Aplanarity of CO3 groups: a theoretical ­investigation

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