Pa. Schroeder et Rj. Pruett, FE ORDERING IN KAOLINITE - INSIGHTS FROM SI-29 AND AL-27 MAS NMR-SPECTROSCOPY, The American mineralogist, 81(1-2), 1996, pp. 26-38
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.