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.