The thermal reactions of montmorillonite studied by high-resolution solid-state²⁹Si and²⁷Al NMR

Abstract

Only very slight changes are observed in the²⁹Si and²⁷Al solid-sate magic-angle spinning NMR spectra of a montmorillonite containing almost equal numbers of octahedral aluminium and magnesium ions when its interlayer water is driven off by heating. The²⁹Si NMR spectra are unaffected by dehydroxylation which begins at ∼ 450° C, but the²⁷Al spectra show a decrease in total intensity, possibly due to the formation of 5-coordinated aluminium, with a slight increase in the intensity of the tetrahedral aluminium resonance. On the basis of these results, a structural model is proposed for the dehydroxylate phase and its formation mechanism is discussed. The destruction of the dehydroxylate X-ray pattern at ∼ 850° C and the subsequent recrystallization of the high-temperature products (β-quartz, enstatite and high-cordierite at ∼ 1100° C;β-cristobalite, enstatite and sapphirine at ∼ 1200° C) is accompanied by changes in the silicon and aluminium NMR spectra and in the⁵⁷Fe Mbssbauer spectra which are fully consistent with the known structural features of these phases.