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Alunogen - Almyras, Agia Varvara, Cyprus.jpg
Category Sulfate minerals
(repeating unit)
Strunz classification 07.CB.45
Crystal symmetry Triclinic pinacoidal
H-M symbol (1)
Space group: P1
Unit cell a = 7.42 Å, b = 26.97 Å, c = 6.062 Å; α = 89.95°, β = 97.566°, γ = 91.888°; Z=2
Colour Colorless; white, pale yellow to red from impurities
Crystal habit Platy to prismatic crystals rare, fibrous masses, crusts, and efflorescences
Crystal system Triclinic
Twinning On {010}
Cleavage Perfect on {010}, probable on {100} and {313}
Fracture Subconchoidal
Mohs scale hardness 1.5 -2
Luster 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 [1][2][3]

Alunogen 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.[1][2]

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.[1] It occurs associated with pyrite, marcasite, halotrichite, pickeringite, epsomite, potash alum, melanterite and gypsum.[1]

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.[citation needed] The second form can be easily removed just by slight grinding of the mineral (preserving the unique structure).