Local and average Fe distribution in trioctahedral micas: Analysis of Fe K-edge XANES spectra in the phlogopite-annite and phlogopite-tetra-ferriphlogopite joins on the basis of single-crystal XRD refinements

Abstract

In Fe-bearing trioctahedral 1M micas Fe2+ occurs essentially in the octahedral O sheet whereas Fe3+ either coexists with Fe2+ in the octahedral M sites or, more rarely, enters the tetrahedral T sites. The Fe K -edge absorption spectra of twelve micas were recorded in an effort to relate the observed multiple-scattering absorption features to the results of single-crystal X-ray structure refinements obtained on crystals from the same rock sample. This study first addresses the three end-members phlogopite, annite and tetra-ferriphlogopite, used as reference materials, and enters later in the interpretation of nine trioctahedral micas intermediate in composition along the joins phlogopite-tetra-ferriphlogopite and phlogopite-annite. A detailed analysis of the observed XANES features singles out the independent effects of coordination and valence, and shows distinctly different position vs. intensity trends that depend upon the local Fe2+ and Fe3+ environments. Bond distances evaluated from XANES spectra in the full multiple-scattering region of the K -edge spectrum agree well with those obtained by XRD refinement. Distinct variations in the local to intermediate degrees of order of certain micas can be determined by the analysis of their intermediate multiple-scattering regions; they supplement the information on their long-range order that is best obtained through the XRD refinement.