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.

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