Meionite: Rietveld structure-refinement, Si-29 MAS and Al-27 SATRAS NMR spectroscopy, and comments on the marialtte-meionite series

Rietveld structure refinement, Si-29 magic angle spinning (MAS) and Al-27 s atellite transition (SATRAS) NMR spectroscopic data are given for three nat ural samples of meionite and for synthetic end-member meionite. Trends in s tructural parameters and NMR spectra are discussed for the solid-solution s eries from marialite to meionite. The a cell dimension and Al-27 SATRAS NMR spectra show a continuous change through the series, whereas the space gro up, c cell dimension and Si-29 MAS NMR spectra show different trends for th e three subseries, Si9.0-8.3, Si8.3-7.4 and Si7.4-6.0 Over the interval Si9 .0-8.3, the space group is I4/m, the c cell dimension increases with Al con tent, and 29Si spectra show multiple well-resolved peaks; the anion site is occupied by Cl-, and the T(1) site, by Si. Si-Al order between the T(2) an d T(3) sites causes the space group for Si8.3-7.4 to be P4(2)/n and the Si- 29 MAS NMR spectra to be dominated by S(1) (3SilAl) and Si(3) (1Si3Al) peak s. Here, there is a negative correlation between c cell dimension and Al co ntent. Over the interval Si7.4-6.01 the c cell parameter increases with Al content, the space group is I4/m, and multiple 29Si MAS NMR peaks become br oad and poorly resolved. Ca2+ continues to replace monovalent cations in th e M site, and Al enters both the T(1) and T(2) sites, producing Al-O-Al bon ds. In the natural end-member meionite, however, the Al-Si order is found t o be 3:1 for T(1) and 3:5 for T(2).