Alum-(K)

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Alum-(K)
Potassium alum octahedral like crystal.jpg
Octahedral crystal of potassium alum
General
CategorySulfate mineral
Formula
(repeating unit)
KAl(SO4)2·12(H2O)
Strunz classification7.CC.20
Crystal systemCubic
Crystal classDiploidal (m3)
H-M symbol: (2/m 3)
Space groupPa3
Unit cella = 12.133 Å; Z = 4
Identification
ColorColorless, white
Crystal habitStalactitic, columnar, granular, massive efflorescences; rare as small octahedral crystals
TwinningRare on {111}
CleavageOn {111} indistinct
FractureConchoidal
Mohs scale hardness2-2.5
LusterVitreous
DiaphaneityTransparent
Specific gravity1.757
Optical propertiesIsotropic
Refractive indexn=1.453
SolubilityWater soluble
References[1][2][3]

Alum-(K) is a hydrous potassium aluminium sulfate mineral with formula KAl(SO4)2·12(H2O). It is the mineral form of potassium alum and is referred to as potassium alum in older sources. It is a member of the alum group.[1]

It occurs as colorless to white, soft isometric crystals and efflorescence coatings.[2] Rare crystals are octahedral in form if occurring as precipitates from neutral water solution, but cubic in form if the solution is alkaline.[1]

It occurs as a precipitate around volcanic fumaroles and solfataras. It also occurs as an alteration in argillaceous sediments or coal beds which contain oxidizing sulfide minerals (pyrite or marcasite). Occurs associated with alunogen, pickeringite, epsomite, melanterite, gypsum and native sulfur.[2]

Occurrences include Mount Vesuvius, Italy and Alum Cave, Sevier County, Tennessee.[2][3]

References[edit]