Understanding the fate of carbonate rocks at mantle depths is crucial for modeling the global carbon cycle. Calcite, CaCO₃, undergoes a pressure-induced transition from calcite-I (R3̅c) to calcite-II (P2₁/c) close to 1.6 GPa, but the effects of these changes on chemical reactivity and electronic properties remain poorly understood. Combining high-pressure X-ray diffraction at the ID27 beamline of the EBS-ESRF synchrotron with charge density analysis reveals subtle electronic and structural changes in the calcite structure with pressure. By determining the integrated charges, atomic basin volumes, and shapes of Ca, C, and O atoms, we effectively decompose the equation of state into individual atomic contributions. The analysis reveals that the phase transition is preceded by a discontinuous charge redistribution between Ca, C, and O atoms, occurring before the symmetry change and reversing upon decompression without hysteresis. Notably, carbon displays negative atomic compressibility, expanding under pressure due to electron uptake, while Ca and O atoms behave conventionally. Our findings demonstrate that experimental charge density methods can resolve pressure-induced electron density rearrangements with unprecedented detail, offering new perspectives on the mineralogy of Earth's interior.