Oxidation experiment of natural megacrystal clinopyroxene: Implications for assignment of Mossbauer spectra

The Massbauer spectra of natural megacrystal clinopyroxene are usually fitted by 4 sets of symmetric doublets, A-A', B-B', C-C' and D-D', respectively, in terms of increasing Qs value in literature. But the assignments of those doublets are quite different, except the D-D' doublet assigned to Fe3+ at the lattice site M1 in previous papers. Particularly, the assignment and interpretation of the C-C' doublet are diverse. The oxidation experiments of natural megacrystal clinopyroxene collected from the Hannuoba basalt, North China, were performed under controlled conditions of temperature at 1000 degreesC and oxygen fugacity of FMQ buffer in 1, 2, 3 and 5 days respectively. The oxidized samples were then measured by X-ray diffraction spectrometry and Mossbauer spectrometry. The oxidation of clinopyroxene is characterized by Fe2+ --> Fe3+ at M1 under the subsolidus conditions, which is consistent with the increase of the area of the D-D' doublet when the heating time increases. Accordingly, the area of the A-A' and B-B' doublets decreases with the increasing heating time. However, the area of C-C' keeps almost constant. Therefore, the 3 sets of doublets can be assigned correctly as follows: A-A' doublet to Fe+2 at M1, B-B' Fe2+ at MI, C-C' Fe2+ at M2, and D-D' Fe3+ at M1. The M1 site splitting is due to the NNN (Next Nearest Neighbour) effect of the M2 site. The amount of Fe3+ at the tetrohedral site of megacrystal clinopyroxene is negligible according to this study.