Original paper

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

Zaitsev, Anatoly N.; Zhitova, Elena S.; Spratt, John; Zolotarev, Anatoly A.; Krivovichev, Sergey V.

Abstract

Isolueshite, a cubic complex oxide with the formula NaNbO3, occurs as euhedral crystals 0.4–0.7 mm 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 (Na0.84 Ca0.07 Sr0.01 La0.01 Ce0.01)Σ0.95 (Nb0.90 Ti0.11)Σ1.01 O3. Minor and trace elements are Ti (4.1–6.8 wt.% TiO2), REEs (1.8–4.0 wt.% REE2 O3), 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.% ThO2), Fe (0.1–0.2 wt.% Fe2 O3) and Ta (0.1 wt.% Ta2 O5). The crystal structure of isolueshite was refined to an agreement index (R 1) of 0.028 for 82 unique reflections with | F 0 | ≥ 4 σ(F). The mineral is cubic, Pm 3m, a = 3.9045(5) Å and V = 59.525(13) Å 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) Å) 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.

Keywords

kovdorcarbonatiteperovskite groupelectron microprobe analysescrystal structureisolueshite