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 (R3̅c), and the high-temperature polymorph (above ∼360 K), β-norsethite (R3̅m). 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 c-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.