FE3+ DISTRIBUTION IN OXIDIZED OLIVINE: A SYNCHROTRON MICRO-XANES STUDY

Abstract

Synchrotron micro-XANES spectroscopy (SmX) is used to examine the amount and distribution of Fe2+ and Fe3+ in five samples of fayalite previously studied by Mossbauer spectroscopy. Rockport fayalite is homogeneous and the Fe in it is completely reduced. Olivine from Qianan is almost completely oxidized, and probably contains finely intergrown silica, laihunite, and hematite with hematite predominating. Pantelleria olivine has an oxidized rim around a reduced core, suggesting either a post-crystallization reaction with fayalite and oxygen going to laihunite and hematite or a change in prevailing oxidation during growth. The texture of olivine from the Mourne Mountains suggests exsolution from a fayalite-laihunite solid solution, based on the substitution of three Fe3+ atoms for two Fe2+ and one vacancy, that was stable at high temperature. Laihunite from the type locality (Lai-He village) shows 1-3 μm mottling between light and dark areas in backscatter images, but these areas cannot correspond to pure fayalite and laihunite exsolution because the bulk Fe3+/ΣFe of such a combination would not correspond to the SmX value of 67% Fe3+. Rather, the texture observed in backscatter is interpreted to represent alternating areas of magnetite-rich and laihunite-rich compositions with a similar (i.e., 67% Fe3+/ΣFe) extent of oxidation.