Influence of hydration on 23Na, 27Al, and 29Si MAS-NMR spectra of sodium saponites and sodium micas

Abstract

Synthetic sodium saponites, NaxMg3(Si4–xAlx)O10(OH)2·nH2O, with 0.33 ≤ x ≤ 1, and trioctahedral sodium micas series, Na(Mg3–yAly)(Si3–yAl1+y)O10(OH)2·nH2O, with 0 ≤ y ≤ 1, have been investigated by MAS-NMR spectroscopy. The presence of anhydrous, one-layer and two-layer hydrates, deduced by X-ray diffraction, has been associated with specific lines detected in 23Na MAS-NMR spectra. In these phyllosilicates, the location of tetra- and octahedral charge has been analyzed by 27Al MAS-NMR spectroscopy. The salient result is the major effect of the interlayer charge on 29Si chemical shift of the four NMR components ascribed to Si3, Si2Al, SiAl2, and Al3 environments. This effect is much more important than the most commonly accepted contribution of the ditrigonal distortion of tetrahedral sheets. In saponites, 29Si MAS-NMR spectra change considerably with the sodium hydration. In dehydrated samples, where Na cations are engaged in two pseudo-hexagonal cavities, 29Si MAS-NMR components split as a consequence of the partial sodium occupancy of three neighboring hexagonal rings that surrounds a particular tetrahedron. In hydrated samples, where Na+ cations interact with water, chemical shifts of resolved components are averaged as a consequence of interlayer water and cation mobilities.