Effects of secondary iron phases on kaolinite 27Al MAS NMR spectra

Abstract--Eight kaolinite and 2 halloysite samples were analyzed using 27A1 magic angle spinning nu-clear 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 nd sample magnetic susceptibilities. Results from high field (11.7 T) NMR studies 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 dif