Ulfanderssonite-(Ce), a new Cl-bearing REE silicate mineral species from the Malmkärra mine, Norberg, Sweden

Ulfanderssonite-(Ce) is a new mineral (IMA 2016-107) from the long-abandoned Malmkärra iron mine, one of the Bastnäs-type Fe-rare earth element (REE) deposits in the Bergslagen ore region, central Sweden. It is named for Ulf B. Andersson, a Swedish geologist and petrologist. In the type specimen, the mineral occurs with västmanlandite-(Ce), bastnäsite-(Ce), phlogopite, talc, magnetite, pyrite, fluorbritholite-(Ce) and scheelite. Ulfanderssonite-(Ce) forms pinkish, translucent subhedral grains, 100–300 μm, in aggregates up to 2 mm. Fracture is uneven, and there is an indistinct cleavage parallel (001). Mohs' hardness is 5–6, D_calc = 4.97 g cm⁻³. Optically, ulfanderssonite-(Ce) is nonpleochroic, biaxial negative, with 2V_meas = 55° and n_calc = 1.82. The ideal composition is Ce₁₅CaMg₂(SiO₄)₁₀(SiO₃OH)(OH,F)₅Cl₃. Electron microprobe and LA-ICP-MS chemical analyses yielded (in wt%) La₂O₃ 11.87, Ce₂O₃ 30.98, Pr₂O₃ 3.99, Nd₂O₃ 17.14, Sm₂O₃ 2.81, Eu₂O₃ 0.18, Gd₂O₃ 1.15, Dy₂O₃ 0.30, Tb₂O₃ 0.10, Y₂O₃ 1.11, CaO 2.26, FeO 0.02, MgO 1.97, P₂O₅ 0.08, SiO₂ 19.13, H₂O_calc 1.07, F 1.09, Cl 2.89, O=(F, Cl) −1.10, sum 97.04. The five strongest powder X-ray diffraction lines are [I (%) d_obs (Å) (hkl)]: 100 2.948 (-421), 47 2.923 (204), 32 2.660 (-225), 26 3.524 (40-1), 25 1.7601 (6-23). Ulfanderssonite-(Ce) is monoclinic, Cm, with a = 14.1403(8), b = 10.7430(7), c = 15.498(1) Å, β = 106.615(6)° and V = 2256.0(2) ų for Z = 2. The crystal structure has been solved by direct methods and refined to R₁ = 2.97% for 5280 observed reflections. It consists of a regular alternation of two layers, designated A and B, along the c-axis: A (ca. 9 Å thickness), with composition [(Ce₈Ca)MgSi₇O₂₂(OH,F)₄]⁸⁺, and B (ca. 6.5 Å), with composition [Ce₇MgSi₄O₂₁(OH,F)₂Cl₃]⁸⁻; the A layer is topologically and chemically closely related to cerite-(Ce). A FTIR spectrum shows strong absorption in the region 2850–3650 cm⁻¹, related to the presence of O–H stretching bands. Ulfanderssonite-(Ce) is interpreted as a primary mineral at the deposit, along with the more common fluorbritholite-(Ce), formed by a magmatic-hydrothermal fluid with REE, Si, F and Cl ion complexes reacting with dolomite marble. The presence of ulfanderssonite-(Ce) is direct evidence of a Cl-rich mineral-forming aqueous solution, normally not reflected in the composition of skarn minerals in Bastnäs-type deposits.