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

Authors: Raade, G.; Grice, J.D.; Erambert, M.; Kristiansson, P.; Witzke, T.

Source: Mineralogical Magazine, Volume 72, Number 5, October 2008 , pp. 1071-1082(12)

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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: (Y_3.70REE_7.54Ca_1.55Na_1.16Mn_0.77Th_0.10Pb₀₀₁)_Σ14.83(Fe²⁺_0.83Mn_0.15Ti_0.02)_Σ1.00Ca_1.00(P_0.70Si_0.26As_0.04)_Σ1.00Si_6.05B_3.20(O_34.55F_13.45)_Σ48. 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 [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 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 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

Document Type: Research article

DOI: http://dx.doi.org/10.1180/minmag.2008.072.5.1071

Publication date: 2008-10-01

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