Dehydration mechanisms in synthetic Fe-bearing enstatite

In this study, we have investigated the relation between hydrogen extraction and Fe valence states by step-wise heating experiments performed on synthetic orthopyroxene. Samples doped with (57F)e, as well as Fe-57 and Al, were synthesised at 25 kbar and 1400-1150 degrees C, and were subsequently subjected to heating experiments in air or H-2 at 700-1000 degrees C and 1 atmosphere pressure. The change in OH content and Fe valence state was traced by infrared and Mossbauer spectra obtained on the same single crystals after the heat treatments. After heat treatments in air, the infrared spectra for Fe-doped samples show a progressive decrease in band intensities coupled to a corresponding increase in the Fe3+ doublet in Mossbauer spectra, indicating that the dehydration follows the redox reaction OH- + Fe2+ = O2- + Fe3+ + 1/2H(2). However, the sample with the maximum Al-content showed a different behaviour, where the dehydration is coupled to a decrease in Fe2+ contents associated with hematite precipitation, as shown by Mossbauer spectra. For samples annealed in H-2 atmosphere (700-1000 degrees C) a correlation of the decrease of the amount of Fell and the intensity of specific IR bands associated with Fe3+, that decrease faster than other bands, was observed. The results indicate that at least two different dehydration steps are active in dehydration processes in orthopyroxene, i.e. one reaction creating a Fe3+-specific OH-defect and one reaction consuming a Fe3+-specific OH-defect. Both dehydration steps exhibit different kinetic characteristics and the previous observation is confirmed that dehydration kinetics in pyroxenes depends on Fe content. For one sample the spatial distribution of the different OH-defects was mapped using synchrotron FTIR micro-spectroscopy, confirming the different stability of Fe-related and Fe-unrelated OH-defects.