Solid-State Mas NMR-Study of Pentameric Aluminosilicate Groups wiht 180-Degrees Intertetrahedral AL-O-SI Angles in Zunyite and Harderite

The minerals zunyite, AL(13)Si(5)O(20)(OH,F)(18)Cl, and harkerite, Ca24Mg8[AlSi4(O,OH)(16)](2)- (CO3)(8)(BO3)(8)(H2O,Cl), have been studied by means of solid-state Al-27 MAS NMR. Zunyite contains Si5O16 pentamers and harkerite contains AlSi4(O,OH)(16) pentamers. These pentameric groups are unique because their T-O-Si angles are almost 180 degrees. Chemical analysis of the zunyite sample shows that it has excess Al: the A-Si ratio is 2.9, compared with the ideal of 2.6. High-speed spinning Al-27 MAS NMR spectra (11-13 kHz) showed two more signals than the spectra obtained by Kunwar et al. (1984). The signal with delta(iso) = 46.8 +/- 0.5 ppm represents the excess Al, which enters the central Si1 site of the Si5O16 pentamer. This assumption is confirmed by the fact that delta(iso) of Al in the AlSi4(O,OH)(16) pentamer in harkerite is 44 +/- 1 ppm. Additional proof comes from comparing the electrostatic energy and the quadrupole interaction of Al in either a Si1 or Si2 configuration. The Al site in the pentamers of zunyite and harkerite can be considered as a q(4)(4Si) site. In this case, Al-O-Si angles are correlated with the Al-27 chemical shift (Lippmaa et al., 1986). This correlation holds well for the Al-27 data for harkerite. The value for zunyite indicates that the structure adapts to the incorporation of Al in the Si1 site by a narrowing of the A1-O-Si angle to 171 +/- 2.5 degrees. The lower limit of the chemical shift range for Al in framework aluminosilicates is decreased by 12 ppm, from 55.8 ppm for mordenite to 44 ppm for harkerite