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Category Sulfate minerals, alum series
(repeating unit)
Strunz classification 7.CC.15
Dana classification
Crystal system Monoclinic
Crystal class Prismatic (2/m)
(same H-M symbol)
Space group C2/c
Formula mass 440.26 g/mol
Color colorless
Cleavage {100} good
{001} indistinct
{010} indistinct
Mohs scale hardness 3
Luster vitreous
Streak white
Diaphaneity transparent to translucent
Density 1.74 g/cm3
Refractive index nα = 1.449
nβ = 1.461
nγ = 1.463
Birefringence δ = 0.014
2V angle 56° (measured)
Solubility soluble
Alters to tamarugite
References [1][2][3][4]

Mendozite is a sulfate mineral, one of the alum series, with formula NaAl(SO4)2·11H2O. It is a hydrated form of sodium aluminium sulfate (soda alum).

It was discovered in western Argentina in 1868, probably near San Juan. The exact location has been lost, but was described as "San Juan, near Mendoza", and it is the latter city that give the mineral its name.[2] It occurs in evaporites, presumably from the oxidation of sulfide minerals in the presence of clays.[2] It is very soluble in water, and so can only be found in dry regions: however, in can still effloresce (lose water of crystallisation) in extremely arid climates, altering to tamarugite (the hexahydrate).[3]


  1. ^ Mineralienatlas
  2. ^ a b c Mendozite,, retrieved 2009-11-28 .
  3. ^ a b Mendozite,, retrieved 2009-11-28 .
  4. ^ Fang, J. H.; Robinson, P. D. (1972), "Crystal structures and mineral chemistry of double-salt hydrates: II. The crystal structure of mendozite, NaAl(SO4)2·11H2O", American Mineralogist, 57: 1081–88 .