Determination of the locations of Mn and Fe in Mn-bearing andalusite by anomalous X-ray scattering and X-ray absorption fine structure analyses

Local structural features around Mn and Fe in Mn-bearing andalusite from Västanåberget, Sweden were investigated by anomalous X-ray scattering (AXS) and X-ray absorption fine structure (XAFS). The X-ray absorption near edge structure (XANES) spectra indicate that all of the Mn and Fe atoms are in +3 oxidation state. Both the electron density map of Mn obtained by AXS and simulated multiple-scattering extended X-ray absorption fine structure (EXAFS) spectra indicate that the Mn atoms exclusively occupy Al1 sites. Further, the EXAFS analysis indicates that the first neighbor coordinated to Mn at the Al1 site shows a complete structural relaxation and Jahn-Teller distortion owing to Mn³⁺. In addition, the EXAFS analyses reveal that Fe occupies both Al1 and Al2 sites. The Fe site occupancy ratio (Fe at Al2 to total Fe) in the Mn-bearing andalusite sample is 40-50%. Employing the local structural information around Mn and Fe, a conventional single-crystal structural refinement converged at R = 2.34% with a structural formula of (Al_0.82Mn_0.16Fe_0.02)^[6](Al_0.99Fe_0.01)^[5]SiO₅ (space group Pnnm with a = 7.8538(9), b = 7.9463(9), c = 5.5867(6) Å).