A neutron-diffraction study of the crystal structure of analcime, NaAlSi2O6· H2O

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Abstract
The crystal structure of the mineral analcime, NaAlSi 2 O 6 · H 2 O, known from x-ray analysis to approximate closely to the cubic space-group symmetry Ia 3 d , has been examined by neutron diffraction. Intensity data from a single crystal were collected for one octant so as to be representative of cubic and some lower symmetries. In general, the systematic absences were consistent with the space group Ia 3 d ; within experimental error, the six reflections equivalent for Laue group m 3 m were equal in intensity. From Fourier difference syntheses and least-squares refinements, it is concluded that the neutron data, like the x-ray data, are consistent with an averaged Ia 3 d structure. Full-matrix least-squares refinement with the constraints required by the Ia 3 d space group gave a final R value of 0.044 over 194 independent reflections. Hydrogen atoms are distributed in 32 e and 96 h positions and are not engaged in hydrogen bonds. Apparent values for the H–O–H angle probably depend on the averaging of arrangements for the water molecules; interpretations are suggested in terms of static and dynamic models.

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