PANICHIITE, NATURAL AMMONIUM HEXACHLOROSTANNATE(IV), (NH4)2SnCl6, FROM LA FOSSA CRATER, VULCANO, AEOLIAN ISLANDS, ITALY

Panichiite, ammonium hexachlorostannate(IV), (NH4)2SnCl6, is a new mineral species found in a medium-temperature intracrater fumarole (~250°C) at La Fossa crater, Vulcano Island, Aeolian Islands, Sicily, Italy. The mineral occurs as aggregates of minute colorless octahedra up to 0.2 mm in diameter on fragments of altered pyroclastic breccia, in association with alunite, anhydrite, bismuthinite, godovikovite, demicheleite-(Br) (BiSBr), BiSCl (IMA 2008–020) and two other new species, a sodium ammonium aluminum sulfate chloride (IMA 2008–057) and an ammonium bismuth chloride that is presently under study. Panichiite is cubic, space group Fm3m (no. 225), with Z = 4; the unit-cell parameter a is 10.064(1) Å, and V is equal to 1019.4(2) Å3. The X-ray powder-diffraction pattern is in excellent agreement with that of the synthetic compound; the strongest six reflections [dobs in Å(I)(hkl)] are: 5.811(100)(111), 5.032(73)(200), 2.516(69)(400), 3.035(48)(311), 1.779(42)(440), and 2.250(39)(420). Chemical analyses obtained with an electron microprobe in energy-dispersion mode gave, on average, Cl 56.57, Sn 31.62, Br 1.53, K 1.05, total 90.77, with NH4+ (by difference) 9.23, for a total of 100.00 wt.%, corresponding to the empirical formula [(NH4)1.90K0.10]S2.00Sn0.99(Cl5.94Br0.07)S6.01. The calculated density for the compound with the above empirical formula is 2.423 g/cm3. The index of refraction n obtained using immersion methods is 1.68(1) (l = 589 nm). The structure, including the positions of the hydrogen atoms of the ammonium ion, was refined from single-crystal diffraction data to a final R of 0.030 for 153 independent observed reflections [I > 2s(I)]. The six equivalent Sn–Cl distances in the octahedral SnCl62– ion are 2.429(1) Å, and the Cl...N distances are 3.559(1) Å. The mineral (IMA 2008–005) is named in honor of Prof. Ugo Panichi, a pioneer in the study of minerals from Vulcano.