High-pressure phase relations in the system TiO2–ZrO2 to 12 GPa: stability of αPbO2-type srilankite solid solutions of (Ti1−x , Zr x )O2 (0 ≤ x ≤ 0.6)

Abstract

Phase relations in the system TiO2–ZrO2 were examined in the pressure range of 3.5–12 GPa at 1,800 °C, using multianvil apparatus. At 1,800 °C, TiO2 rutile transforms to αPbO2 structure at 10 GPa, and the αPbO2-type solid solution is stable in compositional range between TiO2 and about (Ti0.6, Zr0.4)O2 at 3.5–12 GPa. Combination of the present results with the published data at 0–3 GPa demonstrates that continuous solid solution with the αPbO2-type structure is stable between TiO2 and (Ti1−x, Zrx)O2 (x ≈ 0.6) at 0–12 GPa. This indicates that both the αPbO2-type TiO2 and srilankite Ti2ZrO6 with the same structure belong to the continuous solid solution system though the two phases have been regarded as different minerals. With increasing ZrO2 content, lattice parameters of a- and c-axes of the αPbO2-type solid solution increase, but b-axis is almost constant or slightly decreases. At higher pressure, the αPbO2-type solid solution dissociates into two phases, αPbO2-type phase and tetragonal zirconia. Srilankite with more TiO2-rich composition than Ti2ZrO6 might be found in natural rocks derived from the deep upper mantle.