Dehydration mechanisms in synthetic Fe-bearing enstatite

Abstract

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 57Fe, as well as 57Fe and Al, were synthesised at 25 kbar and 1400–1150°C, and were subsequently subjected to heating experiments in air or H2 at 700–1000°C and 1 atmosphere pressure. The change in OH content and Fe valence state was traced by infrared and Mosbauer 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 Mosbauer spectra, indicating that the dehydration follows the redox reaction \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \(OH^{{-}} + Fe^{2+} = O^{2{-}} + Fe^{3+} + \frac{1}{2} H_{2}.\) \end{document} 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 Mosbauer spectra. For samples annealed in H2 atmosphere (700–1000°C) a correlation of the decrease of the amount of Fe3+ 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.