Oxide solution mechanisms in silicate melts: Systematic variations in the activity coefficient of SiO2

Phase equilibria and spectroscopic data are used to develop a simple model for the interaction of various oxide components and molten SiO₂. Network modifying oxides, M_xO_y produce nonbridging oxygens thereby depolymerizing the SiO₂ network. The energetics of nonbridging oxygen formation are least favorable when the field strength of the metal cation is high. This produces relatively strong M-O and Si-O-Si bridging bonds at the expense of weaker Si-O-M bonds (De Jonget al., 1980). This relationship is manifested by an increase in positive deviations from ideality with increasing cation field strength in M_xO_y-SiO₂ systems; the activity coefficient of SiO₂ is inversely correlated with Si-O-M bond strength. Network forming oxides (aluminates, phosphates, titanates. zirconates, etc.) may copolymerize with the SiO₂ network. Mixing on the same quasi-lattice produces solutions which approach ideality. Deviations from ideality in such solutions can be linked to distortions in the SiO₂ network. Discrete anion formers (phosphates, titanates, chromates, zirconates) complex with metal oxides other than SiO₂ to form discrete structural units which do not copolymerize with SiO₂. The SiO₂ network is essentially shielded from the high charge density cations in such systems and unmixing is common. As a result, the relative deviations from ideality in such melts are high. It is important to recognize that oxides such as P₂O₅, TiO₂ and ZrO₂ may act as either network-formers or discrete anion formers depending upon melt composition, and are probably distributed between these two “sites” in most geologically important liquids. The latter structural role is favored in more basic compositions.