Thermodynamic relations among olivine, spinel, and phenacite structures in silicates and germanates: I. Volume relations and the systems NiOMgOGeO₂ and CoOMgOGeO₂

An experimental approach is outlined to systematically obtain free energy differences among olivine, spinel, and phenacite forms of silicates and germanates from the thermodynamics of terminal solid solutions in ternary systems. This is applied to the ternary systems NiOMgOGeO₂ and CoOMgOGeO₂ at 1200°C in air and to the system NiOMgOGeO₂ at 800°C and 0.57 kbar water pressure. From the location of conjugation lines, activity-composition relations along each orthogermanate join are calculated. The free energies of transformation from the olivine to the spinel structure at 1200°C are estimated to be +1.6, -3.5, and -8.2 kcal/mole for Mg₂GeO₄, Co₂GeO₄, and Ni₂GeO₄, respectively. Volume changes for the spinel-olivine and olivine-phenacite transitions are estimated for the silicates and germanates of Mg, Mn, Fe, Co, Ni, and Zn.