Fe ordering in kaolinite; insights from ²⁹Si and ²⁷Al MAS NMR spectroscopy

Kaolinite-rich samples were selected for ²⁷Al and ²⁹Si MAS NMR study to explore the effect of Fe on their spectra and provide insight into the nature of Fe ordering in kaolinite. Initial characterization by chemical analysis, TEM, X-ray diffraction, and magnetic mass susceptibility (χ_g) measurement was conducted to obtain five samples of high purity and with a range of Fe content. Secondary iron oxide and hydroxide phases were extracted using an HCl treatment. TEM study of the samples before and after treatment revealed the effective removal of secondary Fe phases and pristine kaolinite crystal habits.

MAS NMR experiments included measurement of ²⁹Si spin-lattice relaxation times (T₁) at 6.36 T and quantitative ²⁷Al measurements at 8.46 T. The ²⁹Si T₁ studies show a general increase in T₁, with decreasing Fe content. It is probable that the relaxation mechanism occurs dominantly through the dipole-dipole interaction with unpaired electron spins in Fe centers of the dioctahedral sheet. Data fitting indicates that spin relaxations are not best described by a single T₁. Improved fitting using double exponential or power-law behavior to describe ²⁹Si spin-relaxation phenomena indicates a heterogeneous distribution of Fe centers. The relatively longer T₁ values for two samples suggest their dioctahedral Fe domains may have a more clustered distribution within the kaolinite structure. Results from ²⁷Al studies indicate very good correlation between Fe content, χ_g, and spinning sideband (SSB) intensity. There is very little correlation between total integrated²⁷Al intensity and Fe content. When ²⁷Al NMR intensity variations are compared with estimates of ²⁷Al NMR signal loss predicted by a paramagnetic line-broadening wipeout-sphere model that uses an idealized kaolinite structure with regularly ordered dioctahedral Fe sites, the two samples appear to have a more clustered distribution of Fe. The relative increase in SSB intensity is consistent with an increase in the number of antiferromagnetic or ferrimagnetic domains.