Alum-(K)

Alum-(K)
Octahedral crystal of potassium alum
General
Category	Sulfate mineral
Formula (repeating unit)	KAl(SO4)2·12(H2O)
Strunz classification	07.CC.20
Crystal symmetry	Isometric diploidal H-M symbol: (2/m3) Space group: P a3
Unit cell	a = 12.133; Z = 4
Identification
Color	Colorless, white
Crystal habit	Stalactitic, columnar, granular, massive efflorescences; rare as small octahedral crystals
Crystal system	Cubic
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

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.

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

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.

Occurrences include Mount Vesuvius, Italy and Alum Cave, Sevier County, Tennessee.