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.
ASJC Scopus subject areas
- Electronic, Optical and Magnetic Materials
- Ceramics and Composites
- Condensed Matter Physics
- Physical and Theoretical Chemistry
- Inorganic Chemistry
- Materials Chemistry