Leucite-Pollucite Structure-Type Variability and the Structure of a Synthetic End-Member Calcium Wairakite (CaAl2Si4O12·2H2O)

C. M. B. Henderson, A. M. T. Bell1, S. C. Kohn2 and C. S. Page3
Department of Earth Sciences, University of Manchester, Manchester M13 9PL, and Daresbury Laboratory, Warrington WA4 4AD
Daresbury Laboratory, Warrington WA4 4AD
Department of Physics, University of Warwick, Coventry CV4 7AL
Department of Physics, King's College, University of London, London WC2R 2LS, UK
1Department of Chemistry, University of Cambridge, Lensfield Road, Cambridge CB2 1EW
2Department of Geology, University of Bristol, Bristol BS8 1RJ
3Department of Chemistry, Imperial College of Science, Technology and Medicine, University of London, London SW7 2AY

Abstract: The structure of a synthetic end-member wairakite (CaAl2Si4O12·2H2O) has been determined using Rietveld analysis of high-resolution, synchotron X-ray powder diffraction data, and 29Si and 27Al 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. 29Si MAS NMR data are consistent with about 80% of the Si occupying tetrahedral T11 and T12 sites linked to two Al atoms [Q4(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.

Keywords: wairakite • leucite • pollucite • Rietveld analysis • crystal structure

Mineralogical Magazine; April 1998 v. 62; no. 2; p. 165-178; DOI: 10.1180/002646198547558
© 1998, The Mineralogical Society
Mineralogical Society (www.minersoc.org)