Chemistry:Alunogen

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Alunogen
Alunogen - Almyras, Agia Varvara, Cyprus.jpg
Alunogen
General
CategorySulfate minerals
Formula
(repeating unit)
Al2(SO4)3·17H2O
Strunz classification7.CB.45
Crystal systemTriclinic
Crystal classPinacoidal (1)
(same H-M symbol)
Space groupP1
Unit cella = 7.42, b = 26.97
c = 6.062 [Å]; α = 89.95°
β = 97.566°, γ = 91.888°; Z = 2
Identification
ColourColourless; white, pale yellow to red from impurities
Crystal habitPlaty to prismatic crystals rare, fibrous masses, crusts, and efflorescences
TwinningOn {010}
CleavagePerfect on {010}, probable on {100} and {313}
FractureSubconchoidal
Mohs scale hardness1.5–2
Vitreous to silky|re|er}}Vitreous to silky
DiaphaneityTransparent
Specific gravity1.72–1.77
Optical propertiesBiaxial (+)
Refractive indexnα = 1.473 nβ = 1.474 nγ = 1.480
Birefringenceδ = 0.007
2V angleMeasured: 31 to 69°
References[1][2][3]

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.[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 bound (crystallization) form.[5][6]

References