In situ X-ray diffraction of aragonite and dolomite at high pressure and high temperature: Evidence for dolomite breakdown to aragonite and magnesite

Energy-dispersive X-ray powder diffraction spectra have been collected for aragonite (CaCO₃) and dolomite [CaMg(CO₃)₂] at high pressure and high temperature, using synchrotron radiation and a cubic multi-anvil apparatus. Unit-cell volumes were measured up to 7 GPa and 1073 K along several isothermal paths. This study has confirmed previously determined but still debated values of the bulk incompressibilities at 298 K [65.4(5) GPa for aragonite and 90.7(7) GPa for dolomite] and thermal expansivities at 1 bar [〈α〉_{298-1000 K} = 6.7(5) 10^-5 K^-1 for aragonite and 〈α〉_{298-1000 K} = 4.1(5) 10^-5 K^-1 for dolomite]. In addition, new equation-of-state parameters were measured: (∂K/∂T)_P[-0.013(2) and -0.025(4) GPa/K for aragonite and dolomite, respectively] and K′₀ [2.7(7) and 2.3(5) for aragonite and dolomite, respectively]. We suggest that these equation-of-state parameters could be used in the calculation of high-temperature, high-pressure thermodynamic properties of these carbonates. At pressures exceeding 5 GPa, we observed that dolomite breaks down to aragonite and magnesite. We used the equation-of-state parameters measured in this study to calculate the position of the equilibrium curve: dolomite → aragonite + magnesite. This reaction could be important in the ultra-high-pressure metamorphism of carbonates. This decomposition reaction also provides a useful test of thermodynamic data sets of carbonates at high pressures and temperatures.