Västmanlandite-(Ce) - a new lanthanide- and F-bearing sorosilicate mineral from Västmanland, Sweden : description, crystal structure, and relation to gatelite-(Ce)

Västmanlandite-(Ce), ideally (Ce,La)₃CaAl₂Mg₂[Si₂O₇][SiO₄]₃F(OH)₂, is a new mineral species from the Västmanland county, Bergslagen region, Sweden. Together with more Fe-rich, F-poor members, it forms solid solutions that are important for lanthanide sequestration in the Bastnäs-type skarn deposits in Västmanland. At the type locality (Malmkärra, Norberg district) it occurs as anhedral grains 0.2–3 mm across, in association with fluorbritholite-(Ce), tremolite, a serpentine mineral, magnetite, dollaseite-(Ce) and dolomite. The mineral is allanite-like in appearance; it is dark brown, translucent, with vitreous luster, and has good cleavage parallel to {001}, uneven to conchoidal fracture, and a yellowish gray streak; D_calc = 4.51(2) g·cm⁻³ and Mohs hardness ≈ 6. Optically it is biaxial (-), with α = 1.781 (4), β_calc = 1.792, γ = 1.810(4) and 2V_α = 75(5)°. Chemical analysis by electron-microprobe and ⁵⁷Fe Mössbauer spectroscopy yield: La₂O₃ 13.65, Ce₂O₃ 23.90, Pr₂O₃ 2.07, Nd₂O₃ 6.28, Sm₂O₃ 0.42, Gd₂O₃ 0.15, Y₂O₃ 0.18, CaO 4.65, FeO 1.14, Fe₂O₃ 2.69, MgO 5.51, Al₂O₃ 8.58, TiO₂ 0.04, P₂O₅ 0.05, SiO₂ 26.61, F 1.06, H₂O_calc 1.61, O ≡ F −0.45, sum 98.14. Västmanlandite-(Ce) is monoclinic, P2₁/m, with a = 8.939(1), b = 5.706(1), c = 15.855(2) Å, β = 94.58(1)°, V = 806.1(2) ų, and Z = 2 (single-crystal data). The eight strongest lines in the X-ray powder diffraction pattern are [d (in Å) (I/I₀)(hkl)]: 15.81(16)(001̄), 4.65(10)(111̄), 3.50(20)(210), 2.983(100)(014), 2.685(13)(121̄), 2.678(11)(205̄), 2.625(19)(311̄), and 2.187(15)(402̄). The crystal structure was determined using single-crystal X-ray diffraction data and refined to R1(F) = 1.73% for 2864 'observed' reflections with F_o > 4σ(F_o). Västmanlandite-(Ce) is nearly isotypic to gatelite-(Ce), ideally Ce₃CaAl₂AlMg[Si₂O₇][SiO₄]₃O(OH)₂, and can be described as a regular dollaseite-(Ce)-törnebohmite-(Ce) polysome. The occurrence of extremely weak, continuous streaking in the diffraction patterns, and the presence of two pairs of mirror-related atoms in the törnebohmite-(Ce) module indicate that the structure model represents an average structure, unlike the situation in gatelite-(Ce) where there are no continuous streaks but instead, due to atomic ordering, sharp superstructure reflections resulting in a different unit cell.