Alunogen also called feather alum and hair salt is a colourless to white (although often coloured by impurities, like iron substituting for aluminum) fibrous to needle-like aluminium

Aluminium

Aluminium or aluminum is a silvery white member of the boron group of chemical elements. It has the symbol Al, and its atomic number is 13. It is not soluble in water under normal circumstances....

 sulfate

Sulfate

In inorganic chemistry, a sulfate is a salt of sulfuric acid.-Chemical properties:...

 mineral

Mineral

A mineral is a naturally occurring solid chemical substance formed through biogeochemical processes, having characteristic chemical composition, highly ordered atomic structure, and specific physical properties. By comparison, a rock is an aggregate of minerals and/or mineraloids and does not...

 often found on the walls of mines and quarries as 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

Millosevichite

Millosevichite is a rare mineral with the chemical formula Al23. Aluminium is often substituted by iron. It forms white to yellowish cryptocrystalline, often porous, masses. The mineral is mainly known from burning coal dumps, acting as one of the main minerals forming sulfate crust. It can be also...

hydratation).

Alunogen has the chemical formula Al₂(SO₄)₃·17H₂O. More precise, 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.
The second form can be easily removed just by slight grinding of the mineral (by preserving the unique structure).