NMR study of kaolinite. 1. ²⁹Si, ²⁷Al, and ¹H spectra

We have measured ²⁹Si, ²⁷Al, and ¹H NMR spectra of kaolinites with natural and synthetic origins, using high-resolution solid-state techniques such as the magic angle spinning and the multipulse sequences. The ²⁹Si spectra demonstrate clearly that two inequivalent Si sites are present, and the origin of the line width has been discussed quantitatively. The line shape of the ²⁷Al spectra is governed by the second-order quadrupole interaction and suggests the presence of two inequivalent Al sites with different quadrupole coupling constants and asymmetry factors. The ¹H static spectra are broadened by the dipole-dipole interactions between ¹H spins and between ¹H and ²⁷Al spins, indicating that the hydrogen atoms in the hydroxyl groups are in a rigid lattice state at room temperature. The technique of combined rotation and multiple-pulse spectroscopy has been successfully applied to kaolinites. The ¹H chemical shift of the hydroxyl groups is 2.8 ppm from tetramethylsilane.