The occurrence of MnO exsolution in periclase host crystals from Långban, Sweden, is described. The composition of the periclase is Mg_0.942 Mn_0.046Zn_0.012O, and it coexists with exsolved manganosite crystals with a composition of Mn_0.909Mg_0.082Zn_0.008O. Crystals of hausmannite are also present, indicating fo₂ conditions approaching the MnO-Mn₃O₄ equilibrium.

Activity-composition data for the system MgO-MnO is critically reviewed. The data obtained from solid electrolyte measurements are rejected because of the unrealistic calculated equilibration temperatures and the large deviations from the gas equilibration Mn-Pt activity measurements. The large errors associated with the gas equilibration measurements, especially at large X_MnO, also introduces uncertainties associated with the asymmetric interaction parameters. The non-parametric ‘jack-knife’ method was used to determine the asymmetric interaction parameters and their standard deviations from the data of Wood et al. (1994) and Tsai and Muan (1992). These were calculated as W_MnO = 21.2 kJ/mol, σW_MnO = 2.5 kJ/mol and W_MnO = 8.2 kJ/mol, σW_MgO = 3.3 kJ/mol.

Using the asymmetric interaction parameters, W_MnO = 19.9 kJ/mol and W_MgO = 13.7 kJ/mol for the system MgO—MnO as determined by Wood et al. (1994) and the equations for two-phase equilibrium as formulated by Thompson (1967), equilibration temperatures of 334°C and 466°C were calculated. The difference between the calculated temperatures is ascribed to inaccuracies in the experimentally determined interaction parameters.