Dissociation of CaIrO$_3$-type MgSiO$_3$ in the gas giants*

CaIrO$_3$-type MgSiO$_3$ is the planet-forming silicate stable at pressures and temperatures (PTs) beyond those of Earth's core-mantle boundary. We have found using first principles quasiharmonic free energy computations that this mineral dissociates into MgO and SiO$_2$ at PTs expected to occur in the cores of the gas giants ($>\sim$10 Mbar, 10,000 K). This transformation should be important also for modeling the internal structure of two recently discovered terrestrial exoplanets: a dense Saturn orbiting HD149026b and a super Earth orbiting GJ876d. We propose a low pressure route experiment to confirm this dissociation.