Phase Transitions of Earth and Planetary Materials at High Pressure
High-pressure experiments on phase transitions of mantle-constituent minerals and bulk rocks provide indispensable data that clarify the mineralogical constitution of the deep mantle. This paper reviews the results of high-pressure experimental studies carried out in recent years. Phase relations of olivine-wadsleyite-ringwoodite transitions in pyrolite have been precisely determined to compare with seismological observations of the 410 and 520 km discontinuities. Results on the postspinel transition to perovskite + magnesiowüstite, which corresponds to the 660 km discontinuity, still have some controversies in transition pressure as well as the boundary slope. In pyrolite mantle, Ca-poor and Ca-rich pyroxenes are dissolved into garnet to form majorite in the transition zone. Recent studies have indicated that majorite transforms directly to aluminous perovskite in the normal mantle, but that it may transform first to aluminous ilmenite and then to perovskite at relatively low temperatures, such as in subducting slabs. Phase transitions in diopside and wollastonite have recently been examined in detail. The Ca component in majorite is exsolved as CaSiO3-perovskite in the transition zone of the pyrolite mantle. Mg-rich perovskite in the lower mantle contains both Fe and Al components, in which Fe may be present in both ferrous and ferric states. The aluminum in Mg-rich perovskite introduces some vacancies in oxygen sites that may considerably affect elastic properties and possibly incorporate water in the structure. In basalt, Mg-rich perovskite becomes stable at higher pressure than that of the 660 km depth. Because basalt and continental crust materials have higher contents of Al and Si, several aluminous silicate phases that do not appear in pyrolite are stable in lower-mantle conditions. They are calcium ferrite – and hollandite-structured phases and a new hexagonal aluminous phase that can host Na and K in the deep lower mantle. Recent studies on phase transitions in SiO2 have indicated that stishovite transforms to a CaCl2-type phase, which further changes to an α-PbO2-type phase. The Na- and K-hollandites andα-PbO2-type SiO2 were found in shocked meteorites.