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

Iron is an important constituent of kornerupine, (square,Mg,Fe)(Al,Mg,Fe)(9 )(Si,Al,B)(5)O-21(OH,F). We obtained Mossbauer spectra at 300 K on twelve s amples with Sigma Fe = 0.30-1.30 atoms per formula unit (apfu) and Fe3+/Sig ma Fe = 0-0.31; several samples were also run at 77 and 430 K. Models allow ing unequivocal refinement of the spectra and determination of site occupan cies were developed only when single-crystal refinement (SREF) of six of th e samples constrained the number of possibilities. The spectra could then b e fitted to three Fe2+ doublets and one Fe3+ doublet. The Fe(2+)doublets ha ve nearly identical isomer shifts: delta = 1.14-1.19 mm/s for the octahedra l M1 and M2 sites and 1.12-1.20 mm/s for the irregular, eightfold-coordinat ed X site (relative to alpha-Fe at 300 K). However, they differ to a variab le extent in quadrupole splitting, Delta E-Q approximate to 1.06-1.80, 1.83 -2.27, and 2.14-3.41 mm/ s, respectively, to the M1, M2, and X sites. The F e3+ doublet corresponds to the M4 site. The Mossbauer 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 c orresponding 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 increas ing 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 Fe3+/Sigma Fe ratio in kornerupine determined by SREF and Mossbauer spectroscopy increases with increasing Fe2O3 and Fe3 +/Sigma Fe ratio of the associated sillimanite, sapphirine, and ilmenite-he matite, i.e., the measured Fe3+/Sigma Fe ratios are related to the oxygen f ugacity at which the kornerupine crystallized.