Coquimbite - Encyclopedia

    Class : Sulfates, chromates, molybdates
    Subclass : Hydrated sulfates
    Crystal system : Rhombohedral
    Chemistry : Fe2(SO4)3 9H2O
    Rarity : Rare to uncommon

Coquimbite is a rare iron sulphate formed by alteration of iron sulfides in an arid climate. It is a secondary mineral which is part of a classic mineralogical procession to which other iron sulfates belong (copiapite, voltaite, jarosite, amarantite, botryogen, fibroferrite, etc...), resulting like it from precipitation from “natural” sulfuric acid formed by reaction of surface water with iron sulphides. By changing environmental conditions (temperature, pH, redox potential) these sulfates are easily hydrolyzed into ferric hydroxides (goethite and limonite). Its name is linked to its supposed discovery location : the province of Coquimbo (Chile). Coquimbite forms prismatic crystals with a hexagonal outline, purple in color, sometimes greenish or greenish yellow, exceptionally purplish red.

Main photo : Coquimbite from Monte Arsiccio Mine, Toscane, Italie © Quebul Fine Minerals

Coquimbite in the World

Coquimbite is known in beautiful samples in the large copper deposits of Northern Chile (Tierra Amarilla, Chuquicamata), in the underground pyrite mines of the Rio Tinto region in Spain (Rio Tinto, Concepcion...), as well as in several mines in the western American states (Dexter No. 7, Utah), and in Cyprus (Skouriotissa). The best crystals certainly come from Monte Arsiccio Mine (Tuscany, Italy) and the Javier Mine (Peru).

Photo on the right : Coquimbite from Monte Arsiccio Mine, Toscane, Italy © Gianfranco Ciccolini

Coquimbite in France

In France, coquimbite is reported in Fontsante (Var), Jas Roux near Gap (Hautes-Alpes) and Vigan (Gard).

Twinning

No twin known for this mineral species.

Fakes and treatments

No fake identified for this mineral species.


Hardness : 2.5
Density : 2.11
Fracture : Sub-conchoidal
Streak : White

TP : Transparent
RI : 1.539 to 1.548
Birefringence : 0.009
Optical character : Uniaxial +
Pleochroism : None
Fluorescence : None

Solubility : Water and acids


Magnetism : None
Radioactivity : None

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