New solid silicates (or oxides) in the multimegabar range
In 2004, Murakami et al. (2004) reported a new high-pressure phase of magnesium silicate oxide, post-perovskite, which is stable at the base of the Earth’s lower mantle.
It is thus reasonable to think that such a structure is present, if not the dominant structure of super-Earths’ rocky mantles, unless another phase transition occurs with further pressure and temperature increase in such planets.
This question is not very different from the one posed about the MgSiO3 dissociation presented above, which has to be examined at the light of the stabilization of possible new structures.
Umemoto et al. (2011) found the dissociation of MgSiO3 to have a very negative Clapeyron slope and to happen at 0.9 TPa.
The theoretical work of Tsuchiya & Tsuchiya (2011) also showed that dissociation of post-perovskite into oxides was likely at 1 TPa when another hexagonal denser phase of SiO2 was taken into account.
On the other hand, some authors have carried out several studies using compounds isostructural to MgSiO3 perovskite.
With a work on NaMgF3, Grocholski et al. (2010) proposed as an analogue for MgSiO3 a transition toward another stable phase at much higher pressure. With shock experiments on GdGa5O12, Mashimo et al. (2006) showed a possible transition toward a virtually incompressible oxide at 120 GPa, thus suggesting that a similar A3B5O12 oxide with A=Mg or Fe, B=Si might exist in the deep mantle of massive extrasolar rocky planets.
© Dave Martin
Structure of post-perovskite