Wiklundite, ideally Pb2[4](Mn2+,Zn)3(Fe3+,Mn2+)2(Mn2+,Mg)19(As3+O3)2[(Si,As5+)O4]6(OH)18Cl6, a new mineral from Langban, Filipstad, Varmland, Sweden: Description and crystal structure

Wiklundite, ideally Pb-2([4])(Mn2+,Zn)(3)(Fe3+,Mn2+)(2)(Mn2+,Mg)(19)(As3+O3)(2)[(Si,As5+)O-4](6)(OH)(18)Cl-6, is a new arseno-silicate mineral from Langban, Filipstad, Varmland, Sweden. Both the mineral and the name have been approved by the Commission on New Minerals, Nomenclature and Classification of the International Mineralogical Association (IMA 2015-057). Wiklundite and a disordered wiklundite-like mineral form radiating, sheaf-like aggregates (up to 1 mm long) of thin brownish-red and slightly bent lath-shaped crystals. It occurs in a dolomite-rich skarn in association with tephroite, mimetite, turneaurite, johnbaumite, jacobsite, barite, native lead, filipstadite and parwelite. Wiklundite is reddish brown to dark brown, and the streak is pale yellowish brown. The lustre is resinous to sub-metallic, almost somewhat bronzy, and wiklundite does not fluoresce under ultraviolet light. The calculated density is 4.072 g cm(-3). Wiklundite is brittle with an irregular fracture, and has perfect cleavage on {001}; no parting or twinning was observed. Wiklundite is uniaxial (-), orange red and non-pleochroic in transmitted light, but shows incomplete extinction and distorted interference figures, preventing complete determination of optical properties. Electron-microprobe analysis (H2O calculated from the structure) of wiklundite gave SiO2 11.17, Al2O3 0.06, Fe2O3 4.46, As2O5 0.75, As2O3 6.81, MnO 47.89, ZnO 0.78, CaO 0.09, PbO 14.48, Cl 6.65, H2O 5.18, O= Cl-2-1.50, total 97.11 wt.%, As valences and H2O content taken from the crystal-structure refinement, and Fe3+/(Fe2+ + Fe3+) determined by Mossbauer spectroscopy. Wiklundite is hexagonal-rhombohedral, space group R (3) over barc, a = 8.257(2), c = 126.59(4) angstrom, V = 7474(6) angstrom(3), Z = 6. The crystal structure of wiklunditewas solved by direct methods and refined to a final R-1 index of 3.2%. The structure consists of a stacking of five layers of polyhedra: three layers consist of trimers of edge-sharing Mn2+-dominant octahedra linked by (SiO4) tetrahedra, (Fe3+(OH)(6)) dominant octahedra and (AsO3) triangular pyramids; one layer of corner-sharing (SiO4) and (Mn2+O4) tetrahedra; and one layer of (Mn2+Cl6) octahedra and (Pb2+ (OH)(3)Cl-6) polyhedra. The mineral is named after MarkusWiklund (b. 1969) and StefanWiklund (b. 1972), the well-known Swedish mineral collectors who jointly found the specimen containing the mineral.