Adranosite-(Fe), (NH4)4NaFe2(SO4)4Cl(OH)2, a new ammonium sulfate chloride from La Fossa Crater, Vulcano, Aeolian Islands, Italy

Abstract

The new mineral adranosite-(Fe), ideally (NH4)4NaFe2(SO4)4Cl(OH)2, is the Fe3+-analogue of adranosite. It was found on a pyroclastic breccia in two different fumaroles at “La Fossa” crater of Vulcano, Aeolian Islands, Italy, and corresponds to an anthropogenic product previously observed in a burning coal dump at the Anna mine, near Aachen, Germany. The mineral is tetragonal, space group I 41/ acd (no. 142), with a = 18.261(2), c = 11.562(1) A, V = 3855.5(7) A3 (single-crystal data), and Z = 8. The six strongest reflections in the X-ray powder diffraction pattern are [ dobs in A ( I )( hkl )]: 9.134(100)(020), 4.569(83)(040), 3.047(79)(152), 6.462(36)(220), 3.232(29)(251), and 2.891(11)(004). The average chemical composition of the holotype is (wt.%): Na2O 5.01, Fe2O3 15.77, Al2O3 5.11, K2O 0.82, (NH4)2O 15.76, SO3 50.96, Cl 3.71, H2O 2.75, –O=Cl−0.84, total 99.05; the corresponding empirical formula is: [(NH4)3.89K0.11]∑4.00Na1.04[Fe1.27Al0.64]∑1.91S4.10O16.40Cl0.67(OH)1.96. Adranosite-(Fe) forms aggregates of pale yellow acicular crystals up to 1 mm in length, the most common forms most probably being {100}, {110}, and {111}. The measured density is 2.18(1) g/cm3, and the calculated density is 2.195 g/cm3. Adranosite-(Fe) is uniaxial (−) with ω=1.58(1), ɛ=1.57(1) (λ = 589 nm). Using single-crystal X-ray diffraction data from the holotype, the structure was refined to a final R ( F )= 0.0415 for 670 independent observed reflections [ I > 2(σ( I )]. Adranosite-(Fe) is isostructural with its Al-analogue adranosite and contains NaO4Cl2 square tetragonal bipyramids, linked through their opposite Cl corners and helicoidal chains with composition [FeO4(OH)2SO4] n , both extending along [001]. The framework resulting from the sharing of the sulfate ions between the different chains displays cages in which the nine-coordinated hydrogen-bonded NH4+ ions are hosted.