Solid solubility and cation distribution in the system Co2GeO4Mg2GeO4

In the system Co₂GeO₄Mg₂GeO₄, solid solubilities in spinel and olivine structures were studied on samples prepared by solid state reaction at temperatures of 1000–1300°C. The solubility limits were determined from the identification by X-ray powder pattern and the change of the lattice constant of spinel with composition. The relation between temperature and the free energy difference ΔG° which was estimated from the solubility limits agreed qualitatively with the fact that the spinel phase of Mg₂GeO₄ is stable at low temperatures under atmospheric pressure. The spinels were also synthesized at 800°C under the pressure of 20 kb. Over the whole range of composition, the cation distribution was found to be normal with u = 0.375. Above 1000°C under 20 kb in the presence of water, the spinels, except Co₂GeO₄, were found to react with water to form enstatite and probably magnesium hydroxide in an amorphous state.