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In situ measurements of the oxidation state of iron in common minerals have been made by using the synchrotron microXANES (SmX) technique. The results compare very well with wet-chemical and Mossbauer spectroscopic analyses of the same samples. Areas of 10 x 20 [[micro]meter] have been measured successfully, and the results demonstrate that both zoning of [Fe.sup.3+] and the effect of oxide inclusions on bulk analyses can be quantified. Such sample heterogeneity cannot be detected by conventional bulk analytical techniques. The ability to measure [Fe.sup.3+]/[Sigma]Fe with spatial resolution comparable to that of the electron probe further enhances the importance of microbeam techniques in the earth and planetary sciences. Direct measurements of elemental oxidation states that can constrain oxidation-reduction processes are now possible with close to the spatial resolution of traditional compositional analyses derived from electron, or ion, beam and optical microscopic techniques. New, more intense, synchrotron sources such as the Advanced Photon Source at Argonne National Laboratory, are becoming available for analyses of 1-10 [[micro]meter] areas.