Detection of imogolite in soils using solid state 29Si NMR

Abstract

High resolution solid state 29Si-NMR has recently been used to examine silicates, and synthetic and natural zeolites1–4. This has been possible through the use of dipolar decoupling, magicangle spinning and cross-polarization techniques which have enabled line narrowing and also considerable signal enhancement for 29Si nuclei in close proximity to protons. It has been found that isotropic 29Si chemical shifts in solids and solutions are generally similar and depend mainly on the degree of condensation of silicon–oxygen tetrahedra1 For silicates, increasing condensation from single (Q0, −66 to −72 p.p.m.) to double tetrahedra (Q1, −78 to −82 p.p.m.), to chains (Q2, −86 to −88 p.p.m.) and cyclic layered structures (Q3, −107 to −109 p.p.m.) leads to increasing diamagnetic shielding. In solid aluminosilicates, the 29Si chemical shift is sensitive to the substitution of aluminium. A further important point was that the 29Si linewidth in aluminosilicates is sensitive to the regularity of aluminium distribution in the lattice. Thus 29Si-NMR has obvious potential for the elucidation of the coordination of silicon in soils and mineral components. We report here the first 29Si-NMR study of the clay mineral imogolite, and show that 29Si-NMR is an excellent analytical technique for the identification of imogolite (or imogolite like materials, that is proto-imogolite) in clay fractions. The observed chemical shift of imogolite (−78 p.p.m.) is consistent with the proposal that imogolite is a tubular hydrated aluminosilicate in which silicon tetrahedra are isolated by coordination through oxygen with three aluminium atoms and one proton.