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

Synthetic sodium saponites, Na_xMg₃(Si_4–xAl_x)O₁₀(OH)₂·nH₂O, with 0.33 ≤ x ≤ 1, and trioctahedral sodium micas series, Na(Mg_3–yAl_y)(Si_3–yAl_1+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.