THE SUBSTITUTION OF AL AND F IN TITANITE AT HIGH PRESSURE AND TEMPERATURE: EXPERIMENTAL CONSTRAINTS ON PHASE RELATIONS AND SOLID SOLUTION PROPERTIES

Abstract

Experimental studies were carried out to evaluate phase relations involving titanite–F–Al-titanite solid solution in the system CaSiO3–Al2SiO5–TiO2–CaF2. The experiments were conducted at 900–1000°C and 1·1–4·0 GPa. The average F/Al ratio in titanite solid solution in the experimental run products is 1·01 ± 0·06, and X Al ranges from 0·33 ± 0·02 to 0·91 ± 0·05, consistent with the substitution [TiO 2+ ] –1 [AlF 2+ ] 1 . Analysis of the phase relations indicates that titanite solid solutions coexisting with rutile are always low in X Al , whereas the maximum X Al of titanite solid solution occurs with fluorite and either anorthite or Al 2 SiO 5 . Reaction displacement experiments were performed by adding fluorite to the assemblage anorthite + rutile = titanite + kyanite. The reaction shifts from 1·60 GPa to 1·15 ± 0·05 GPa at 900°C, from 1·79 GPa to 1·375 ± 0·025 GPa at 1000°C, and from 1·98 GPa to 1·575 ± 0·025 GPa at 1100°C. The data show that the activity of CaTiSiO 4 O is very close to the ideal molecular activity model ( X Ti ) at 1100°C, but shows a negative deviation at 1000°C and 900°C. The results constrain ΔG ° f,298·15 of CaAlSiO4F to be −2595 ± 3 kJ/mol and S ° 298·15 to be in the range of 105·2–109·6 J/mol K, which in turn can be used to calculate petrogenetic grids involving titanite solid solutions in the system CaTiSiO4O–CaAlSiO4F.