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Aphthitalite, collected from Ghom Salt Dome, Qom Province, Iran
Category Sulfate mineral
(repeating unit)
Strunz classification 7.AC.35
Crystal system Trigonal
Crystal class Hexagonal scalenohedral (3m)
H-M symbol: (3 2/m)
Space group P3m
Unit cell a = 5.67, c = 7.33 [Å]; Z = 1
Color White, colorless; gray, blue, green due to inclusions and impurities
Crystal habit Tabular crystals (with distorted pseudo-orthorhombic habit); as bladed aggregates and in crusts
Twinning On {0001} or repeated on {1120}
Cleavage Fair on {1010}, poor on {0001}
Fracture Conchoidal to uneven
Tenacity Brittle
Mohs scale hardness 3
Luster Vitreous to resinous
Diaphaneity Transparent to opaque
Specific gravity 2.66–2.71
Optical properties Uniaxial (+) (anomalously biaxial)
Refractive index nω = 1.487 - 1.491 nε = 1.492 - 1.499
Birefringence δ = 0.005
Solubility In water
References [1][2][3]

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 Mt. 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]