Iron in kornerupine: A 57Fe Mössbauer spectroscopic study and comparison with single-crystal structure refinement

Iron is an important constituent of kornerupine, (M,Mg,Fe)(Al,Mg,Fe)9(Si,Al,B)5O21(OH,F). We obtained Mössbauer spectra at 300 K on twelve samples with SFe 5 0.30–1.30 atoms per formula unit (apfu) and Fe31/SFe 5 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 Fe21 doublets and one Fe31 doublet. The Fe21doublets have nearly identical isomer shifts: d 5 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 a-Fe at 300 K). However, they differ to a variable extent in quadrupole splitting, DEQ ø 1.06–1.80, 1.83–2.27, and 2.14–3.41 mm/ s, respectively, to the M1, M2, and X sites. The Fe31 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 Fe31/SFe ratio in kornerupine determined by SREF and Mössbauer spectroscopy increases with increasing Fe2O3 and Fe31/SFe ratio of the associated sillimanite, sapphirine, and ilmenite-hematite, i.e., the measured Fe31/SFe ratios are related to the oxygen fugacity at which the kornerupine crystallized.