MnO exsolution in periclase from Långban, Sweden: an evaluation of the activity-composition relations in the system MgO–MnO

The occurrence of MnO exsolution in periclase host crystals from Långban, Sweden, is described. The composition of the periclase is Mg 0.942 Mn 0.046 Zn 0.012 O, and it coexists with exsolved manganosite crystals with a composition of Mn 0.909 Mg 0.082 Zn 0.008 O. Crystals of hausmannite are also present, indicating f O 2 conditions approaching the MnO–Mn 3 O 4 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 MgO = 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.