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 Bastnästype 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, 100–300mm, in aggregates up to 2 mm. Fracture is uneven, and there is an indistinct cleavage parallel (001). Mohs’ hardness is 5–6, Dcalc = 4.97 g cm3. Optically, ulfanderssonite-(Ce) is nonpleochroic, biaxial negative, with 2Vmeas=55° and ncalc = 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, H2Ocalc 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 (%) dobs (A) (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)A, b = 106.615(6)° and V = 2256.0 (2)A 3 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. 9A thickness), with composition [(Ce8Ca) MgSi7O22(OH,F)4]8þ, and B (ca. 6.5A), 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.