Alunogen also called feather alum and hair salt is a colorless to white (although often colored by impurities, such as iron substituting for aluminum) fibrous to needle-like aluminum sulfate mineral. It has the chemical formula Al2(SO4)3·17H2O. The crystallochemical formula can be written as: [Al(H2O)6]2(SO4)3.5H2O. The second formula shows that H2O in the alunogen formula occurs both as ligand (coordinative form) and loosely bounded (crystallization) form.

General Information:

  • Category: Sulfate minerals
  • Formula: Al2(SO4)317H2O
  • Crystal system: Triclinic
  • Transparency: Transparent.

It is often found on the walls of mines and quarries as a secondary mineral. It can be found in the oxidation zones of some ore deposits as well as on burning coal dumps (i.e., as the product of millosevichite hydration). It also forms a low-temperature deposit in fumaroles. It occurs associated with pyrite, marcasite, halotrichite, pickeringite, epsomite, potash alum, melanterite, and gypsum.


  • Color: Colorless; white, pale yellow to red from impurities
  • Crystal habit: Platy to prismatic crystals rare, fibrous masses, crusts, and efflorescences
  • Twinning: On {010}
  • Cleavage: Perfect on {010}, probable on {100} and {313}
  • Fracture: Subconchoidal
  • Mohs scale hardness: 1.5 -2
  • Lustre: Vitreous to silky
  • Diaphaneity: Transparent
  • Specific gravity: 1.72 – 1.77.


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