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Siderite (FeCO 3 ) and rhodochrosite (MnCO 3 ) 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 (ν i ) 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 ν i - 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 MnCO 3 -I to MnCO 3 -II at ~46 GPa for rhodochrosite at room temperature. Furthermore, we determined the isobaric and isothermal equivalents of the mode Grüneisen parameter (γ iT , γ iP ) and the anharmonic parameter ( a  i ) for each Raman mode of siderite and rhodochrosite. The δν i /δ P , δν i /δ T , γ iT , γ iP , and  a  i  span a much larger value range for the external lattice modes (T, L) than internal modes (ν 4 , ν 1 ) 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  (K  T ) for siderite and rhodochrosite, respectively.

In situ Raman vibrational spectra of siderite (FeCO 3 ) and rhodochrosite (MnCO 3 ) up to 47 GPa and 1100 K