ARRHENIUSITE-(Ce), CaMg[(Ce7Y3)Ca5](SiO4)3(Si3B3O18)(AsO4)(BO3)F11, A NEW MEMBER OF THE VICANITE GROUP, FROM THE O¨ STANMOSSA MINE, NORBERG, SWEDEN

Arrheniusite-(Ce) is a new mineral (IMA 2019-086) from the ¨ Ostanmossa mine, one of the Bastn¨as-type deposits in the Bergslagen ore region, Sweden. It occurs in a metasomatic F-rich skarn, associated with dolomite, tremolite, talc, magnetite, calcite, pyrite, dollaseite-(Ce), parisite-(Ce), bastn¨asite-(Ce), fluorbritholite-(Ce), and gadolinite-(Nd). Arrheniusite-(Ce) forms anhedral, greenish-yellow translucent grains, exceptionally up to 0.8 mm in diameter. It is optically uniaxial (–), with x ¼ 1.750(5), e¼1.725(5), and non-pleochroic in thin section. The calculated density is 4.78(1) g/cm3. Arrheniusite-(Ce) is trigonal, space group R3m, with unit-cell parameters a¼10.8082(3) °A, c¼27.5196(9) ° A, and V¼2784.07(14) °A3 for Z¼3. The crystal structure was refined from X-ray diffraction data to R1 ¼ 3.85% for 2286 observed reflections [Fo . 4r(Fo)]. The empirical formula for the fragment used for the structural study, based on EPMA data and results from the structure refinement, is: (Ca0.65As3þ 0.35)R1(Mg0.57Fe2þ 0.30As5þ 0.10Al0.03)R1[(Ce2.24Nd2.13La0.86Gd0.74Sm0.71Pr0.37)R7.05(Y2.76Dy0.26Er0.11Tb0.08Tm0.01 Ho0.04Yb0.01)R3.27Ca4.14]R14.46(SiO4)3[(Si3.26B2.74)R6O17.31F0.69][(As5þ 0.65Si0.22P0.13)R1O4](B0.77O3)F11; the ideal formula obtained is CaMg[(Ce7Y3)Ca5](SiO4)3(Si3B3O18)(AsO4)(BO3)F11. Arrheniusite-(Ce) belongs to the vicanite group of minerals and is distinct from other isostructural members mainly by having a Mg-dominant, octahedrally coordinated site (M6); it can be considered a Mg-As analog to hundholmenite-(Y). The threefold coordinated T5 site is partly occupied by B, like in laptevite-(Ce) and vicanite-(Ce). The mineral name honors C.A. Arrhenius (1757–1824), a Swedish officer and chemist, who first discovered gadolinite-(Y) from the famous Ytterby pegmatite quarry.