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Journal of Materials Science

, Volume 43, Issue 14, pp 4693–4700 | Cite as

The role of protons in ionic diffusion in (Mg, Fe)O and (Mg, Fe)2SiO4

  • David L. KohlstedtEmail author
  • Stephen J. Mackwell
Reactivity of Solids

Abstract

The presence of hydrogen dissolved within iron-magnesium oxides and silicates results in an increase in the rate of Fe–Mg interdiffusion. Experimental data and point defect models suggest that the increased interdiffusivity is due to an increase in the total metal-vacancy concentration through stabilization of proton-vacancy defect associates in a hydrous environment. In the case of (Mg1–xFex)O, interdiffusion experiments under hydrothermal conditions at a fluid pressure of ∼0.3 GPa yield similar dependencies of interdiffusivity on Fe-content, oxygen fugacity, and temperature as under dry conditions, but interdiffusion coefficients are a factor of ∼3 larger. These data suggest that the increased interdiffusivities in (Mg1–xFex)O result from incorporation of defect associates formed between a metal vacancy and a single proton, \(\hbox{p}_{\rm Me}^{\prime} \equiv \{\hbox{p}^{\bullet}-\hbox{V}_{\rm Me}^{\prime\prime} \}^{\prime}.\) For (Mg1–xFex)2SiO4, interdiffusion under hydrothermal conditions over a range of fluid pressures reveals a significant difference in the dependence of interdiffusivity on Fe content than obtained under dry conditions, combined with a strong dependence on water fugacity. These data indicate that the increased diffusivities in (Mg1–xFex)2SiO4 result from incorporation of defect associates involving a metal vacancy and 2 protons, \(\hbox{(2p)}_{\rm Me}^\times \equiv \{2\hbox{p}^{\bullet} -\hbox{V}_{\rm Me}^{\prime\prime} \}^{\times}.\) It is anticipated that, at higher water fugacities, Fe–Mg interdiffusion in both materials will become dominated by these latter defects and that the interdiffusivity will increase linearly with water fugacity but will be independent of oxygen fugacity and iron concentration.

Keywords

Olivine Oxygen Fugacity Hydrous Condition Cation Vacancy Interdiffusion Coefficient 

Notes

Acknowledgments

Support from the National Science Foundation through grants EAR-0439747 (DLK) and EAR-0337012 (SJM) is gratefully acknowledged. The authors thank Dr. Sylvie Demouchy for her help. This article is LPI publication #1381.

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Copyright information

© Springer Science+Business Media, LLC 2008

Authors and Affiliations

  1. 1.Department of Geology and GeophysicsUniversity of MinnesotaMinneapolisUSA
  2. 2.Lunar and Planetary InstituteHoustonUSA

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