Thermessaite-(NH4), ideally (NH4)2AlF3(SO4), is a new mineral found as a medium- to high-temperature (~250–300°C) fumarole encrustation at the rim of La Fossa crater, Vulcano, Aeolian Islands, Italy. The mineral deposited as aggregates of minute (<0.2 mm) sharp prismatic crystals on the surface of a pyroclastic breccia in association with thermessaite, sulfur, arcanite, mascagnite, and intermediate members of the arcanite–mascagnite series.

The new mineral is colourless to white, transparent, non-fluorescent, has a vitreous lustre, and a white streak. The calculated density is 2.185 g/cm3. Thermessaite-(NH4) is orthorhombic, space group Pbcn, with a = 11.3005(3) Å, b = 8.6125(3) Å, c = 6.8501(2) Å, V = 666.69(4) Å3 and Z = 4. The eight strongest reflections in the powder X-ray diffraction data [d in Å (I)(hkl)] are: 5.65 (100)(200), 4.84 (89)(111), 6.85 (74)(110), 3.06 (56)(112), 3.06 (53)(221), 3.08 (47)(311), 2.68 (28)(022) and 2.78 (26)(130). The average chemical composition, determined by quantitative SEM-EDS (N by difference), is (wt.%): K2O 3.38, Al2O3 25.35, SO3 36.58, F 26.12, (NH4)2O 22.47, O = F –11.00, total 102.90. The empirical chemical formula, calculated on the basis of 7 anions per formula unit, is [(NH4)1.85K0.15]Σ2.00Al1.06F2.94S0.98O3.06. The crystal structure, determined from single-crystal X-ray diffraction data [R(F) = 0.0367], is characterised by corner-sharing AlF4O2 octahedra which form [001] octahedral chains by sharing two trans fluoride atoms [Al–F2 = 1.8394(6) Å]. Non-bridging Al–F1 distances are shorter [1.756(1) Å]. The two trans oxygen atoms [Al–O = 1.920(2) Å] are from SO4 tetrahedra. NH4+ ions occur in layers parallel to (100) which alternate regularly with (100) layers containing ribbons of corner-sharing AlF4O2 octahedra and associated SO4 groups. The NH4+ ions are surrounded by five oxygen atoms and by four fluorine atoms. The mineral is named as the (NH4)-analogue of thermessaite, K2AlF3(SO4), and corresponds to an anthropogenic phase found in the burning Anna I coal dump of the Anna mine, Aachen, Germany. Both mineral and mineral name have been approved by the International Mineralogical Association Commission on New Minerals, Nomenclature and Classification (IMA2011-077).

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