Långbanshyttanite, a new low-temperature arsenate mineral with a novel structure from Långban, Sweden

The new mineral långbanshyttanite was discovered in a specimen from the Långban mine (59.86°N, 14.27°E), Filipstad district, Värmland County, Bergslagen ore province, Sweden. Associated minerals are calcite, Mn-bearing phlogopite, spinels of the jacobsite-magnetite series, antigorite and trigonite. The mineral is named after the old name of the mine, smelter and mining village: Långbanshyttan. Långbanshyttanite is transparent, colourless. It occurs in late-stage fractures or corroded pockets, forming soft, radial and random aggregates (up to 1 mm) of acicular crystals up to 5 × 20 × 400 μm. D_calc is 3.951 g/cm³. The new mineral is biaxial (+), α = 1.700(5), β = 1.741(5), γ = 1.792(5), 2V (meas.) ≈ 90°, 2V (calc.) = 86°. Dispersion is strong, r < v. The IR spectrum is given. The chemical composition is (electron microprobe, mean of five analyses, wt%): PbO 44.71, MgO 3.79, MnO 13.34, FeO 1.89, P₂O₅ 0.65, As₂O₅ 22.90, H₂O (determined by gas chromatographic analysis of the products of ignition at 1200 °C) 14.4; total 101.68. The empirical formula based on 18 O atoms is: Pb_1.97Mn_1.85Mg_0.93Fe_0.26(AsO₄)_1.96(PO₄)_0.09(OH)_3.87·5.93H₂O. The simplified formula is: Pb₂Mn₂Mg(AsO₄)₂(OH)₄·6H₂O. Single-crystal diffraction data obtained using synchrotron radiation indicate that långbanshyttanite is triclinic, P1̅, a = 5.0528(10), b = 5.7671(6), c = 14.617(3) Å, α = 85.656(14), β = 82.029(17), γ = 88.728(13)°, V = 420.6(2) ų, Z = 1, and is a representative of a new structure type. In the structure, edge-sharing MnO₂(OH)₄ octahedra form zig-zag columns that are linked by isolated AsO₄ tetrahedra. Pb cations having six-fold coordination are located between the AsO₄ tetrahedra. Isolated Mg(H₂O)₆ octahedra are located in the inter-block space. The strongest lines of the powder diffraction pattern [d, Å (I, %) (hkl)] are: 14.48 (100) (001), 7.21 (43) (002), 4.969 (34) (100, 101), 4.798 (28) (003), 3.571 (54) (112, 1-1-1, 01–3, 11-1), 2.857 (45) (020, 021, 114), 2.800 (34) (11-3). Parts of the holotype specimen are deposited in the Fersman Mineralogical Museum of Russian Academy of Sciences, Moscow, Russia, with the registration number 4032/1 and in the collections of the Swedish Museum of Natural History, Stockholm, Sweden, under catalogue number NRM 20100076.