Connection between spectral features of B K edge XANES of minerals and the local structure

The potential of XANES spectroscopy for studying the structure of boron-containing compounds is assessed by theoretical modelling ofB K edge spectra of minerals. Spectra arising from three-fold coordinated boron sites differ considerably from spectra arising from four-fold coordinated sites. The characteristic shape of the spectra is governed first of all by short range order and, to a lesser extent, by medium range order. Areas of main spectral peaks depend only on the coordination of the photoabsorbing site and can thus be used for quantifying the portion of boron atoms in BO 3 and in BO 4 units, in minerals as well as in glasses. The width of the main peak of spectrum at a four-fold coordinated boron site generally increases with the the spread of the B-O distances but deviations from this correlation may be substantial. In contrast to spectra of minerals, XANES of BPO 4 crystal is governed by long range order and is thus not a suitable model for studying XANES of borate glasses.