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

Carlos Pimentel,C Ignacio Sainz-Díaz,Carlos M Pina

doi:10.1021/acsearthspacechem.1c00058

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.

Highlights

IntroductionIts idealized structure can be described as layers of CO32− anionic groups, which separate alternate layers of Mg2+ and Ca2+ along the c-axis of a hexagonal cell
Dolomite, CaMg(CO3)[2], the second most abundant carbonate in the Earth’s crust, crystallizes in the R3̅ space group (SG) with Z = 3
We found that the parameters of the optimized structures are in good agreement with those experimentally determined[14,16] and previously calculated.[27,33]

Summary

Introduction

Its idealized structure can be described as layers of CO32− anionic groups, which separate alternate layers of Mg2+ and Ca2+ along the c-axis of a hexagonal cell. These layers can consist of just one type of cation (Mg2+ or Ca2+) or, alternatively, each layer can contain a more or less disordered distribution of Ca2+ and Mg2+ cations. In spite of its abundance and geological relevance, the mechanisms of the formation of dolomite at low temperatures remain elusive. Dolomite cannot be directly synthesized in laboratories at temperatures below 100 °C

Methods

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Discussion

Conclusion