Iron and titanium solution properties in peraluminous and peralkaline rhyolitic liquids

Abstract

The saturation surface of pseudobrookite (Fe₂TiO₅) was determined for melts in the system SiO₂-Al₂O₃-K₂O-FeO-Fe₂O₃-TiO₂ at 1400° C and 1 atm. The variation in concentrations of Fe₂O₃, TiO₂ and Fe₂TiO₅ in liquids can be used to infer relative changes in activity coefficients of these components with changing K₂O/(K₂O+Al₂O₃) of the melts. Saturation concentrations of these components are low and relatively constant in the peraluminous melts and increase with increasing K₂O/(K₂O+Al₂O₃) in peralkaline liquids. The activity coefficients of Fe₂O₃ and TiO₂ and Fe₂TiO₅, therefore, are higher in peraluminous liquids than in peralkaline liquids in this system. In addition, the iron redox ratio was measured as a function of K₂O/(K₂O+Al₂O₃) for liquids just below the saturation surface; $f_{{\text{O}}_{\text{2}} }$ was fixed so all variations in redox ratio are entirely due to changes in melt composition. The redox ratio from unsaturated liquids was applied to saturated liquids where redox analysis of the glass is impossible. The homogeneous equilibrium experiments indicate that the activity coefficient of Fe₂O₃ relative to that of FeO is significantly greater in peraluminous melts than peralkaline melts. Both the heterogeneous and homogeneous equilibria suggest that in peralkaline liquids K⁺in excess of that required to charge balance tetrahedral Al⁺³ is used to stabilize both Fe⁺³ and Ti⁺⁴. Calculations show that ferric iron and titanium compete equally effectively for charge-balancing potassium but neither can outcompete aluminum. The observed changes in solution properties of Fe₂O₃ and TiO₂ in the synthetic melts are used to explain variations in Fe-Ti oxide stabilities in natural peraluminous and peralkaline rhyolites and granites. Since the activity coefficients of both ferric iron and titanium are significantly higher in peraluminous liquids than in peralkaline liquids, Fe-Ti oxides should occur earlier in the crystallization sequence in peraluminous rhyolites than in peralkaline rhyolites. In addition, iron will be reduced in peraluminous granites and rhyolites relative to peralkaline ones under comparable P, T, and $f_{{\text{O}}_{\text{2}} }$. Finally, observed crystallization patterns for minerals containing highly charged cations other than ferric iron and titanium are evaluated in the context of this and other experimental studies.