Effects of Spin Transition and Cation Substitution on the Optical Properties and Iron Partitioning in Carbonate Minerals

The high-pressure behavior of deep carbonate dictates the state and dynamics of oxidized carbon in the Earth's mantle, playing a vital role in the global carbon cycle and potentially influencing long-term climate change. Optical absorption and Raman spectroscopic measurements were carried out on two natural carbonate samples in diamond-anvil cells up to 60 GPa. Mg-substitution in high-spin siderite FeCO₃ increases the crystal field absorption band position by approximately 1000 cm^–1 but such an effect is marginal at >40 GPa when entering the low-spin state. The crystal field absorption band of dolomite cannot be recognized upon compression to 45.8 GPa at room temperature but, in contrast, the high-pressure polymorph of dolomite exhibits a strong absorption band at frequencies higher than (Mg,Fe)CO₃ in the low-spin state by 2000–2500 cm^–1. Additionally, these carbonate minerals show more complicated features for the absorption edge, decreasing with pressure and undergoing a dramatic change through the spin crossover. The optical and vibrational properties of carbonate minerals are highly correlated with iron content and spin transition, indicating that iron is preferentially partitioned into low-spin carbonates. These results shed new light on how carbonate minerals evolve in the mantle, which is crucial to decode the deep carbon cycle.