Figure 1. Stability diagram of FeCO₃ at high P-T.

Symbols and phase regions identified in experiments: magnesite-structured FeCO₃ (orange dots), oxide(s) and recrystallized FeCO₃ (orange-blue dots), tetrairon(III) orthocarbonate Fe₄C₃O₁₂ (red triangles), diiron(II) diiron(III) tetracarbonate Fe₄C₄O₁₃+Fe₄C₃O₁₂ (red-green dots), Fe₄C₄O₁₂+oxides (red-blue dots), Fe₄C₄O₁₃+oxides (green-blue dots), FeCO₃ decomposition to Fe₃O₄+C+CO₂ (refs ²⁹, ³⁰; dark blue area), high- and low- spin FeCO₃ (dark and light orange area, respectively), incongruent melting of FeCO₃ (light blue area), and formation of HP-carbonates Fe₄C₃O₁₂ and Fe₄C₄O₁₃ (red and green areas). The grey dashed curve is the expected mantle geotherm⁵⁹. The black solid lines are from refs ²⁹, ³⁰. Black dashed lines indicate the region delimiting the spin transition in magnesio-siderite at HPHT from Liu et al.¹³. The vertical dashed black line separates the regions in which the formation of α-Fe₂O₃ and HP-Fe₃O₄ was observed upon incongruent melting of FeCO₃.