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Proshchenkoite-(Y) from Russia - a new mineral species in the vicanite group: descriptive data and crystal structure

G. Raade^1,,*, J. D. Grice², M. Erambert³, P. Kristiansson⁴ and T. Witzke⁵

¹ Department of Geology, Natural History Museum, University of Oslo, P.O. Box 1172, Blindern, NO-0318 Oslo, Norway
² Research Division, Canadian Museum of Nature, P.O. Box 3443, Station D, Ottawa, Ontario K1P 6P4, Canada
³ Department of Geosciences, University of Oslo, P.O. Box 1047, Blindern, NO-0316 Oslo, Norway
⁴ Department of Nuclear Physics, Lund Institute of Technology, Box 118, SE-221 00 Lund, Sweden
⁵ Institut für Chemie/Technische Chemie, Universität Rostock, Albert-Einstein-Strasse 3a, DE-18059 Rostock, Germany

^* E-mail: gunn-ra{at}online.no

A REE-bearing fluorosilicate from the Tommot REE-Nb deposit in Yakutia, Russia, described without a name in 1966, is characterized here as a new species, proshchenkoite-(Y), of the vicanite group of borosilicates. Wavelength-dispersive electron probe analyses gave the following empirical formula: (Y_3.70REE_7.54Ca_1.55Na_1.16Mn_0.77Th_0.10Pb_0.01)_14.83 (Mn_0.15Ti_0.02)_1.00Ca_1.00 (P_0.70Si_0.26As_0.04)_1.00Si_6.05B_3.20(O_34.55F_13.45)₄₈. Boron was analysed with a nuclear microprobe method based on the nuclear reaction ¹¹B(p,)2. The simplified formula is (Y,REE,Ca,Na,Mn)₁₅ (Fe²⁺,Mn)Ca(P,Si)Si₆B₃O₃₄F₁₄. The mineral is trigonal, R3m, with a = 10.7527(7)Å, c = 27.4002(18) Å, V = 2743.6(6)ų, Z = 3. The crystal structure was refined to R1 = 0.042 for 1819 observed reflections. Proshchenkoite-(Y) is isostructural with okanoganite-(Y), vicanite-(Ce) and hundholmenite-(Y), and the differences in site occupancies are discussed. The strongest six reflections of the X-ray powder-diffraction pattern [d_obs in Å, (I), (hkl)] are: 4.441, (36), (202); 3.144, (77), (214); 3.028, (45), (009); 2.968, (100), (027); 1.782, (32), (330); and 1.713, (32), (1.2.14). The mineral is optically uniaxial (—) with 1.734(2) and 1.728(2). The Mohs hardness is about 5; density measured on material subject to incipient metamictization is 4.72 g/cm³, as compared to D_calc = 4.955 g/cm³.

The result of electron microprobe analyses of alleged okanoganite-(Y) from the type locality in Okanogan County, Washington, USA, is also presented. We find here also that P > Si at one of the sites, whereas the analytical data of Boiocchi et al. (2004) indicate Si > P. Consequently, the mineral we have analysed is the P analogue of okanoganite-(Y), another new species.

KEYWORDS: proshchenkoite-(Y), new mineral species, okanoganite-(Y), vicanite group, REE borosilicate, electron-microprobe data, nuclear microprobe data, crystal structure, Tommot REE-Nb deposit, Russia