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

Abstract

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 (R3¯ c), and the high-temperature polymorph (above ∼360 K), β-norsethite (R3¯ m). 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.