Experimental and theoretical XANES and EXAFS study of tetra-ferriphlogopite

Experimental and theoretical EXAFS and XANES data on Fe coordination and bond distances have been obtained for a natural tetra-ferriphlogopite. XANES data show a predominance for trivalent iron in tetrahedral coordination. The use of theoretical multiple scattering calculations helps distinguish the contributions to the total spectrum coming from the different iron atoms. EXAFS spectra were reduced and analyzed using the GNXAS programs and Fe has been confirmed to be in tetrahedral coordination, with refined structural data Fe-O = 1.86 Å, Fe-Si = 3.24 Å, Fe-O-Si = 138°. Moroever, a further contribution from Fe is detected corresponding to a Fe-O distance equal to 2.22 Å. This latter contribution (20 mol % of the total Fe ca.) is interpreted to arise from divalent Fe in octahedral coordination, in excellent agreement with the structural and chemical data.