High-PT study of solid solutions in the system ZrO₂-TiO₂: The stability of srilankite

The ZrO₂-TiO₂ phase diagram was studied with synthesis experiments between 1200°C and 1650°C, 1 atm and 28 kbar, investigating the effect of pressure on the compositions of rutile [TiO₂], zirconia [ZrO₂] , and zirconium titanate [(Zr,Ti)₂O₄) solid solutions. All three phases become more Ti-rich with increasing pressure, which is in good agreement with the smaller ionic radius of Ti^4- compared to Zr⁴⁺, and consistent with the observed unit-cell volume of (Zr,Ti)₂O₄. The range of zirconium titanate solid solution, which is limited to X_Ti < 0.58 at room pressure, was extended to X_Ti = 0.68 at 28 kbar and 1500°C. Thus the compounds (Zr,Ti)₂O₄ [also referred to ZrTiO₄ when X_Ti = 0.5] and ZrTi₂O₆ (srilankite), which were previously regarded as the respective high- and low-temperature forms of zirconium-titanate, are both stable at high temperatures (< 1200°C), and are part of the same solid solution that is continuous with pressure. Srilankite (X_Ti = 0.67) formed at 1440°C and 28 kbar in equilibrium with rutile. This is the first synthesis of the phase at high pressures under equilibrium conditions. The stability of srilankite at high pressures and temperatures in our experiments contrasts with earlier studies that proposed a hydrothermal origin for the mineral, with an upper stability limit of 900°C. Our results, however, are consistent with the natural occurrence of srilankite in high-grade rocks such as eclogites, granulites, lamprophyres and chromitites. Our experiments show that the stability of zirconium titanate is also strongly dependent fugacity.