FE ORDERING IN KAOLINITE - INSIGHTS FROM SI-29 AND AL-27 MAS NMR-SPECTROSCOPY

Kaolinite-rich samples were selected for Al-27 and Si-29 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 c hemical analysis, TEM, X-ray diffraction, and magnetic mass susceptibi lity (chi(g)) measurement was conducted to obtain five samples of high purity and with a range of Fe content. Secondary iron oxide and hydro xide phases were extracted using an HCl treatment. TEM study of the sa mples before and after treatment revealed the effective removal of sec ondary Fe phases and pristine kaolinite crystal habits. MAS NMR experi ments included measurement of Si-29 spin-lattice relaxation times (T-1 ) at 6.36 T and quantitative Al-27 measurements at 8.46 T. The Si-29 T -1 studies show a general increase in T-1 with decreasing Fe content. It is probable that the relaxation mechanism occurs dominantly through the dipole-dipole interaction with unpaired electron spins in Fe cent ers of the dioctahedral sheet. Data fitting indicates that spin relaxa tions are not best described by a single T-1. Improved fitting using d ouble exponential or power-law behavior to describe Si-29 spin-relaxat ion phenomena indicates a heterogeneous distribution of Fe centers. Th e relatively longer T-1 values for two samples suggest their dioctahed ral Fe domains may have a more clustered distribution within the kaoli nite structure. Results from Al-27 studies indicate very good correlat ion between Fe content, chi(g), and spinning sideband (SSB) intensity. There is very little correlation between total integrated Al-27 inten sity and Fe content. When Al-27 NMR intensity variations are compared with estimates of Al-27 NMR signal loss predicted by a paramagnetic li ne-broadening wipeout-sphere model that uses an idealized kaolinite st ructure with regularly ordered dioctahedral Fe sites, the two samples appear to have a more clustered distribution of Fe. The relative incre ase in SSB intensity is consistent with an increase in the number of a ntiferromagnetic or ferrimagnetic domains.