UNSPECIFIED. (1998) Leucite-pollucite structure-type variability and the structure of a synthetic end-member calcium wairakite (CaAl2Si4O12 center dot 2H(2)O). MINERALOGICAL MAGAZINE, 62 (2). pp. 165-178. ISSN 0026-461X

Abstract

The structure of a synthetic end-member wairakite (CaAl2Si4O12. 2H(2)O) has been determined using Rietveld analysis of high-resolution, synchrotron X-ray powder diffraction data, and Si-29 and Al-27 magic angle spinning nuclear magnetic resonance spectroscopy. The framework in the synthetic sample is more disordered than that in natural wairakite. Ca is distributed over the cavity cation sites M2: M12A, M12B in the approximate proportions 0.8:0.1:0.1, respectively, with M11 being vacant. Si-29 MAS NMR data are consistent with about 80% of the Si occupying tetrahedral T11 and T12 sites linked to two Al atoms [Q(4)(2Al) silicons]. Tetrahedral and cavity cation site disorder are coupled so that Al mainly occupies T2 sites, with Ca in M12A and M12B being balanced by Al in T12A and T12B; T11A and T11B sites appear to only contain Si, in agreement with the M11 site being vacant. The crystal chemistries of the wide range of stoichiometries which crystallize with the leucite/pollucite structure-type are also reviewed, with particular attention being paid to the tetrahedral ordering configurations present in these phases, and the implications to crystallographic phase transitions.

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