Wiklundite, ideally Pb2
[4](Mn2+,Zn)3(Fe3+,Mn2+)2(Mn2+,Mg)19(As3+O3)2[(Si,As5+)O4]6(OH)18Cl6,
isa new arseno-silicate mineral from Långban, Filipstad, Värmland, 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 andparwelite.
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=Cl2 –1.50, total 97.11 wt.%, As valences and H2O content
taken from the crystal-structure refinement, and
Fe3+/(Fe2+ + Fe3+) determined by
Mössbauer spectroscopy. Wiklundite is hexagonal-rhombohedral, space group
R3c, a = 8.257(2),
c = 126.59(4) Å, V = 7474(6) Å3,
Z = 6. The crystal structure of wiklundite was 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 layerof corner-sharing
(SiO4) and (Mn2+O4) tetrahedra; and one
layer of (Mn2+Cl6) octahedra and
(Pb2+(OH)3Cl6) polyhedra. The mineral is
named after Markus Wiklund (b. 1969) and Stefan Wiklund
(b. 1972), the well-known Swedish mineral collectors who
jointly found the specimen containing the mineral.