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Aphthitalite, collected from Ghom Salt Dome, Qom Province, Iran
CategorySulfate mineral
(repeating unit)
Strunz classification7.AC.35
Crystal systemTrigonal
Crystal classHexagonal scalenohedral (3m)
H-M symbol: (3 2/m)
Space groupP3m1 (no. 164)
Unit cella = 5.67, c = 7.33 [Å]; Z = 1
ColorWhite, colorless; gray, blue, green due to inclusions and impurities
Crystal habitTabular crystals (with distorted pseudo-orthorhombic habit); as bladed aggregates and in crusts
TwinningOn {0001} or repeated on {1120}
CleavageFair on {1010}, poor on {0001}
FractureConchoidal to uneven
Mohs scale hardness3
LusterVitreous to resinous
DiaphaneityTransparent to opaque
Specific gravity2.66–2.71
Optical propertiesUniaxial (+) (anomalously biaxial)
Refractive indexnω = 1.487 - 1.491 nε = 1.492 - 1.499
Birefringenceδ = 0.005
SolubilityIn water

Aphthitalite (IMA symbol: Att[4]) is a potassium sulfate mineral with the chemical formula: (K,Na)3Na(SO4)2.

It was first described in 1835 for an occurrence on Mount Vesuvius, Italy. The name is from the Greek άφθητος, "unalterable", and άλας, "salt", for its stability in air.[1] It occurs as fumarolic incrustations in volcanic environments, as small crystals and masses in evaporite deposits and in guano deposits.[2][3] It occurs associated with thenardite, jarosite, sylvite and hematite in fumaroles; with blodite, syngenite, mirabilite, picromerite, borax and halite in evaporites; and with syngenite, whitlockite, monetite, niter and gypsum in guano deposits.[3]


  1. ^ a b Mindat.org
  2. ^ a b Webmineral data
  3. ^ a b c Handbook of Mineralogy
  4. ^ Warr, L.N. (2021). "IMA-CNMNC approved mineral symbols". Mineralogical Magazine. 85: 291–320.