Abstract
Zunyite [Al13Si5O20(OH,F)18Cl] possesses an unusual combination of an Al 13O16(OH)24 Keggin structure and a Si 5O16 pentamer unit. We used 19 F, 27 Al, 29 Si MAS and 27 Al MQMAS NMR spectroscopy of a series of samples with varying Al:Si and F:OH ratios to investigate the degree of atomic order in zunyite. The quadrupole coupling constant (C Q) and asymmetry parameter () obtained from simulations of 27 Al MAS and MQMAS spectra were compared with those derived from the electric field gradients (EFG), which were calculated from an ab initio cluster quantum model using the program Gaussian 98W. This comparison led to unequivocal assignments for 29 Si, 27 Al and 19 F MAS NMR peaks and showed that the number of Al substituting for Si(1) in the pentamer increases with Al content. The number of Al octahedra with one F atom increases with F content, whereas the number with two F atoms remains negligible, indicating that F preferentially occupies AlO 4OHF octahedra.
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