Theoretical and experimental evidence for a post-perovskite phase of MgSiO$_3$ in Earth's D$''$ layer.

The Earth's lower mantle, the largest region within our planet (670-2890 km depths), is believed to contain $\sim $75 vol.{\%} of (Mg,Fe)SiO$_{3}$ perovskite, $\sim $20{\%} (Mg,Fe)O, and $\sim $5{\%} CaSiO$_{3}$. This mineralogy was unable to explain many unusual properties of the D'' layer, the lowermost $\sim $150 km of the mantle. Using \textit{ab initio} simulations and high-pressure experiments we have demonstrated [1] that at pressures and temperatures of the D$''$ layer, MgSiO$_{3}$ transforms from perovskite into a layered CaIrO$_{3}$--type structure (space group \textit{Cmcm}); this structure was also independently found in [2]. The elastic properties of the new phase and its stability field explain most of the previously puzzling properties of the D$''$ layer: its seismic anisotropy [3], strongly undulating shear-wave discontinuity at its top$^{ }$[4], and the anticorrelation between shear and bulk sound velocities [5]. This new phase is therefore likely to be a major Earth-forming mineral, and its discovery will change our understanding of the deep Earth's interior. Latest studies of the effects of impurities [6,7] on the stability of this phase, and similar phases of other compounds will be discussed. \\ \\ REFERENCES: \\1. Oganov A.R., Ono S. (2004). \textit{Nature} \textbf{430}, 445-448. \\2. Murakami M., et al. (2004). \textit{Science} \textbf{304}, 855-858. \\3. Panning M., Romanowicz B. (2004). \textit{Science }\textbf{303}, 351-353. \\4. Sidorin I., et al. D.V. (1999). \textit{Science} \textbf{286}, 1326-1331. \\5. Su W.J., Dziewonski A.M. (1997). \textit{Phys. Earth Planet. Inter.} \textbf{100}, 135-156. \\6. Mao W.L., et al. (2004). \textit{Proc. Natl. Acad. Sci.} \textbf{101}, 15867-15869. \\7. Ono S., Oganov A.R., Ohishi Y. (2004).\textit{ Submitted.}