Acmonidesite, a new ammonium sulfate chloride from La Fossa crater, Vulcano, Aeolian Islands, Italy

摘要

The new mineral acmonidesite, (NH4,K,Pb2+,Na)(9)Fe-4(2+)(SO4)(5)Cl-8, was found in an active fumarole (fumarole FA, temperature similar to 250 degrees C) at La Fossa crater, Vulcano, Aeolian Islands, Sicily, Italy. It occurs on a pyroclastic breccia as brown prismatic crystals up to 0.10 mm long, in association with salammoniac, alunite and adranosite. The mineral is orthorhombic, space group C222(1) (no. 20) with a = 9.841(1), b = 19.448(3) c = 17.847(3) angstrom, V = 3415.7(9) angstrom(3) and Z = 4. The six strongest reflections in the powder X-ray diffraction pattern are: [d(obs) in angstrom(I)(hkl)] 8.766(100)(110), 1.805(88)(390), 5.178(45)(131), 4.250(42)(221), 2.926(42)(330) and 2.684(32)(261). The empirical formula (based on 28 anions per formula unit [pfu]) is (NH4)(5.77)K1.42Pb0.62Na1.24Fe3.96Mn0.08S5.04O20.16Cl7.97Br0.08. The idealised formula is (NH4,K,Pb2+,Na)(9)Fe-4(2+)(SO4)(5)Cl-8. The calculated density is 2.551 g cm(-3). Using single-crystal diffraction data, the structure was refined to a final R(F) = 0.0363 for 4614 independent observed reflections [I > 2 sigma(I)]. The structure contains two independent, distorted octahedral iron sites, Fe1 and Fe2, with the iron atoms in the 2+ oxidation state, as confirmed by the interatomic distances and bond-valence calculations (2.06 and 1.94 vu, respectively). Fe1 is surrounded by two chlorine atoms and four oxygens of the sulfate ions, with the following average distances (angstrom): Fe1-O 2.125 and Fe1-Cl 2.472; and Fe2 is surrounded by three chlorine atoms and three oxygens of the sulfate ions, with the following average distances (angstrom): Fe2-O 2.110 and Fe2-Cl 2.531. Three independent sulfate anions are also present and are connected with the iron polyhedra to form a three-dimensional structure containing voids occupied by four independent ammonium ions (two of them partially replaced by K+), one Na+/Pb2+ site and one Cl- ion.