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 1 . 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 ( III ) to total iron content (Fe 3+ /ΣFe) 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 , 3 , 4 , 5 , 6 , 7 , 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 Mössbauer 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+ /ΣFe in minerals at the nanometre scale. This procedure is efficient for determining Fe 3+ /ΣFe 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.