Density functional theory-based calculations have been used to demonstrate that the aplanarity of CO3 groups in some carbonates such as dolomite, CaMg(CO3)2, aragonite, CaCO3, and norsethite, BaMg(CO3)2, is a ground-state property. This distortion stabilizes dolomite by approximately 500 J mol(-1). Up to at least 6 GPa, the aplanarity of CO3 groups in dolomite is independent of pressure. In aragonite the aplanarity increases slightly on increasing pressure, while a significant tilting of the CO3 groups occurs. The calculations do not support previous findings of anomalously low values for the pressure derivative of the bulk moduli, B', of aragonite and dolomite. Instead, the computed pressure dependences of the unit-cell volumes correspond to B' = 5.0 (5) for aragonite and B' = 4(1) for dolomite, when fitted with a third-order Birch-Murnaghan equation-of-state.