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. Khomyakov3
1Department 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 770oC [1] has been solved by Rietveld refinement from X-ray powder data: a focusing STOE X-ray diffractometer, MoKa1, 0.3mm capillary, step/time of 0.02o/10s, 5.0<2 q<54.98o, 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, Rp=0.029, Rwp=0.039, RB=0.048, RF=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, 90o. 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, 1st EMU School on Modular Aspects of
Minerals, 40, Budapest, Hungary, 1997.