Theoretical prediction of new mineral phases in Earth's mantle and core (Invited)

After theoretical-experimental discovery of MgSiO3 post-perovskite [1,2], many other important mineral phases have been proposed in the deep Earth’s interior. We have developed [3] and further enhanced [4] an evolutionary method for predicting the most stable crystal structure at given thermodynamic conditions. Here, I will illustrate several examples from our recent works. For example, we have predicted new phases of CaCO3, MgCO3 and CO2 at Earth’s mantle pressures, and many of these phases have already found experimental support [5-7]. These results shed new light on the behavior of carbon in the Earth’s mantle [7]. More recently, we have studied the behavior of methane at high pressures and temperatures [8], and we confirm that indeed CH4 should break down under pressure - first, into hydrocarbons (ethane, butane) and hydrogen, and then into diamond and hydrogen. Crucial role here is played by lattice vibrations (zero-point vibrations and entropic factor). These vibrational effects are frequently neglected, but we have demonstrated that without them the decomposition into diamond and hydrogen would not be possible. Considering variable-composition systems, we have demonstrated [9] that FeSi with the CsCl-type structure is the only iron silicide stable at pressures of the Earth’s inner core. Similar studies can be performed also for Fe-O, Fe-S, Fe-O and Fe-H systems, addressing the common assumptions on their behavior at ultrahigh pressures of the inner core. REFERENCES: [1] Murakami M., et al., Science 304, 855-858 (2004). [2] Oganov A.R., Ono S., Nature 430, 445-448 (2004). [3] Oganov A.R., Glass C.W., J. Chem. Phys. 124, 244704 (2006). [4] Lyakhov A.O., Oganov A.R., Valle M. Comp. Phys. Comm. 181, 1623-1632 (2010). [5] Oganov A.R., Glass C.W., Ono S., Earth Planet. Sci. Lett. 241, 95-103 (2006). [6] Ono S., Kikegawa T., Ohishi Y. Am. Mineral. 92, 1246-1249 (2007). [7] Oganov A.R., et al., Earth Planet. Sci. Lett. 273, 38-47 (2008). [8] Gao G., Oganov A.R., et al., J. Chem. Phys., in press (2010). [9] Zhang F., Oganov A.R., Geophys. Res. Lett. 37, art. L02305 (2010).