AL K-EDGE XANES SPECTRA OF ALUMINOSILICATE MINERALS

Al K-edge X-ray absorption near-edge structure (XANES) spectra of a ra nge of aluminosilicate and aluminum oxide minerals were collected usin g synchrotron radiation. The Al K-edge spectra of aluminosilicates con taining fourfold-coordinated Al (Al-[4]) and six-fold-coordinated Al ( Al-[6]) are qualitatively interpreted on the basis of a comparison wit h the Si K-edge spectra of alpha quartz and stishovite and MO calculat ions for tetrahedral and octahedral clusters. Some near-edge features are attributed to the multiple scattering (MS) effect from the more di stant shell atoms. The Al K-edge (peak C) shifts toward higher energy with an increase in the coordination number (CN) of Al, from 1566.7 eV for Al-[4] (averaged for eight samples) to 1567.8 eV for Al-[5] and t o 1568.3 eV for Al-[6] (averaged for 17 samples). For Al-[4] and Al-[6 ] aluminosilicates, respectively, the Al K-edge shifts to higher energ y with increase in the Al-O bond distance (d(Al-O)), distortion of the Al polyhedron (Delta(Al-O)), and decrease in the Al-O bond valence (s (Al-O)). Also for Al-[4] and Al-[6] aluminosilicates, the relative int ensity of the Al K-edge is correlated with the content (in weight perc ent) of Al in tetrahedral and octahedral sites, respectively. This cor relation therefore establishes Al K-edge spectroscopy as a potential t echnique for semiquantitatively determining the distribution of Al bet ween fourfold- and sixfold-coordinated sites.