ABSTRACT

The high-resolution ²⁹Si- and ²⁷Al-NMR spectra and ESR spectra for five allophanes (SiO₂/Al₂O₃=1.19, 1.22, 1.75, 1.77 and 1.90) are reported, together with the spectra for two imogolites (SiO₂/Al₂O₃ = 0.96 and 1.0) and a halloysite (SiO₂/Al₂O₃ = 2.0). It is apparently seen that ²⁹Si-NMR spectra for allophanes vary depending on SiO₂/Al₂O₃ ratios, indicating structural variations of allophanes with the change of the SiO₂/Al₂O₃ ratios. Allophanes with SiO₂/Al₂O₃ molar ratios close to 1.0 show a broad absorption at -79 ppm with a second, minor resonance around 90 ppm in the ²⁹Si-NMR spectra; this suggests that orthosilicate is a main component of allophanes with lower SiO₂/Al₂O₃ ratios. On the other hand, allophanes with higher SiO₂/Al₂O₃ ratios show complicated ²⁹Si-NMR spectra with several peaks between -79 and -97 ppm; these spectra indicate that these allophanes contain Si atoms in the orthosilicate structure as well as several different states of polymerized silicates with various Si-O-Si angles and Si-O bond strengths. Further, it is indicated that the relative abundances of polymerized silicates to orthosilicate tend to increase with the increase of the SiO₂/Al₂O₃ ratio of allophane samples up to 1.75 and decrease at higher ratios. ²⁷Al-NMR spectra for four allophanes studied show the resonance mainly at the chemical shift of 2.9-4.1 ppm, indicative of six-coordinated Al.