Iron oxidation state variations in zoned micro-crystals measured using micro-XANES

The determination of the oxidation state of transition metals at high spatial resolution is a crucial issue for many fields of science, including solid state physics, earth sciences, biology, bio-chemistry and catalysis. Among the other available analytical methods, micro-XANES allows to probe in situ the oxidation state with high lateral resolution, enabling an unprecedented level of description in heterogeneous samples. In geological samples the determination of the Fe3+/Fe ratio is of particular interest since it can be used as an indicator of the oxygen fugacity (fO2) at which a mineral formed. With this respect, we performed a micro-XANES aiming to investigate the Fe-redox state variation across single-crystals of both garnet and omphacite exploiting the X-ray microprobe available at the ESRF ID22 beamline to reach a spot size of 1.7 m × 5.3 m. For garnet, the absolute Fe3+ content was determined in a space-resolved way. In the case of omphacite, the analysis of the XANES data is not straightforward owing to the presence of a significant dichroism effect and to the random orientation of the different grains in the mineral assemblage. The investigated samples are highly complex materials which represent a challenge for the micro-XANES technique. These zoned micro-crystals are therefore ideal systems to develop analytical procedures which can be subsequently generalized to other relevant fields of science such as the Fe speciation in a single cell or a single grain for life science and catalysis applications, respectively.