Abstract

Biraite-(Ce), ideally Ce2Fe2+(CO3)(Si2O7), has been found in the Biraia deposit (Irkutsk district, Russia), associated with cordylite-(Ce) and -(La), aragonite, strontianite, Sr,Fe- bearing dolomite, ancylite-(Ce) and -(La), hydroxy1-bastnäsite-(Ce), daqingshanite-(Ce) and -(La), tremolite, winchite, ferriallanite-(Ce), törnebohmite-(Ce), cerite, chevkinite-(Ce), belkovite, humite, fergusonite-(Ce) and -(Nd), pyrochlore, barite, monazite-(Ce) and other unknown minerals. Biraite-(Ce) occurs as irregular to well-shaped grains from 0.1 to 3 mm in length, has brown colour with a white streak, is semi-transparent with a vitreous luster and brittle.The hardness (Mohs) is 5, and the calculated density is 4.76 g/cm3. Optically, biraite-(Ce) is biaxial (-), with α 1.785(1),β 1.810(2),γ 1.820(1), 2V 66°(1). Electron microprobe and wet chemical analyses gave the following empirical formula based on 10 O+F:(Ce1.01La0.57Nd0.25 Pr0.09Sm0.02Ca0.07 Na0.02Ba0.01)Σ=2.04(Fe0.60Mg0.25Mn0.11Ti0.01)Σ=0.97 (CO3)0.99 [Si1.97(O6.87F0.17)Σ=7.04].The simplified formula is Ce2Fe2+(CO3)(Si2 O7). The IR spectrum confirmed the presence of [CO3] groups. The strongest lines of the X-ray powder pattern [d in Å (I) (hkl)] are: 3.30 (5) (021), 2.92 (10) (006, 21–2), 2.65 (5) (202, 12–4), 2.23 (5) (116, 031). Biraite-(Ce) is monoclinic, space group P21/c, with a 6.505(7), b 6.744(2), c 18.561(4) Å,β 108.75(2)°. Its crystal structure was refined from single-crystal X-ray diffraction data to R(F) = 0.033. Biraite-(Ce) displays a new structure type, based on polyhedral (001) sheets composed of pairs of edge-sharing [FeO6] octahedra, [Si2O7] groups, and [CO3] triangles. Ce3+ cations in ten-fold coordination provide the linkage between neighbour polyhedral sheets. Both the mineral and its name have been approved by the IMA Commission on New Minerals and Mineral Names.

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