EFFECTS OF SECONDARY IRON PHASES ON KAOLINITE AL-27 MAS NMR-SPECTRA

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.