Eight kaolinite and 2 halloysite samples were analyzed using Al-27 mag
ic angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy, c
hemical analysis and magnetic susceptibility to understand the effect
of isomorphously substituted Fe3+ and secondary Fe phases on the NMR s
ignal. Known additions of goethite and hematite were made to determine
the response of kaolinite Al-27 MAS NMR spectra and sample magnetic s
usceptibilities. Results from high field (11.7 T) NMR studies show pos
itive correlations between 1) Fe content, 2) magnetic susceptibility a
nd 3) relative intensity of the spinning side band (SSB) to central ba
nd (CB) ratio. No correlation is observed between the mass-corrected N
MR spectral intensity and Fe content. Comparative high/low field (11.7
T/8.46 T) NMR studies show a decrease in the relative ratio of line b
roadening with increasing Fe content. Projected trends of known additi
ons of hematite and goethite versus magnetic susceptibility extrapolat
e back to zero y intercepts that have Fe concentrations higher than ac
tually measured. Absolute intensity observations have negative implica
tions for the use of Al-27 MAS NMR spectroscopy in assessing Fe-orderi
ng in kaolinites. First, high-energy, short (1/6 of pi/2 solutions) pu
lse sequences do not produce reliable quantitative data needed to asse
ss paramagnetic line-broadening affects caused by different Fe-orderin
g clustering scenarios. The lack of perfect correlation between SSB/CB
, Fe content and magnetic susceptibility indicates that differences ex
ist with respect to 1) the amount of isomorphously substituted Fe, 2)
the ordering of the Fe within kaolinite, 3) the concentration of secon
dary Fe phases and 4) magnetic susceptibility of the secondary Fe asse
mblage. Variability of line-width ratios at different field strengths
indicates an increasing second-order quadrupole effect (SOQE) with inc
reasing Fe. Finally, the difference between the observed Fe content an
d that predicted from magnetic susceptibility measurements suggest tha
t magnetic domain properties of secondary Fe phases behave differently
from Fe domains bound in kaolinite.