria (Spain) and the Greek mines of Laurion. Jarosite is abundant in earth masses in the sulphide deposits of Rio Tinto (Andalusia). In the Bisbee district of Arizona, jarosite was particularly abundant, notably at the Southwest mine, where masses of several thousand tons were extracted. Masses of comparable sizes were also extracted in Chuquicamata (Chile).

Jarosite in France

In France, only the concretioned masses of St-Félix-de-Pallières constituted interesting samples. However, it is known in micro-crystals in many localities such as Echassières (Allier), Lantignié (Rhône), Valcroze (Gard), Chessy (Rhône), Les Rompas (Nièvre), etc...


No twin known for this mineral species.

Fakes and treatments

No fakes recorded for this mineral species.

Hardness : 2.5 to 3.5
Density : 2.9 to 3.26
Fracture : Irregular to conchoidal
Streak : Pale yellow

TP : Translucent to transparent
RI : 1.713 to 1.820
Birefringence : 0.102 to 0.105
Optical character : Uniaxial -
Pleochroism : Visible
Fluorescence : None

Solubility : Hydrochloric acid

Magnetism : NoneRadioactivity : None


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