| Aphthitalite | |
| Category: | Sulfate mineral |
| Formula: | (K,Na)3Na(SO4)2 |
| Imasymbol: | Att[1] |
| Strunz: | 7.AC.35 |
| 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 or repeated on |
| Cleavage: | Fair on, poor on |
| Fracture: | Conchoidal to uneven |
| Tenacity: | Brittle |
| Mohs: | 3 |
| Luster: | Vitreous to resinous |
| Diaphaneity: | Transparent to opaque |
| Gravity: | 2.66–2.71 |
| Opticalprop: | Uniaxial (+) (anomalously biaxial) |
| Refractive: | nω = 1.487 - 1.491 nε = 1.492 - 1.499 |
| Birefringence: | δ = 0.005 |
| Solubility: | In water |
| References: | [2] [3] [4] |
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.[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]