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Hydrogen diffusion in synthetic Fe-free diopside

¹ Sektionen för Mineralogi, Naturhistoriska Riksmuséet, Box 50007, 10405 Stockholm, Sweden
² Institutionen för Geologi och Geokemi, Svante Arrhenius väg 8C, 106 91 Stockholm, Sweden
³ Institut für Mineralogie und Petrographie, Universität Innsbruck, Innrain 52f, A-6020 Innsbruck, Austria

^* Corresponding author, e-mail: rickard.sundvall{at}nrm.se

Hydrogen is a widespread trace element in many nominally anhydrous minerals (NAMs) in the Earth’s crust and mantle and has profound influence on the physical properties of the host mineral. Of all NAMs from the upper mantle, clinopyroxenes have been shown to contain the highest amount of hydrogen. This study focuses on the dehydration kinetics of pure diopside along [010] and [100]* by thermal annealing under normal atmospheric pressure. The diopside crystals used were synthesized at high pressure under water-saturated conditions. FTIR spectra were obtained after each step, including untreated samples. The Arrhenius equation gives an activation energy (E_a) of –331 ± 50 kJ mol^–1 and D₀ = 10^{0.9 ± 2.3} m² s^–1 for diffusion along [010]. Diffusion along [100]* gives an E_a-value of –312 ± 55 kJ mol^–1 with D₀ = 10^{0.5 ± 2.4} m² s^–1. Therefore, our experimental results show no difference between diffusion along [010] and [100]* (within error limits). The diffusion rate in pure synthetic diopside is about one order of magnitude faster than for synthetic diopside with very low Fe contents. A suitable explanation for this behavior is that in the case of low Fe diopside, the rate-limiting process for the protons associated with Fe is probably Fe-diffusion. In contrast, in pure diopside all protons are associated to Mg-defects, which are more mobile than Fe. Nevertheless, compared to natural diopside with appreciable Fe contents, diffusion rates in these synthetic samples are several orders of magnitude slower.

Key-words: diopside, FTIR spectroscopy, thermal annealing, hydrogen diffusion, defects.