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

Minerals with mixed valence states are widespread and form in many different rock types'. They can contain, for example, Fe(2+)-Fe(3+) and Mn(2+)-Mn(3+)-Mn(4+), with the ratios of oxidation states reflecting the redox conditions under which the host materials crystallized. The distribution of the ratio of iron (nr) to total iron content (Fe(3+)/Sigma Fe) in minerals reflects the oxidation states of their host rocks and is therefore important for answering fundamental questions about the Earth's evolution and structure(2-8). Iron is the most sensitive and abundant indicator of oxidation state, but many mineral samples are too fine-grained and heterogeneous to be studied by standard methods such as Mossbauer spectroscopy, electron microprobe, and wet chemistry. Here we report on the use of electron energy-loss spectroscopy with a transmission electron microscope to determine Fe(3+)/Sigma Fe in minerals at the nanometre scale. This procedure is efficient for determining Fe(3+)/Sigma Fe ratios of minor and major amounts of iron on a scale heretofore impossible and allows information to be obtained not only from ultra-fine grains but also, for example, at reaction fronts in minerals.