Sharp colorless crystals of the rare mineral thermassaite from the famous La Fossa Crater locality in Eolian Islands (Sicily, Italy). — Photo: David Hospital · CC BY-SA 4.0
Thermessaite
Thermessaite is an IMA-recognized mineral species in the sulfate class, with the chemical formula K₂AlF₃(SO₄).
It crystallizes in the orthorhombic crystal system (space group Pbcn). It is a member of the Thermessaite structural group.
Key properties: specific gravity 2.77; White streak; Vitreous luster.
IMA symbol Tms, IMA status: approved, named in 2007.
Found in altered pyroclastic breccias in an active fumarole (300-350° C).
It is found associated with Alunite, sassolite, anhydrite, metavoltine.
The name refers to Thermessa (warm island), an ancient Greek name for the island of Vulcano.