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

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)(Sigma 14.83)(Fe0.832+M0.15Ti0.02)(Sigma 1.00)Ca-1.00 (P0.70Si0.26As0.04)(Sigma 1.00)Si6.05B3.20(O34.55F13.45)(Sigma 48). Boron was analysed with a nuclear microprobe method based on the nuclear reaction B-11(p,alpha)2 alpha. 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) angstrom, c = 27.4002(18) angstrom, V = 2743.6(6) angstrom(3), 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 angstrom, (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 omega 1.734(2) and epsilon 1.728(2). The Mohs hardness is about 5; density measured on material subject to incipient metarnictization is 4.72 g/cm(3), as compared to D-calc = 4.955 g/cm(3). 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.