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 zeolites^1–4. 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.