Fe3+ distribution in oxidized olivine: A synchrotron micro-XANES study

Synchrotron micro-XANES spectroscopy (SmX) is used to examine the amount an d distribution of Fe2+ and Fe3+ in five samples of fayalite previously stud ied by Mossbauer spectroscopy. Rockport fayalite is homogeneous and the Fe in it is completely reduced. Olivine from Qianan is almost completely oxidi zed, and probably contains finely intergrown silica, laihunite, and hematit e with hematite predominating. Pantelleria olivine has an oxidized rim arou nd a reduced core, suggesting either a post-crystallization reaction with f ayalite and oxygen going to laihunite and hematite or a change in prevailin g 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 st able at high temperature. Laihunite from the type locality (Lai-He village) shows 1-3 mu m mottling between light and dark areas in backscatter images , but these areas cannot correspond to pure fayalite and laihunite exsoluti on because the bulk Fe3+/Sigma Fe of such a combination would not correspon d to the SmX value of 67% Fe3+. Rather, the texture observed in backscatter is interpreted to represent alternating areas of magnetite-rich and laihun ite-rich compositions with a similar (i.e., 67% Fe3+/Sigma Fe) extent of ox idation.