Phase equilibria in partially open systems under pressure: the decomposition of stoichiometric GeO2 oxide

The behavioral peculiarities of a highly compressed substance in partially open thermodynamic systems is discussed. Unlike for their closed counterparts, for open systems the time factor and effective pressure differences between various parts of the system should be taken into account. This proves to be relevant to many high-pressure experiments and especially so to geophysics, where celestial bodies are not closed systems with respect to their components. To demonstrate the unusual behavior of partially open systems, results on the decomposition of the stoichiometric GeO₂ oxide under heating and pressure are presented. It is shown that under a moderate pressure of 100 kbar or less, in most cases the high entropy of the volatile component is the determining factor for the plausible decomposition reaction of compounds involving light elements, whereas at megabar pressures the ratio of specific volumes of the reaction components becomes more important. The results of the work suggest that the decomposition of compounds with volatile components at ultrahigh pressures and high temperatures might be a source of gaseous planetary atmospheres. In particular, the decomposition of Fe, Si, and Mg oxides in the Earth interior can serve as an additional geological source of oxygen beyond the familiar biogenic source. An alternative model for the formation of the Earth metal core is proposed within the framework of this hypothesis.