High-pressure, high-temperature phase stability of iron-poor dolomite and the structures of dolomite-IIIc and dolomite-V

The stability of Fe-poor dolomite, CaMg0.98Fe0.02(CO3)2, was studied by Raman spectroscopy and single-crystal X-ray diffraction at high pressures (P < 60 GPa) and high temperatures (T < 2300 K). Density functional theory calculations were employed to complement the experimental study. Between 40 and 60 GPa and 1800–2300 K, we observed the formation of a high P,T phase, “dolomite-V”, which, after quenching to ambient temperature, remained stable down to 12 GPa. The dolomite-V phase crystallizes in the space group C2/c with Z = 4 formula units. The structure of the high pressure polymorph “dolomite-IIIc” was solved, which crystallizes in the space group P1¯ with Z = 8 formula units. The combined experimental and theoretical findings show that at high pressures and low to moderate temperatures Dol-IIIc is formed, while at high pressures and high temperatures Dol-V becomes stable. Assuming that thermodynamic equilibrium is obtained at high-pressure, high-temperature conditions, the current study extends our understanding of the phase stability of Fe-poor dolomite polymorphs at upper and lower mantle conditions.