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

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Marco E. Ciriotti
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Isolueshite from the Kovdor carbonatite, Kola peninsula, Russia: composition, crystal structure and formation scenarios

Messaggio da Marco E. Ciriotti » gio 20 apr, 2017 21:38

Prossima pubblicazione.

Referenza:
▪ Zaitsev, A.N., Zhitova, E.S., Spratt, J., Zolotarev, A.A., Krivovichev, S.V. (2017): Isolueshite, NaNbO3, from the Kovdor carbonatite, Kola peninsula, Russia: composition, crystal structure and possible formation scenarios. Neues Jahrbuch für Mineralogie - Abhandlungen: Journal of Mineralogy and Geochemistry, 194, (in press).

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 (Na0.84Ca0.07Sr0.01La0.01Ce0.01)Σ0.95(Nb0.90Ti0.11)Σ1.01O3. 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.% ThO2), Fe (0.1– 0.2 wt.% Fe2O3) and Ta (0.1 wt.% Ta2O5). The crystal structure of isolueshite was refined to an agreement index (R1) of 0.028 for 82 unique reflections with |F0| ≥ 4 σ(F). The mineral is cubic, Pm3-m, a = 3.9045(5) Å and V = 59.525(13) Å3. 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.
Marco E. Ciriotti

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