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

Ulfanderssonite-(Ce) is a new mineral (IMA 2016-107) from the long-abandoned Malmkarra iron mine, one of the Bastnas-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 vastmanlandite-(Ce), bastnasite-(Ce), phlogopite, talc, magnetite, pyrite, fluorbritholite-(Ce) and scheelite. Ulfanderssonite-(Ce) forms pinkish, translucent subhedral grains, 100-300 mu m, in aggregates up to 2 mm. Fracture is uneven, and there is an indistinct cleavage parallel (0 0 1). Mohs' hardness is 5-6, D (sub calc) = 4.97 g cm (super -3) Optically, ulfanderssonite-(Ce) is nonpleochroic, biaxial negative, with 2V (sub meas) = 55 degrees and n (sub calc) = 1.82. The ideal composition is Ce (sub 15) CaMg (sub 2) (SiO (sub 4) ) (sub 10) (SiO (sub 3) OH)(OH,F) (sub 5) Cl (sub 3) Electron microprobe and LA-ICP-MS chemical analyses yielded (in wt%) La (sub 2) O (sub 3) 11.87, Ce (sub 2) O (sub 3) 30.98, Pr (sub 2) O (sub 3) 3.99, Nd (sub 2) O (sub 3) 17.14, Sm (sub 2) O (sub 3) 2.81, Eu (sub 2) O (sub 3) 0.18, Gd (sub 2) O (sub 3) 1.15, Dy (sub 2) O (sub 3) 0.30, Tb (sub 2) O (sub 3) 0.10, Y (sub 2) O (sub 3) 1.11, CaO 2.26, FeO 0.02, MgO 1.97, P (sub 2) O (sub 5) 0.08 SiO (sub 2) 19.13, H (sub 2) O (sub 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 (sub obs) (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, beta = 106.615(6) degrees and V = 2256.0(2) A (super 3) for Z = 2. The crystal structure has been solved by direct methods and refined to R (sub 1) = 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 A thickness), with composition [(Ce (sub 8) Ca)MgSi (sub 7) O (sub 22) (OH,F) (sub 4) ] (super 8+) , and B (ca 6.5 A), with composition [Ce (sub 7) MgSi (sub 4) O (sub 21) (OH,F) (sub 2) Cl (sub 3) ] (super 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 (super -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 Bastnas-type deposits.