Isolueshite, NaNbO3, from the Kovdor carbonatite, Kola peninsula, Russia: composition, crystal structure and possible formation scenarios

Isolueshite, a cubic complex oxide with the formula NaNbO3, occurs as euhedral crystals 0.4-0.7mm in size in calcite carbonatite, Kovdor ultrabasic-alkaline complex (Kola, Russia). Average composition of isolueshite, based on 40 analyses by wavelength-dispersive electron microprobe is (Na(0.84)Ca(0.07)Sr(0.01)La(0.01)Ceo(0.01))(Sigma 0.95)(Nb0.90Ti0.11)(Sigma 1.01)O-3. Minor and trace elements are Ti (4.1-6.8 wt.% TiO2), REEs (1.8-4.0 wt.% REE2O3), Ca (1.7-3.3 wt.% CaO), Zr (0.1-0.8 wt.% ZrO2), Sr (0.3-0.4 wt.% SrO), Th (0.1-0.5 wt.% TiO2), Fe (0.1-0.2 wt.% Fe2O3) and Ta (0.1 wt.% Ta2O5). The crystal structure of isolueshite was refined to an agreement index (R-1) of 0.028 for 82 unique reflections with vertical bar F-0 vertical bar > 4 sigma(F). The mineral is cubic, Pm (3) over barm, a= 3.9045(5) angstrom and V= 59.525(13) angstrom(3). The diffraction pattern of the crystal contains only regular and strong Bragg reflections with no signs of diffuse scattering. There are two sites in the crystal structure: A is 12-coordinated (A-O = 2.556(3) angstrom) and located at the corners of the cubic primitive cell and B is situated in the center of the unit-cell and has an octahedral coordination. The crystal-chemical formula based on the structure refinement is (Na0.84(1)Ca0.16(1))(Nb0.88(1)Ti0.12(1)O3. We suggest that isolueshite is a quenched (kinetically favored) polymorph of lueshite that formed as a result of rapid crystallization due to the sudden drop in temperature and/or pressure.