Al coordination and local structure in minerals: XAFS determinations and multiple-scattering calculations for K-feldspars

The Al K-edge spectra of three potassium feldspars $\rm KAlSi_3O_8$: sanidine, microcline and orthoclase, all with four-fold–coordinated Al, have been measured by X-ray absorption near-edge structure spectroscopy and calculated according to the multiple-scattering formalism using clusters of different sizes and the $X_{\alpha}$ exchange-correlation potential. Although these structures are quite complex, and clusters as large as containing ca. 150 atoms are needed to calculate them, a significant agreement has been obtained between experimental spectra and calculated ones up to 60 eV above the threshold, both for energy positions and for relative intensities. Moreover, while the gross features of all spectra appear to be dominated mainly by the first coordination sphere around the absorber, effects due to higher shells are detected