In situ Raman vibrational spectra of siderite (FeCO3) and rhodochrosite (MnCO3) up to 47 GPa and 1100 K

Abstract Siderite (FeCO3) and rhodochrosite (MnCO3) are two interesting carbonate minerals, which normally occur in hydrothermal deposits on deep-sea altered oceanic crust. Despite the ubiquity of carbonates in the slab, little is known of the physicochemical behavior of siderite and rhodochrosite at high-pressure (P) and high-temperature (T) conditions during slab subduction. In this study, we characterized the Raman vibrational spectra of natural siderite and rhodochrosite up to 47 GPa and 1100 K in an externally heated diamond-anvil cell (DAC). Experimental results show that the Raman frequency shifts (νi) for siderite and rhodochrosite are a function of both P and T, and the effect of the P-T cross derivative term cannot be neglected, especially at high-P and high-T conditions. Based on the functional relationship of νi-P-T, the P-T calibrants of siderite and rhodochrosite are developed, respectively. This is significant for studying the water-carbonate interaction at high P-T conditions in a DAC because the undesired change of the experimental system from traditional pressure sensors (e.g., ruby, quartz) in a reaction chamber can be avoided. Like previous studies, we observed a sharp spin transition at ~45 GPa in siderite and a phase transition from MnCO3-I to MnCO3-II at ~46 GPa for rhodochrosite at room temperature. Furthermore, we determined the isobaric and isothermal equivalents of the mode Grüneisen parameter (γiT, γiP) and the anharmonic parameter (ai) for each Raman mode of siderite and rhodochrosite. The δνi/δP, δνi/δT, γiT, γiP, and ai span a much larger value range for the external lattice modes (T, L) than internal modes (ν4, ν1) in both siderite and rhodochrosite. Combining Raman frequency shifts and the first-order Murnaghan equation of state, we also developed a method to calculate the temperature dependence of the bulk modulus (KT) for siderite and rhodochrosite, respectively.