Iron oxidation dynamics vs. temperature of synthetic potassic-ferro-richterite: a XANES investigation

We investigated the oxidation behaviour of a synthetic potassic-ferro-richterite up to 750 degrees C by using simultaneous X-ray absorption spectroscopy and X-ray diffraction experiments with synchrotron radiation. From the X-ray diffraction results, we observed an abrupt decrease of cell dimensions at approximate to 335 degrees C accompanied by an anomalous increase in the monoclinic cell angle . From the analysis of the XANES spectra at the iron K-edge, we observed that the structural shrinkage is due to the iron oxidation process, coupled to hydrogen loss, occurring at approximate to 315 degrees C, slightly before the cell contraction. Combining these results with previous studies performed on similar samples by single-crystal structure refinement, Mossbauer, high temperature-Fourier transform IR and Raman spectroscopies, we show that the temperature evolution in Fe-amphiboles is a multi-step process. Following the iron oxidation driven by temperature, the structural dynamics in this double-chain silicate is ruled by local strains in the ribbon of iron-filled octahedra, mainly due to the contraction of the M(1) site.