Octahedral cation ordering in Mg, Fe²⁺-olivine: an optical absorption spectroscopic study

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• 作者：Michail N. Taran and Monika Koch-M眉ller
• 关键词：Olivine ; Optical spectroscopy ; Ferrous ion ; Cation ordering
• 刊名：Physics and Chemistry of Minerals
• 出版时间：November, 2006
• 出版年：2006
• 期刊代码：117_0342-1791
• 类别：ear
• 卷：33
• 期：8-9
• 页码：511-518
• 数据来源：sp

摘要

Polarized optical absorption spectra of natural olivine, Fa₁₀Fo₉₀, were measured before and after annealing/quenching experiments performed at 650, 800, 1,000 and 1,200掳C under controlled oxygen fugacity. It was found that the annealing induces weak but definite changes in the olivine spectra. The intensity of the spin-allowed Z > X-polarized band at 9,560 cm−1 and shoulder at ∼8,300 cm−1 attributed to Fe2+(M2), continuously decreases with annealing temperature, whereas a weaker band at ∼11,600 cm−1 assigned to electronic spin-allowed transitions of Fe2+(M1), increases. This evidently shows that annealing treatments cause a redistribution of Fe2+ from M2 to M1. The fractionation increases with increasing temperature. This observation is in good correspondence with many diffraction structural studies of natural and synthetic olivines, as well as with recent Raman and M枚ssbauer investigations by Kolesov and Geiger (Mitt 脰sterr Mineral Ges 149:48, 2004) and Morozov et al. (Eur J Mineral 17:495–500, 2005) evidencing a weak tendency of Fe to order into the M1 site with increasing temperature. However, this deduction is incompatible with the results of the in situ neutron power diffraction study of synthetic FeMgSiO₄ by Redfern et al. (Phys Chem Minerals 27:630–637, 2000). Polarization properties of the UV absorption edge, attributed to ligand-to-metal charge-transfer transitions in Fe3+, changes from Y > X ≫ Z in natural samples to a weak Y ≥ X ≥ Z-pleochroism in annealed ones. This may be due to redistribution of a small content of Fe3+ among M1 and M2 structural sites.