NUCLEAR MAGNETIC RESONANCE STUDY OF Al:Si AND F:OH ORDER IN ZUNYITE

Zunyite [Al13Si5O20(OH,F)18Cl] possesses an unusual combination of an Al13O16(OH)24 Keggin structure and a Si5O16 pentamer unit. We used 19F, 27Al, 29Si MAS and 27Al 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 (CQ) and asymmetry parameter ( ) obtained from simulations of 27Al 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 29Si, 27Al and 19F 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 AlO4OHF octahedra.