• journal_title：Mineralogical Magazine
• Contributor：G. Raade ; J. D. Grice ; M. Erambert ; P. Kristiansson ; T. Witzke
• Publisher：Mineralogical Society of Great Britain and Ireland
• Date：2008-
• Format：text/html
• Language：en
• Identifier：10.1180/minmag.2008.072.5.1071
• journal_abbrev：Mineralogical Magazine
• issn：0026-461X
• volume：72
• issue：5
• firstpage：1071
• section：Articles

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,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)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 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.