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 Fe^2+-Fe²⁺ and Fe²⁺-Fe³⁺ electranic interactions along the directions of the electric field gradients of these ions is suggested.