MnO exsolution in periclase from Langban, Sweden: An evaluation of the activity-composition relations in the system MgO-MnO

The occurrence of MnO exsolution in periclase host crystals from L (a) over circle ngban, Sweden, is described. The composition of the periclase is Mg-0.942 Mn0.046Zn0.012O, and it coexists with exsolved manganosite crystals with a composition of Mn0.909Mg0.082Zn0.008O. Crystals of hausmannite are also present, indicating fo(2) 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, sigma W-Mno = 2.5 kJ/mol and W-MgO = 8.2 kJ/mol, sigma W-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 degrees C and 466 degrees C were calculated. The difference between the calculated temperatures is ascribed to inaccuracies in the experimentally determined interaction parameters.