Gibbs energy changes of reactions of Mg-Fe distribution and oxidation in coexisting (MgFe)₂SiO₄ and (MgFe) SiO₃, with special references to its petrogenetic significance

Abstract

Gibbs energy changes of (Mg-Fe) distribution reaction (12) and oxidation reaction (i), (ii), (iii) are calculated on the basis of thermodynamic constant (Table 2) and experimental data (Table 1, Nafziger et al., 1967). Also it is discussed that the error of Gibbs energy changes calculated from experimental data are smaller than the other from thermodynamic constant. As a result of correspondence between Gibbs energy changes obtained from Nafziger et al. (1967) and calculated from thermodynamic data, the thermodynamic constant used in the calculation may be preferable. For the representaiton of Mg-Fe distribution constant, the equation (2) is more suitable in comparison with the equation (13) because of unity on the consideration of the Gibbs energy changes.
The stability relations of Fe-oxide and silicate are given in Fig. 2. Fe₂SiO₄ is more strongly persist to Po₂ than FeSiO₃ at high temperature but reverse at low temperature. Mg- rich olivine coexisting with Ferich pyroxene in calc-alkaline rock (Fig. 3) may be explained by the above relation.
Also it is discussed that the Mg-Fe distribution in coexisting olivine and pyroxene may be difficult to discuss simply because of non ideality of these minerals.