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

The variation of the oxygen content, x(O), of synthetic fayalite (Fe(2)SiO(4)) single crystals was investigated thermogravimetrically at 1130 degrees C as a function of the oxygen activity, a(O)(2) (= P(O2)/P degrees(O2) approximate to f(O2)/f degrees(O2) with P degrees(O2) approximate to f degrees(O2) = 1 bar approximate to 1 atm). It was found that x(O) varies less in fayalite single crystals than in polycrystalline Fe(2)SiO(4) studied earlier. The majority defects are most likely cation vacancies, (V(Me2-))'', ferric ions on M-sites, (Fe(Me2+)(3+))(center dot) and ferric ions on Si-sites, (Fe(si4-)(3+))'. Furthermore, the diffusion of iron in synthetic olivine single crystals ((Fe(x)Mg(1 - x))(2)SiO(4)) was studied at 1130 degrees 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(Fe2-))'', and possibly neutral associates, [2(Fe(Me2-)(3+))(center dot) (V(Me2+))'')(x), the latter being minority defects. In addition, the electrical conductivity, sigma, 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(Me2+))'', (Fe(Me2+)(3+))(center dot), and (Fe(Si4+)(3+))' being the majority point defects at high a(O2) and with If and e' as the majority defects at low a(O2) 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. (C) 2011 Elsevier B.V. All rights reserved.