LEUCITE-POLLUCITE STRUCTURE-TYPE VARIABILITY AND THE STRUCTURE OF A SYNTHETIC END-MEMBER CALCIUM WAIRAKITE (CAAL2SI4O12-CENTER-DOT-2H(2)O)

The structure of a synthetic end-member wairakite (CaAl2Si4O12. 2H(2)O ) has been determined using Rietveld analysis of high-resolution, sync hrotron X-ray powder diffraction data, and Si-29 and Al-27 magic angle spinning nuclear magnetic resonance spectroscopy. The framework in th e synthetic sample is more disordered than that in natural wairakite. Ca is distributed over the cavity cation sites M2: M12A, M12B in the a pproximate proportions 0.8:0.1:0.1, respectively, with M11 being vacan t. Si-29 MAS NMR data are consistent with about 80% of the Si occupyin g tetrahedral T11 and T12 sites linked to two Al atoms [Q(4)(2Al) sili cons]. 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 leucit e/pollucite structure-type are also reviewed, with particular attentio n being paid to the tetrahedral ordering configurations present in the se phases, and the implications to crystallographic phase transitions.