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Biraite-(Ce), Ce₂Fe ²⁺(CO₃)(Si₂O₇), a new mineral from Siberia with a novel structure type

¹ Institute of the Earth's Crust, Siberian Branch of the Russian Academy of Sciences, Lermontova 128,Irkutsk 664033, Russia
² Dipartimento di Scienze della Terra, Università di Pisa, Via S. Maria 53,I–56126 Pisa, Italy
³ Dipartment of Geology, Moscow State University, Moscow 119992, Russia
⁴ Institute of Geochemistry, Siberian Branch of the Russian Academy of Sciences, P.O. Box 4019,Irkutsk 664033, Russia
⁵ Institute of Problems of Chemical Physics, Russian Academy of Sciences, Chernogolovka,Moscow district 142432, Russia

^* E-mail: pasero{at}dst.unipi.it

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 1.785(1),ß 1.810(2), 1.820(1), 2V 66°(1). Electron microprobe and wet chemical analyses gave the following empirical formula based on 10 O+F:(Ce_1.01La_0.57Nd_0.25 Pr_0.09Sm_0.02Ca_0.07 Na_0.02Ba_0.01)=_2.04(Fe_0.60Mg_0.25Mn_0.11Ti_0.01)=0.97 (CO₃)_0.99 [Si_1.97(O_6.87F_0.17)=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,ß 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.

Key-words: biraite-(Ce), new mineral, crystal structure, alkaline pegmatites, Biraia carbonatite dyke.

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