Quantitative local structure determination in mica crystals: ab initio simulations of polarization XANES at the potassium K-edge.

An attempt to refine the local structure of a layered structure such as mica is made by combining angle-resolved XANES (AXANES) and single-crystal X-ray diffraction (SC-XRD) experiments. Ab initio calculations of AXANES spectra of several tri-octahedral micas have been used to further interpolate experimental data and to deduce physico/chemical effects. Structural distortions have been found highly correlated with the compositional disordering that arises from electronic interactions between anions and cations, and extend the interlayer entering deep into nearby tetrahedral and octahedral sheets. Multiple occupations at the same atomic site have been investigated in detail both in the parallel and perpendicular components of AXANES spectra. Finally, the best fit obtained, computed in the framework of the multiple-scattering theory, is presented and the limitations of the muffin-tin potential in layered systems are briefly discussed.