IRON IN KORNERUPINE: A 57FE MOSSBAUER SPECTROSCOPIC STUDY AND COMPARISON WITH SINGLE-CRYSTAL STRUCTURE REFINEMENT

Abstract

Iron is an important constituent of kornerupine, (,Mg,Fe)(Al,Mg,Fe) 9 (Si,Al,B) 5 O 21 (OH,F). We obtained Mössbauer spectra at 300 K on twelve samples with Fe 0.30–1.30 atoms per formula unit (apfu) and Fe 3 /Fe 0–0.31; several samples were also run at 77 and 430 K. Models allowing unequivocal refinement of the spectra and determination of site occupancies were developed only when single-crystal refinement (SREF) of six of the samples constrained the number of possibilities. The spectra could then be fitted to three Fe 2 doublets and one Fe 3 doublet. The Fe 2 doublets have nearly identical isomer shifts: 1.14–1.19 mm/s for the octahedral M1 and M2 sites and 1.12–1.20 mm/s for the irregular, eightfold-coordinated X site (relative to -Fe at 300 K). However, they differ to a variable extent in quadrupole splitting, E Q 1.06–1.80, 1.83–2.27, and 2.14–3.41 mm/ s, respectively, to the M1, M2, and X sites. The Fe 3 doublet corresponds to the M4 site. The Mössbauer and SREF occupancies are in excellent agreement for the six samples. The M1 doublet is split in B-bearing kornerupine and the proportion of Fe corresponding to each doublet, as well as quadrupole splitting, varies with B content. Similarly, the X doublet is split in F-bearing kornerupine, and quadrupole splitting of the X site increases with increasing F content. In contrast to most silicates, resolution of the spectra improves with increasing temperature. Quadrupole splitting of the X, M1, and M2 sites decreases with temperature, the X site at a lesser rate consistent with its being the most distorted site. To a first approximation, the Fe 3 /Fe ratio in kornerupine determined by SREF and Mössbauer spectroscopy increases with increasing Fe 2 O 3 and Fe 3 /Fe ratio of the asso-ciated sillimanite, sapphirine, and ilmenite-hematite, i.e., the measured Fe 3 /Fe ratios are related to the oxygen fugacity at which the kornerupine crystallized.