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3.7 h. Measurement of oxide activities in silicate melts in the CaO-MgO-Al₂O₃-SiO₂ system by equilibration with Pd (L. Chamberlin)

It is possible to determine directly the activities of the oxide components MgO, Al₂O₃, or SiO₂ in melts and minerals through analysis of Pd alloys equilibrated with them at fixed T and fO₂. The activity of each component relative to the pure solid oxide is a simple function of the ratio of the Mg, Al, or Si dissolved in Pd equilibrated with the sample to the Mg, Al, or Si in Pd equilibrated with the corresponding oxide. Previous experimentation suggested a dependence of the activity coefficients of MgO, Al₂O₃, and SiO₂ on composition for simple silicate liquids in the system CaO-MgO-Al₂O₃-SiO₂-TiO₂ (CMAST) that were synthetic analogs of Ca,Al-rich inclusions in carbonaceous chondrites. Measurement of oxide activities have now been performed on compositions in the "basalt tetrahedron" (forsterite-diopside-CaTschermaks-silica) in the system CaO-MgO-Al₂O₃-SiO₂ (CMAS), with the intent of extending the study to more silicic compositions and lower Al:Si ratios, and providing a simple model system in which changes in thermodynamic properties as a function of differentiation in an Fe-free basaltic system may be observed. Comparison between activities determined experimentally at 1250° - 1400 °C and those predicted by published models shows good agreement overall, although the SiO₂ activities are systematically high. Plots were made of the activity coefficients γ_i of the various oxide components as a function of composition using both the prior data on the meteoritic liquids and the new data on the Fe-free basaltic liquids. A strong positive correlation exists between γ_SiO2 and X_SiO2, which probably indicates the controlling influence of SiO₂ as a network former (Fig. 3.7-7). There is also a good negative correlation between γ_Al2O3 and the number of non-bridging oxygens per total oxygens in the melt (NBO/Σ O), implying that
 

Fig. 3.7-7: Activity coefficient of SiO2 in silicate melt as a function of mole fraction of SiO2.

 
depolymerization of the melt results directly in the breakup of Al - O bonds. However, γ_SiO2 increases either with decreasing NBO/Σ O or decreasing Al:Si ratio, such that there is no correspondingly simple relationship between γ_SiO2 and NBO/Σ O. This implies that network modifiers preferentially attack Al - O bonds as they break up the silicate framework. The activity coefficient of MgO exhibits a well-defined negative correlation with X_SiO2 and a positive correlation with X_CaO, implying an overall negative relationship between g_MgO and the degree of polymerization. However, there is no clear correlation between γ_MgO and NBO/Σ O, perhaps due to the presence of the second network modifier CaO. The effects of differentiation in these liquids may be observed in the increase of γ_SiO2 as the silica-poor phases forsterite and pigeonite crystallize from the melt.