SOLVING THE CRYSTAL STRUCTURE OF SYNTHETIC NACAPHITE DIMORPH FROM POWDER X-RAY DATA USING A POLYSOMATIC APPROACH

E. Sokolova¹, Yu. Kabalov¹, E. Gobechiya¹, J. Schneider², and A. Khomyakov<sup>3

1</sup>Department of Crystallography, Faculty of Geology, Moscow State University, Vorob'evy Gory, 119899 Moscow, Russia
²Institute fr Kristallographie und Angewandte Mineralogie Universitt Mnchen, Theresienstrae 41, D-80333 Muenchen, Germany
³Institute of Mineralogy, Geochemistry, and Crystal Chemistry of Rare-Earth Elements, Veresaeva Ul. 15, 121357 Moscow, Russia

Keywords: Rietveld refinement, powder diffraction, polysomatism, phosphate minerals

Crystal structure of a synthetic compound Na₂Ca[PO₄]F obtained from a melt of Ca₅[PO₄]₃F - NaF at 770^oC [1] has been solved by Rietveld refinement from X-ray powder data: a focusing STOE X-ray diffractometer, MoK_a1, 0.3mm capillary, step/time of 0.02^o/10s, 5.0<2 q<54.98^o, 560 Bragg reflections, Wyriet, version 3.3 Rietveld program [2]. This compound was named "synthetic nacaphite dimorph" with a formula of (Na,Ca)₄(Ca,Na)[PO₄]₅F₅. It is of a hexagonal symmetry, a=7.01790(7), c=40.5653(4), sp. gr. R 3m, z=3, _calc.=2.88gcm^-3, R_p=0.029, R_wp=0.039, R_B=0.048, R_F=0.054, s=2.14.

Two initial models of nacaphite dimorph structure were proposed basing on polysomatic relationships between crystal structures of alkaline phosphates from the Khibina massif, Kola Peninsula: nacaphite Na(Na,Ca)₂[PO₄]F (a=10.654(4), b=24.443(6), c=7.102(2)Ä, a=89.99(2), b=90.01(2), g=90.01(2)^o, sp.gr. C1, z=16) [3] and arctite (Na₅Ca)Ca₆Ba(PO₄)₆F₃ (a=7.094(1), c=41.32(1) Ä, sp. gr. R3m, z=1)[4]. In the nacaphite structure, (Na,Ca)-polyhedra and [PO₄]-tetrahedra constitute a framework. Symmetry of nacaphite structure is triclinic, its pseudosymmetry is hexagonal when seen along the a axis. Two types of modules have been established in the nacaphite structure: (NI) of triclinic (pseudoorthorhombic) symmetry and (NII) of pseudohexagonal symmetry [5]. The latter one has dimensions of 7.1 x 7.1 x 5.3Ä, 120, 90, 90^o. Crystal structure of arctite consists of three different modules: one of them being equivalent to NII module and two of them being original. Let's call one of them NIII module. Similarity of the unit-cell parameters of synthetic nacaphite dimorph and arctite allowed us to propose two initial models of a synthetic nacaphite dimorph structure. These models differ in stacking modes of modules NII and NIII and space groups: (1) R¯3m and (2) R 3m.

In a powder sample, quantitative analysis revealed 15.9(1) wt. % of NaF, 8.11(7) wt. % of CaF₂, 3.4(1) wt. % of Ca₅(PO₄)₃F, and 72.5(4) wt. % of Na₂Ca[PO₄]F. Multiphase Rietveld refinement testified to the second structural model of synthetic nacaphite dimorph (sp. gr. R 3m). Polymorphism of Na₂Ca[PO₄]F has been established: nacaphite and synthetic nacaphite dimorph demonstrate different degrees of ordering due to the formation temperature.

1. E.A. Kirova, L.V.Delitsina, T.V. Zubkova et al.: Mineralogicheskii J., 9 (1987), 15 - 22.
2. J. Schneider, Int.Workshop on the Rietveld Method, Petten 1989, 48.
3. E.V. Sokolova, Yu.K. Egorov-Tismenko, and A.P. Khomyakov: Dokl. Akad. Nauk SSSR, 304 (1989), 610 - 615. 4. E.V. Sokolova, N.A. Yamnova, Yu.K. Egorov-Tismenko et al.: Dokl. Akad. Nauk SSSR, 274 (1984), 78 - 83.
5. E.V. Sokolova, A.P. Khomyakov, G. Ferraris: Abstracts, 1^st EMU School on Modular Aspects of Minerals, 40, Budapest, Hungary, 1997.