Gibbs free-energy-of-mixing surface, ﹩{\hat{G}}_{{\rm{mix}}}﹩, in the ternary system MgSiO₃–Fe–H₂ for T = 3500 K and P = 4 GPa. The heavy black lines show the spinodals denoting the regions of spontaneous decomposition into two phases. Points comprising the binodals are shown in blue connected by tie lines. The blue binodal points define the coexistence curves. Between the binodals and spinodals lies the region of metastability for two phases.