Pressure-Induced Phase Transitions of Natural Brookite

The structures and stabilities of the high-pressure phases of titania (TiO2) are of great interest in the earth sciences, as these phases are the accessible analogs of component minerals in the earth's mantle. Brookite is a natural titania mineral, whose bulk phase is hard to synthesize in the laboratory, and its phase behavior at very high pressure remains unknown. Thus, in this work, using phase-pure natural brookite as the sample, we studied the phase transition of bulk brookite under compression up to ∼60 GPa in three different pressure transmitting media. Results show that, at room temperature and in near hydrostatic conditions, brookite undergoes a series of transitions, i.e., brookite (∼0-12 GPa) → TiO2-II and minor rutile (∼1-21 GPa) → baddeleyite (∼12-60 GPa) → TiO2-OI (∼33-60 GPa). Taking into account that all transitions occur at finite rates and that solidification of a pressure medium can influence transition kinetics, we show that the observed transition sequence is consistent with the expected high-pressure phase stability of TiO2 calculated from density functional theory. The knowledge obtained from this work may be used to infer the geological fate of brookite in nature