Experimental determination of activity-composition relations in Ni 2SiO 4− Mg 2SiO 4 and Co 2SiO 4-Mg 2SiO 4 olivine solid solutions at 1200 K and 0.1 MPa and 1573 K and 0.5 GPa

Abstract

Activity-composition relations in the olivine solid solutions Ni 2SiO 4 − -Mg 2SiO 4 and Co 2SiO 4-Mg 2SiO 4 have been determined at 1200 K and 0.1 MPa and at 1573 K and 0.5 GPa by equilibration with the corresponding oxide solutions. Both olivine solutions show small positive deviations from ideal (two site) mixing, which, within the limits of accuracy of the method, may be described by the simple regular solution model with parameters W Ni+ Mg ol = 0.35 ± 1.0 kJ/ g-atom and W Co- Mg ol = 1.37 ± 0.9 kJ/ g-atom. The requirements of internal consistency between the two systems also show that the recent determination by Brousse et al. (1984) of the enthalpy of formation of Mg 2SiO 4is to be preferred over earlier work, and that their value is also probably more accurate than the uncertainty in their own measurements indicates; activities in the NiO-MgO system are close to ideal.