P-V-T equation of state of CaCO₃ aragonite to 29 GPa and 1673 K: In situ X-ray diffraction study

Pressure–volume–temperature relations have been measured to 29 GPa and 1673 K for CaCO₃ aragonite using synchrotron X-ray diffraction with a multianvil apparatus at the ‘SPring-8’ facility. A least-squares fit of the room-temperature compression data to the Vinet-Rydberg equation of state (EOS) yielded K_{T 0} = 65.7 ± 0.8 GPa and K_T' = 5.1 ± 0.1, with fixed V₀ = 227.11 Å³. Further analysis of the high-temperature compression data led to the temperature derivative of the bulk modulus (∂K_T/∂T)_P = −0.016 ± 0.001 GPa/K and zero-pressure thermal expansion α = a₀ + a₁T with a₀ = 4.98 (22) × 10⁻⁵ K⁻¹ and a₁ = 2.81(38) × 10⁻⁸ K⁻². The Mie-Gruneisen-Debye approach revealed the Gruneisen parameter γ₀ = 1.39 at a fixed Debye temperature θ₀ = 516 K and the parameter q = 1. Analysis of axial compressibility and thermal expansion indicates that the c-axis is two times more compressible than the b-axis and four times more compressible than the a-axis, whereas zero-pressure thermal expansion of the a-axis (a_{0 a} = 2.6 × 10⁻⁵ K⁻¹ and a_{1 a} = 2.3 × 10⁻⁸ K⁻²) is weaker than that of the b-axis axis (a_{0 b} = 6.3 × 10⁻⁵ K⁻¹ and a_{1 b} = 0.1 × 10⁻⁸ K⁻²) and c-axis axis (a_{0 c} = 5.2 × 10⁻⁵ K⁻¹ and a_{1 c} = 9.5 × 10⁻⁸ K⁻²). A full set of thermodynamic parameters (including heat capacity, enthalpy and free energy) for aragonite and updated equations of state for magnesite and siderite was obtained using the Kunc-Einstein approach. The new EOS parameters were used for thermodynamic calculations for aragonite decarbonation reactions. The present thermal EOS provides accurate calculations of aragonite density to deep mantle. Decarbonation of subducting oceanic crust containing 2 wt% aragonite would result in a 0.5% density reduction at 30 GPa and 1273 K. Aragonite becomes denser than magnesite at pressures about 16 GPa along the 1500 K isotherm and at 9 GPa along the 298 K isotherm.