Aphthitalite is a potassium sulfate mineral with the chemical formula: (K,Na)₃Na(SO₄)₂.

Aphthitalite
Aphthitalite, collected from Ghom Salt Dome, Qom Province, Iran
General
Category	Sulfate mineral
Formula (repeating unit)	(K,Na)3Na(SO4)2
IMA symbol	Att[1]
Strunz classification	7.AC.35
Crystal system	Trigonal
Crystal class	Hexagonal scalenohedral (3m) H-M symbol: (3 2/m)
Space group	P3m1 (no. 164)
Unit cell	a = 5.67, c = 7.33 [Å]; Z = 1
Identification
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	[2][3][4]

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.^[2] It occurs as fumarolic incrustations in volcanic environments, as small crystals and masses in evaporite deposits and in guano deposits.^[3][4] 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.^[4]