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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- 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. [ABSTRACT FROM AUTHOR]

The equations of state of different phases in the CaSiO3 system (wollastonite, pseudowollastonite, breyite (walstromite), larnite (Ca2SiO4), titanite-structured CaSi2O5 and CaSiO3-perovskite) are constructed using a thermodynamic model based on the Helmholtz free energy. We used known experimental measurements of heat capacity, enthalpy, and thermal expansion at zero pressure and high temperatures, and volume measurements at different pressures and temperatures for calculation of parameters of equations of state of studied Ca-silicates. The used thermodynamic model has allowed us to calculate a full set of thermodynamic properties (entropy, heat capacity, bulk moduli, thermal expansion, Gibbs energy, etc.) of Ca-silicates in a wide range of pressures and temperatures. The phase diagram of the CaSiO3 system is constructed at pressures up to 20 GPa and temperatures up to 2000 K and clarifies the phase boundaries of Ca-silicates under upper mantle conditions. The calculated wollastonite–breyite equilibrium line corresponds to equation P(GPa) = −4.7 × T(K) + 3.14. The calculated density and adiabatic bulk modulus of CaSiO3-perovskite is compared with the PREM model. The calcium content in the perovskite composition will increase the density of mineral and it good agree with the density according to the PREM model at the lower mantle region. [ABSTRACT FROM AUTHOR]