Changes in the oxygen content of Mn2SiO4 + delta in equilibrium with MnSiO3 were measured thermogravimetrically at 1000, 1100 and 1200 degrees C at a total pressure of about 0.5 atm as a function of the oxygen activity, a(O2), and were used to determine absolute values for delta. The obtained values for delta vary from about 15 . 10(-4) at 1200 degrees C and about 5 . 10(-4) at 1000 degrees C at high oxygen activities, to below 1 . 10(-4) at log(10) a(O2) approximate to -8.5 at both temperatures. Thus Mn2SiO4 + delta 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, (Mn-Mn2+(3+))(center dot), manganese vacancies, (VMn2+)'', and Mn4+ ions on silicon sites, (Mn-Si4+(4+))(x). This defect model accounts for the observed oxygen activity dependence of delta. 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 Kroger-Vink diagrams for a generic orthosilicate Me2SiO4 + delta for constant Si/(Me + Si)-ratios and for being in equilibrium with a second oxide of the type MeSiO3. (c) 2010 Elsevier B.V. All rights reserved.