Aplanarity of CO3 groups: a theoretical investigation

Abstract

Density functional theory-based calculations have been used to demonstrate that the aplanarity of CO_{3} groups in some carbonates such as dolomite, CaMg(CO_3)_2, aragonite, CaCO_3, and norsethite, BaMg(CO_3)_2, is a ground-state property. This distortion stabilizes dolomite by ∼500 J mol^{-1}. Up to at least 6 GPa, the aplanarity of CO_3 groups in dolomite is independent of pressure. In aragonite the aplanarity increases slightly on increasing pressure, while a significant tilting of the CO_3 groups occurs. The calculations do not support previous findings of anomalously low values for the pressure derivative of the bulk moduli, B^\prime, of aragonite and dolomite. Instead, the computed pressure dependences of the unit-cell volumes correspond to B^\prime = 5.0 (5) for aragonite and B^\prime = 4 (1) for dolomite, when fitted with a third-order Birch–Murnaghan equation-of-state.