Pressure and Temperature Effects on Low-Density Mg3Ca(CO3)4Huntite Carbonate

Abstract

Pressure (P)-volume (V)-temperature (T) relations of huntite [Mg3Ca(CO3)4] have been determined in situ up to 5 GPa and 500 °C using a resistive-heated diamond-anvil cell and synchrotron X-ray diffraction. Three runs were carried out: (i) compression at room temperature, (ii) heating at room pressure, and (iii) heating under compression. Experiments have been complemented with density functional theory calculations. The experimental (theoretical) bulk modulus of huntite is B0 = 102.6(2) GPa (99.55 GPa) with a first-pressure derivative of B′0 = 4.5(2) (4.483). The atomic distribution in space and the different compressibilities of [MgO6] octahedra and [CaO6] trigonal prisms in the R32 rhombohedral unit cell cause a strongly anisotropic compressibility. The axial compressibilities are 1.91(2) × 10-3 GPa-1 (2.395 × 10-3 GPa-1) and 4.52(5) × 10-3 GPa-1 (4.405 × 10-3 GPa-1) for the a and c axes, respectively. The volumetric thermal expansion of huntite at low pressures is estimated to be 2.21(4) × 10-5 K-1 (2.95 × 10-5 K-1), slightly lower than for calcite, magnesite, and dolomite. No phase transition was observed in the studied P-T range. The decomposition of huntite into CaCO3, MgO, and CO2 occurs at ∼410 °C independently of pressure when the sample is heated at 2 °C/min, but it is strongly dependent on the heating rate (i.e., the higher the heating rate, the higher the temperature at which decomposition starts). The crystal chemistry and phase stability of huntite are compared to those of other carbonates.