Phase Relations in CaSiO₃ System up to 100 GPa and 2500 K

Phase relations in one of the key petrological systems, CaSiO₃, have been comprehensively investigated for the first time in the pressure range 0–100 GPa and temperatures 0–2500 K within the density functional theory using the method of lattice dynamics in the quasi-harmonic approximation. The results showed that at atmospheric pressure and 0 K CaSiO₃ is stable in the wollastonite structure, which above 1250 K transforms to the high-temperature pseudowollastonite modification. Above a pressure of 4 GPa, CaSiO₃ is stable in the breyite structure. The phase equilibrium curve has a negative slope of dP/dT = –0.6 MPa/K. At 8 GPa, CaSiO₃ decomposes into an assemblage of Ca₂SiO₄-larnite and titanite-structured CaSi₂O₅. The phase equilibrium curve has a positive slope of dP/dT = 1.35 MPa/K. At a pressure of 13 GPa, Ca₂SiO₄-larnite reacts with CaSi₂O₅, forming a phase with a perovskite-like structure – CaSiO₃-perovskite. The pressure of this phase transition is practically independent of temperature. In the low-temperature region, Ca-perovskite is stable in the tetragonal modification CaSiO₃-I4/mcm. Above 340 K at 13 GPa, Ca-perovskite is stable in the cubic modification CaSiO₃-$$Pm\bar {3}m.$$ The phase transition temperature increases to 755 K with pressure increase to 100 GPa. The thermodynamic parameters were also calculated for the first time for wollastonite, pseudowollastonite, and titanite-structured CaSi₂O₅.