Point defects and orientation-dependent transport of matter and charge in iron-containing olivines

Abstract

The variation of the oxygen content, x O, of synthetic fayalite (Fe 2SiO 4) single crystals was investigated thermogravimetrically at 1130 °C as a function of the oxygen activity, a O 2 (= P O 2 /P O 2 ° ≈ f O 2 /f O 2 ° with P O 2 ° ≈ f O 2 ° = 1 bar ≈ 1 atm). It was found that x O varies less in fayalite single crystals than in polycrystalline Fe 2SiO 4 studied earlier. The majority defects are most likely cation vacancies, (V Me 2+ )″, ferric ions on M-sites, (Fe 3+ Me 2+ ) , and ferric ions on Si-sites, (Fe 3+ Si 4+ )′. Furthermore, the diffusion of iron in synthetic olivine single crystals ((Fe xMg 1 − x ) 2SiO 4) was studied at 1130 °C as a function of orientation, oxygen activity, and cationic composition. The observed oxygen activity dependencies suggest that cations move via different types of cation vacancies, most likely isolated vacancies, (V Fe 2+ )″, and possibly neutral associates, {2(Fe 3+ Me 2+ ) ⋅ (V Me 2+ ) ′ ​ ′ } x, the latter being minority defects. In addition, the electrical conductivity, σ, of fayalite single crystals was investigated as a function of orientation and oxygen activity within the stability field of fayalite at 1130 °C. The observed oxygen activity dependencies are compatible with (V Me 2+ ) ′ ​ ′ , (Fe 3+ Me 2+ ) , and (Fe 3+ Si 4+ )′ being the majority point defects at high a O 2 and with h ⋅ and e′ as the majority defects at low a O 2 . The electrical conduction in fayalite is governed by contributions of electrons and holes. This extended point defect model for fayalite is also compatible with data for the variation of the oxygen content and for the iron tracer diffusion.