Phase Stability and Vibrational Properties of Iron-Bearing Carbonates at High Pressure

The spin transition of iron can greatly affect the stability and various physical properties of iron-bearing carbonates at high pressure. Here, we reported laser Raman measurements on iron-bearing dolomite and siderite at high pressure and room temperature. Raman modes of siderite FeCO3 were investigated up to 75 GPa in the helium (He) pressure medium and up to 82 GPa in the NaCl pressure medium, respectively. We found that the electronic spin-paring transition of iron in siderite occurred sharply at 42-44 GPa, consistent with that in the neon (Ne) pressure medium in our previous study. This indicated that the improved hydrostaticity from Ne to He had minimal effects on the spin transition pressure. Remarkably, the spin crossover of siderite was broadened to 38-48 GPa in the NaCl pressure medium, due to the large deviatoric stress in the sample chamber. In addition, Raman modes of iron-bearing dolomite Ca1.02Mg0.76Fe0.20Mn0.02(CO3)(2) were explored up to 58 GPa by using argon as a pressure medium. The sample underwent phase transitions from dolomite-I to -Ib phase at similar to 8 GPa, and then to -II at similar to 15 and -IIIb phase at 36 GPa, while no spin transition was observed in iron-bearing dolomite up to 58 GPa. The incorporation of FeCO3 by 20 mol% appeared to marginally decrease the onset pressures of the three phase transitions aforementioned for pure dolomite. At 55-58 GPa, the nu(1) mode shifted to a lower frequency at similar to 1186 cm(-1), which was likely associated with the 3 + 1 coordination in dolomite-IIIb. These results shed new insights into the nature of iron-bearing carbonates at high pressure.