Alunogen

Alunogen (from French alun, "alum"), also called feather alum and hair salt is a colourless to white (although often coloured by impurities, such as iron substituting for aluminium) fibrous to needle-like aluminium sulfate mineral. It has the chemical formula Al₂(SO₄)₃·17H₂O.

Alunogen
Alunogen
General
Category	Sulfate minerals
Formula (repeating unit)	Al2(SO4)3·17H2O
IMA symbol	Alg
Strunz classification	7.CB.45
Crystal system	Triclinic
Crystal class	Pinacoidal (1) (same H-M symbol)
Space group	P1
Unit cell	a = 7.42, b = 26.97 c = 6.062 [Å]; α = 89.95° β = 97.566°, γ = 91.888°; Z = 2
Identification
Colour	Colourless; 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
Optical properties	Biaxial (+)
Refractive index	nα = 1.473 nβ = 1.474 nγ = 1.480
Birefringence	δ = 0.007
2V angle	Measured: 31 to 69°
References

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 as a low temperature deposit in fumaroles. It occurs associated with pyrite, marcasite, halotrichite, pickeringite, epsomite, potash alum, melanterite and gypsum.

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 bound (crystallization) form.