Ferromagnesite (Mg, Fe)CO₃ with 20 mol% iron is a potential host mineral for carbon transport and storage in the Earth mantle. The high-pressure behavior of synthetic ferromagnesite (Mg_0.81Fe_0.19)CO₃ up to 53 GPa was investigated by synchrotron X-ray diffraction (XRD) and Raman spectroscopy. The iron bearing carbonate underwent spin transition at around 44–46 GPa accompanied by a volume collapse of 1.8%, which also demonstrated a variation in the dν_i/dP slope of the Raman modes. The pressure-volume data was fitted by a third-order Birch-Murnaghan equation of state (BM-EoS) for the high spin phase. The best-fit K₀ = 108(1) GPa and K^′₀ = 4.2(1). Combining the dν_i/dP and the K₀, the mode Grüneisen parameters of each vibrational mode (T, L, ν₄ and ν₁) were calculated. The effects of iron concentration on the Mg_1−xFe_xCO₃ system related to high-pressure compressibility and vibrational properties are discussed. These results expand the knowledge of the physical properties of carbonates and provide insights to the potential deep carbon host.