Single-crystal 57Fe Mössbauer studies of the site positions in cordierite

Abstract

Mossbauer (57Fe) absorption spectra of different orientations of single crystals of cordierite (Mg,Fe)2AI3 [Si5AIO18] have been determined. The variation of the ratio of the peak areas of the quadrupole split lines with orientation has enabled the determination of the electric field gradient directions, and the site positions and occupancies of the Fe2+ and Fe3+ ions. Of a total iron content of 1.85% w/w, 79% is present as Fe2+ replacing Mg2+ in the octahedral sites; 19% as Fe2+ occupying the larger of two channel sites; and 1% as Fe3+ replacing Al3+ in the larger of the two tetrahedral sites. A possible interpretation of the observed pleochroism in terms of Fe2+-Fe2+ and Fe2+-Fe3+ electranic interactions along the directions of the electric field gradients of these ions is suggested.