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Potassium alum octahedral like crystal.jpg
Octahedral crystal of potassium alum
Category Sulfate mineral
(repeating unit)
Strunz classification 7.CC.20
Crystal system Cubic
Crystal class Diploidal (m3)
H-M symbol: (2/m 3)
Space group Pa3
Unit cell a = 12.133 Å; Z = 4
Color Colorless, white
Crystal habit Stalactitic, columnar, granular, massive efflorescences; rare as small octahedral crystals
Twinning Rare on {111}
Cleavage On {111} indistinct
Fracture Conchoidal
Mohs scale hardness 2-2.5
Luster Vitreous
Diaphaneity Transparent
Specific gravity 1.757
Optical properties Isotropic
Refractive index n=1.453
Solubility Water 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]