DFT Simulations of the Structure and Cation Order of Norsethite, BaMg(CO₃)₂

Norsethite, BaMg­(CO₃)₂, is an interesting mineral that can be used to investigate processes leading to the formation of dolomite and other dolomite-type structures. To this end, it is first necessary to study in detail the Ba–Mg cation arrangement in the crystal structure of norsethite. In this work, first-principles calculations based on density functional theory (DFT) have been used to simulate cation ordering for the crystal structures of two BaMg­(CO₃)₂ polymorphs: the low-temperature polymorph (up to ∼360 K), α-norsethite (<i>R</i>3̅<i>c</i>), and the high-temperature polymorph (above ∼360 K), β-norsethite (<i>R</i>3̅<i>m</i>). We found that for both structural variants of norsethite, the most stable cation arrangements are those with the alternation of barium and magnesium layers along the <i>c</i>-axis. Furthermore, we have adequately simulated nonstoichiometric β-norsethite structures since some synthetic norsethites were found to have an excess of magnesium, which seems to favor the crystallization of β-norsethite at room temperature.