A 23Na NMR study of hydrous layered silicates

Abstract

Sodium-23 single-pulse NMR spectra have been obtained for the layered sodium polysilicate hydrates, makatite, kanemite, octosilicate, magadiite and kenyaite. The kanemite spectra were severely distorted by second-order effects from strong quadrupolar interactions. However, the regular behaviour of the linewidth with frequency allowed the isotropic chemical shift and quadrupole interaction constant for the single peak to be deduced. The spectra of octosilicate, magadiite and kenyaite contain either one or two peaks. Powder X-ray diffraction indicates that the spectra with two lines correspond to samples contaminated by sodium chloride, which is a result of relevance to the conclusions of two previous authors. A 1H,23Na heteronuclear shift correlation spectrum for magadiite shows that strongly hydrogen-bonded protons are responsible for the single magadiite 23Na cross-polarisation signal. The presence of these hydrogen-bonded protons, and the detection of a single sodium site, indicate that there are significant differences between the interlayer conformations of kanemite, octosilicate, magadiite and kenyaite and the known structure of makatite, which has no hydrogen-bonded protons and two sodium sites. The 23Na isotropic chemical shifts of kanemite, octosilicate, magadiite and kenyaite indicate that the relevant sodium species have coordination numbers of five or six; both arrangements are known to occur with makatite. The estimated quadrupole interaction constants infer that the value is five for kanemite and six for octosilicate, magadiite and kenyaite.