AL-27 MAS NMR AND ALUMINUM X-RAY-ABSORPTION NEAR-EDGE STRUCTURE STUDY OF IMOGOLITE AND ALLOPHANES

This paper compares the results of Al-27 nuclear magnetic resonance spectroscopy (NMR) and Al-K-edge X-ray Absorption Near Edge Structure (XANES) of natural imogolite and allophanes and some crystalline reference minerals. All soil allophanes studied contain 4-coordinated Al (Al(IV)). The highest relative proportion of Al(IV), 21% of the total Al, was found in Si-rich allophane. This value is close to that found in spring allophanes, which were previously considered to be different from soil allophanes. For a quantitative determination of the Al(IV)/Al(total) ratio, NMR is more reliable than XANES, because of the sensitivity of the chemical shift to low Al(IV) concentrations, but XANES may be used even if paramagnetic impurities (mostly Fe) are present. Al-K XANES also yields more information than NMR on the local environment of Al(VI) and especially site multiplicity. Al(VI) XANES of imogolite and allophanes are similar regardless of the Al/Si ratio. They yield two well-resolved resonances with maxima near 1568 and 1570 eV, which indicates the presence of a unique Al(VI) site by comparison with crystalline references. The presence of only one Al(VI) site indicates that imogolite and allophanes have an octahedral sheet with a structure similar to 2/1 dioctahedral phyllosilicates but different from gibbsite or kaolinite, previously considered as structural analogues. The Al-27(IV) MAS NMR peak maxima of allophanes are between 58.6 and 59.8 ppm, in the range observed for crystalline and amorphous framework aluminosilicates, and less positive than those of sheet silicates, which are typically in the range 65-75 ppm. Al-27-H-1 CPMAS NMR spectra suggest that both Al(IV) and Al(VI) have Al-O-H linkages.