Abstract

Iron is an important constituent of kornerupine, ([]Mg,Fe) (Al,Mg,Fe) 9 (Si,Al,B) 5 O 21 (OH,F). We obtained Mossbauer spectra at 300 K on twelve samples with Sigma Fe = 0.30-1.30 atoms per formula unit (apfu) and Fe (super 3+) /Sigma 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 (super 2+) doublets and one Fe (super 3+) doublet. The Fe (super 2+) doublets have nearly identical isomer shifts: delta = 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 alpha -Fe at 300 K). However, they differ to a variable extent in quadrupole splitting, Delta E Q nearly equal 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 (super 3+) 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 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 (super 3+) /Sigma Fe ratio in kornerupine determined by SREF and Mossbauer spectroscopy increases with increasing Fe 2 O 3 and Fe (super 3+) /Sigma Fe ratio of the associated sillimanite, sapphirine, and ilmenite-hematite, i.e., the measured Fe (super 3+) /Sigma Fe ratios are related to the oxygen fugacity at which the kornerupine crystallized.

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