alum-(K)

{{infobox mineral | name = Alum-(K) | image = Potassium alum octahedral like crystal.jpg | imagesize = 260px | alt = | caption = Octahedral crystal of potassium alum | category = Sulfate mineral | formula = KAl(SO4)2·12(H2O) |IMAsymbol=Aum-K | molweight = | strunz = 7.CC.20 | dana = | system = Cubic | class = Diploidal (m) H-M symbol: (2/m ) | symmetry = Pa | unit cell = a = 12.133 Å; Z = 4 | color = Colorless, white | habit = Stalactitic, columnar, granular, massive efflorescences; rare as small octahedral crystals | twinning = Rare on {111} | cleavage = On {111} indistinct | fracture = Conchoidal | tenacity = | mohs = 2-2.5 | luster = Vitreous | streak = | diaphaneity = Transparent | gravity = 1.757 | density = | polish = | opticalprop = Isotropic | refractive = n=1.453 | birefringence = | pleochroism = | 2V = | dispersion = | extinction = | length fast/slow = | fluorescence = | absorption = | melt = | fusibility = | diagnostic = | solubility = Water soluble | impurities = | alteration = | other = | 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.