Abstract

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: (Y3.70REE7.54Ca1.55Na1.16Mn0.77Th0.10Pb0.01)∑14.83 (

\(\mathrm{Fe}_{0.83}^{2+}\)
Mn0.15Ti0.02)∑1.00Ca1.00 (P0.70Si0.26As0.04)∑1.00Si6.05B3.20(O34.55F13.45)∑48. Boron was analysed with a nuclear microprobe method based on the nuclear reaction 11B(p,α)2α. 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)Å3, 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 [dobs 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 ω 1.734(2) and ϵ 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 Dcalc = 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.

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