aphthitalite

{{infobox mineral | name = Aphthitalite | image = Aphthitalite-180031.jpg | alt = | caption = Aphthitalite, collected from Ghom Salt Dome, Qom Province, Iran | category = Sulfate mineral | formula = (K,Na)3Na(SO4)2 |IMAsymbol=Att | molweight = | strunz = 7.AC.35 | dana = | system = Trigonal | class = Hexagonal scalenohedral (m) H-M symbol: ( 2/m) | symmetry = Pm1 (no. 164) | unit cell = a = 5.67, c = 7.33 [Å]; Z = 1 | color = White, colorless; gray, blue, green due to inclusions and impurities | habit = Tabular crystals (with distorted pseudo-orthorhombic habit); as bladed aggregates and in crusts | twinning = On {0001} or repeated on {110} | cleavage = Fair on {100}, poor on {0001} | fracture = Conchoidal to uneven | tenacity = Brittle | mohs = 3 | luster = Vitreous to resinous | streak = | diaphaneity = Transparent to opaque | gravity = 2.66–2.71 | density = | polish = | opticalprop = Uniaxial (+) (anomalously biaxial) | refractive = nω = 1.487 - 1.491 nε = 1.492 - 1.499 | birefringence = δ = 0.005 | pleochroism = | 2V = | dispersion = | extinction = | length fast/slow = | fluorescence = | absorption = | melt = | fusibility = | diagnostic = | solubility = In water | impurities = | alteration = | other = | references = }} Aphthitalite 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. It occurs as fumarolic incrustations in volcanic environments, as small crystals and masses in evaporite deposits and in guano deposits. It occurs associated with thenardite, jarosite, sylvite and hematite in fumaroles; with blödite, syngenite, mirabilite, picromerite, borax and halite in evaporites; and with syngenite, whitlockite, monetite, niter and gypsum in guano deposits.