Specific gravity: 2.185 calculated
Colour: Colourless to white
At the type locality, the La Fossa crater, Vulcano Island, Lipari, Eolie Islands, Messina Province, Sicily, Italy, thermessaite-(NH4) is found as a medium to high temperature (~250–300°C) fumarole encrustation at the rim of the crater. It deposited as aggregates of minute (<0.2 mm) sharp prismatic crystals on the surface of a pyroclastic breccia in association with thermessaite, görgeyite, sulphur, arcanite, mascagnite and intermediate members of the arcanite–mascagnite series (MM 85.5.665-672, Mindat).
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