a nuclear magnetic resonance study of hydrous layer silicates

Abstract

This is a study of five silicates, namely makatite, kanemite, octosilicate, magadiite and kenyaite. The silicates have been analysed using a range of techniques, principally solid-state NMR spectroscopy. High-quality (^29)Si NMR spectra have been obtained for samples of all five layered sodium polysilicate hydrates. Amongst other findings, these have revealed four crystallographically distinct sites in makatite and a Q(^4):Q(^4) site ratio in kenyaite of ca. 5. Proton MAS NMR studies can produce well-resolved spectra, particularly for carefully-dried samples. Distinct water and strongly hydrogen-bonded proton species have been detected. The latter are particularly noteworthy and they are present in kanemite, octosilicate, magadiite and kenyaite, but not in makatite or layered silicic acids. Interactions between the resolved proton species have been investigated with a series of 1- and 2-dimensional experiments resulting in the detection of mixing, via spin-diffusion or chemical exchange. Sodium-23 NMR studies were complicated by second-order effects from strong quadrupolar interactions and the presence of a significant signal from a sodium chloride contamination in many samples. The latter had fooled previous authors. Acidification products of kanemite, octosilicate and magadiite were characterised by (^29)Si CP NMR, thermogravimetric analysis, 1H MAS NMR and powder X-ray diffraction. A single H-kanemite sample proved to be H(_2)Si(_2)O(_5), but two types of H-octosilicate and H-magadiite sample were prepared. These differed in the presence of interlayer water. Several CP experiments were used to investigate the relationship between (^29)Si and (^23)Na nuclei and protons in the silicates. Cross-polarisation mechanisms tended to involve magnetisation transfer from the H-bonded protons only. Their determination was possible with a consideration of the extent of spin-diffusion or chemical exchange over the time-scale of the relevant experiments. Finally, new model structures for kanemite and the interlayer space in kanemite, octosilicate, magadiite and kenyaite have been suggested, while previously- proposed silicate layers have been reconsidered.