Variation of the oxygen content and point defects in tephroite, Mn2SiO4 + δ

Changes in the oxygen content of Mn2SiO4 + δ in equilibrium with MnSiO3 were measured thermogravimetrically at 1000, 1100 and 1200 °C at a total pressure of about 0.5 atm as a function of the oxygen activity, aO2, and were used to determine absolute values for δ. The obtained values for δ vary from about 15 · 10− 4 at 1200 °C and about 5 · 10− 4 at 1000 °C at high oxygen activities, to below 1 · 10− 4 at log10 aO2 ≈ − 8.5 at both temperatures. Thus Mn2SiO4 + δ is non-stoichiometric over the entire oxygen activity range investigated. The deviation from stoichiometry in manganese silicate is modeled based on the formation of localized holes, (MnMn2+3+), manganese vacancies, (VMn2+)″, and Mn4+ ions on silicon sites, (MnSi4+4+)x. This defect model accounts for the observed oxygen activity dependence of δ. Furthermore, to better understand oxygen activity dependencies of point defect concentrations at experimentally relevant conditions, model calculations using a more general defect model were performed to derive Kröger–Vink diagrams for a generic orthosilicate Me2SiO4 + δ for constant Si/(Me + Si)-ratios and for being in equilibrium with a second oxide of the type MeSiO3.