Effects of Secondary Iron Phases on Kaolinite 27Al MAS NMR Spectra
Clays and clay minerals
Eight kaolinite and 2 halloysite samples were analyzed using 27A1 magic angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy, chemical analysis and magnetic susceptibility to understand the effect of isomorphously substituted Fe 3+ and secondary Fe phases on the NMR signal. Known additions of goethite and hematite were made to determine the response of kaolinite 27A1 MAS NMR spectra and sample magnetic susceptibilities. Results from high field (11.7 T) NMR studies show positive
... show positive correlations between 1) Fe content, 2) magnetic susceptibility and 3) relative intensity of the spinning side band (SSB) to central band (CB) ratio. No correlation is observed between the mass-corrected NMR 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 broadening with increasing Fe content. Projected trends of known additions of hematite and goethite versus magnetic susceptibility extrapolate back to zero y intercepts that have Fe concentrations higher than actually measured. Absolute intensity observations have negative implications for the use of 27A1 MAS NMR spectroscopy in assessing Fe-ordering in kaolinites. First, high-energy, short (~/6 of ~/2 solutions) pulse sequences do not produce reliable quantitative data needed to assess paramagnetic line-broadening affects caused by different Fe-ordering clustering scenarios. The lack of perfect correlation between SSB/CB, Fe content and magnetic susceptibility indicates that differences exist with respect to 1) the amount of isomorphously substituted Fe, 2) the ordering of the Fe within kaolinite, 3) the concentration of secondary Fe phases and 4) magnetic susceptibility of the secondary Fe assemblage. Variability of line-width ratios at different field strengths indicates an increasing second-order quadrupole effect (SOQE) with increasing Fe. Finally, the difference between the observed Fe content and that predicted from magnetic susceptibility measurements suggest that magnetic domain properties of secondary Fe phases behave differently from Fe domains bound in kaolinite.