THERMESSAITE, K2[AlF3|SO4], A NEW INO-ALUMINOFLUORIDE–SULFATE FROM LA FOSSA CRATER, VULCANO, AEOLIAN ISLANDS, ITALY

Thermessaite, ideally K2[AlF3|SO4], is a new ino-aluminofluoride-sulfate found in active medium- to high-temperature fumaroles (300–350°C) at the rim of La Fossa crater, Vulcano Island, Aeolian archipelago, Sicily, Italy. The mineral occurs as sharp translucent crystals up to 0.25 mm in an altered pyroclastic breccia, together with alunite, sassolite, anhydrite and metavoltine. It is orthorhombic (mmm), space group Pbcn, with a 10.810(2), b 8.336(2), c 6.822(1) Å, V 614.8(2) Å3, Z = 4; the crystal habit is prismatic, with {110} and the pinacoid {100} as the dominant forms, terminated by minor faces of the bipyramid {111}.The mineral is colorless to white; the streak is white, the luster, vitreous, and the tenacity, brittle. It is nonfluorescent. Cleavage and fracture were not observed. The calculated density is 2.790 g/cm3, the observed density is 2.77(2) g/cm3, and the mean index of refraction is 1.445. The chemical analysis leads to the empirical formula K2.02[Al1.05F3.17S0.94O3.83], corresponding to the ideal formula K2[AlF3|SO4]. The structure was refined to a final R = 0.0286 for 685 observed reflections [I > 2s(I)]. It contains chains of corner-sharing AlF4O2 octahedra running along [001]; these octahedra share two vertices with SO4 tetrahedra [Al–F distances 1.752(1) 2 and 1.844(1) Å 2, and Al–O 1.906(1) Å 2]; this structural motif is new for aluminofluoride minerals. The K+ ions are surrounded by five oxygen atoms [<K–O> 2.642(2)–3.247(2) Å] and by four fluorine atoms [<K–F> 2.682(2)–2.926(2) Å]. The strongest six lines in the X-ray powder-diffraction pattern [dobs in Å(I)hkl] are: 2.983(100)(311), 2.702(82)(130), 6.631(70) (110), 1.712(58)(530), 2.208(30)(330) and 3.317(28)(310). The mineral is named after Thermessa (urmssa), an ancient Greek name of the Island of Vulcano meaning “the warm island”. Both the mineral and the mineral name have been approved by the IMA Commission on New Minerals, Nomenclature and Classification (No. 2007–030).