Aluminium X-ray absorption Near Edge Structure in model compounds and Earth’s surface minerals

Abstract 

Aluminium K-edge X-ray absorption near edge spectra (XANES) of a suite of silicate and oxides minerals consist of electronic excitations occurring in the edge region, and multiple scattering resonances at higher energies. The main XANES feature for four-fold Al is at around 2 eV lower energy than the main XANES feature for six-fold Al. This provides a useful probe for coordination numbers in clay minerals, gels, glasses or material with unknown Al-coordination number. Six-fold aluminium yields a large variety of XANES features which can be correlated with octahedral point symmetry, number of aluminium sites and distribution of Al-O distances. These three parameters may act together, and the quantitative interpretation of XANES spectra is difficult. For a low point symmetry (1), variations are mainly related to the number of Al sites and distribution of Al-O distances: pyrophyllite, one Al site, is clearly distinguished from kaolinite and gibbsite presenting two Al sites. For a given number of Al-site (1), variations are controlled by changes in point symmetry, the number of XANES features being increased as point symmetry decreases. For a given point symmetry (1) and a given number of Al site (1), variations are related to second nearest neighbours (gibbsite versus kaolinite). The amplitude of the XANES feature at about 1566 eV is a useful probe for the assessment of Al^IV/Al^total ratios in 2/1 phyllosilicates. Al-K XANES has been performed on synthetic Al-bearing goethites which cannot be studied by ²⁷Al NMR. At low Al content, Al-K XANES is very different from that of α-AlOOH but at the highest level, XANES spectrum tends to that of diaspore. Al-K XAS is thus a promising tool for the structural study of poorly ordered materials such as clay minerals and natural alumino-silicate gels together with Al-subsituted Fe-oxyhydroxides.