Influence of charge location on 29Si NMR chemical shift of 2:1 phyllosilicates

A series of synthetic sodium-rich saponites and trioctahedral Na, K, and Ba mica solid solutions have been investigated by 29Si MAS-NMR spectroscopy. The salient result is the major effect of the interlayer charge on 29Si chemical shift variations detected in NMR components ascribed to tetrahedral Si3, Si2Al, SiAl2, and Al3 environments. In this analysis, the effect of the octahedral charge is considerably lower. The contribution of the interlayer charge is much more important than the previously reported contribution of the ditrigonal distortion of tetrahedral layers, generated by the misÞ t between tetrahedral and octahedral layers. This observation rests on the peculiar disposition of tetrahedra in phyllosilicates, with three out of four oxygen atoms exposed to the interlamellar space. A similar effect could be operative in other silicates, like zeolites, in which most of tetrahedra interact with charge compensating cations.