ALUNOGEN

    Class : Sulphates, chromates, molybdates
    Subclass : Hydrated sulphates
    Crystal System : Triclinic
    Chemistry : Al2(SO4)3 17H2O
    Rarity : Uncommon

Alunogen is an hydrated aluminum sulphate, it is above all a mineral of arid regions where its formation results from the reaction of sulfuric acid formed by oxidation of pyrite on aluminous rocks. It forms efflorescences, fibrous encrustations or rare and small prismatic crystals of pseudohexagonal appearance, in the oxidation zones of sulphide deposits. This mineral has been known since Antiquity, its name comes from the Latin alumen (alun) and the Greek genos (origin), in reference to its use as a source of alum. It is also found in coal mines, where it is formed by the same process by the destruction of pyrite, as well as in fumaroles and solfataras. It is a theoretically colorless mineral, but usually tinted yellow or red by impurities.

Alunogen in the World

Very soluble, alunogen does not keep well on outcrops. It is a mineral that was abundant at Alum Mountain (New Mexico) where it formed crusts exceeding one meter thick, and at Smoky Mount (North Carolina). Alunogen is common in volcanic emanations from Pozzuoli, Miseno and Vesuvius in Italy. It is also known in Chile, Slovakia, and various other world sites.

Photo on the right: alunogen from the Handlová Coal Mine (Slovakia) © Pavol Myslan

Alunogen in France

In France, alunogen is reported near St-Nazaire (Loire-Atlantique), at Capdenac (Lot), in the Aure valley (Hautes-Pyrénées), in the coal basin of Decazeville (Aveyron) and at Ronchamp (Haute-Saône), etc...

Twinning

This species twins with {010}, but they are not visible to the naked eye.

Fakes and treatments

No fake reported for this species.



Hardness : 1.5 to 2
Density : Not measured
Fracture : Fibrous
Trace : White




TP : Translucent to transparent
RI : 1.473 to 1.480
Birefringence : 0.007
Optical character : Biaxial +
Pleochroism : None
Fluorescence : None


Solubility : Water

Magnetism : None
Radioactivity : None