Ratios of ferrous to ferric iron from nanometre-sized areas in minerals

Minerals with mixed valence states are widespread and form in many differen t rock types'. They can contain, for example, Fe2+-Fe3+ and Mn2+-Mn3+-Mn4+, with the ratios of oxidation states reflecting the redox conditions under which the host materials crystallized. The distribution of the ratio of iro n (nr) to total iron content (Fe3+/Sigma Fe) in minerals reflects the oxida tion states of their host rocks and is therefore important for answering fu ndamental questions about the Earth's evolution and structure(2-8). Iron is the most sensitive and abundant indicator of oxidation state, but many min eral samples are too fine-grained and heterogeneous to be studied by standa rd methods such as Mossbauer spectroscopy, electron microprobe, and wet che mistry. Here we report on the use of electron energy-loss spectroscopy with a transmission electron microscope to determine Fe3+/Sigma Fe in minerals at the nanometre scale. This procedure is efficient for determining Fe3+/Si gma Fe ratios of minor and major amounts of iron on a scale heretofore impo ssible and allows information to be obtained not only from ultra-fine grain s but also, for example, at reaction fronts in minerals.