Aphthitalite

Aphthitalite
Aphthitalite, collected from Ghom Salt Dome, Qom Province, Iran
General
Category	Sulfate mineral
Formula	(K,Na)3Na(SO4)2
IMA symbol	Att
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

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 Ancient Greek ἄφθιτος, "unalterable", and ἅλς, "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.

References

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