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Dehydration mechanisms in synthetic Fe-bearing enstatite

¹ Geowissenschaftliches Zentrum der Universität Göttingen, Abteilung Angewandte und Experimentelle Mineralogie, Goldschmidtstraße 1, 37077 Göttingen, Germany
² Naturhistoriska riksmuseet, Sektionen för mineralogi, Box 50007, 10405 Stockholm, Sweden

^* Corresponding author, e-mail: rstalde{at}gwdg.de

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 Mößbauer 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 Mößbauer spectra, indicating that the dehydration follows the redox reaction

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 Mößbauer 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.

Key-words: enstatite, dehydration mechanism, IR spectroscopy, hematite precipitation, Mößbauer spectroscopy.

This article has been cited by other articles:

				R. Sundvall, H. Skogby, and R. Stalder Hydrogen diffusion in synthetic Fe-free diopside European Journal of Mineralogy, October 1, 2009; 21(5): 963 - 970. [Abstract] [Full Text] [PDF]

				R. Sundvall, H. Skogby, and R. Stalder Dehydration-hydration mechanisms in synthetic Fe-poor diopside European Journal of Mineralogy, January 1, 2009; 21(1): 17 - 26. [Abstract] [Full Text] [PDF]

				R. Stalder, H. Purwin, and H. Skogby Influence of Fe on hydrogen diffusivity in orthopyroxene European Journal of Mineralogy, December 1, 2007; 19(6): 899 - 903. [Abstract] [Full Text] [PDF]