(10.4) Face of Ordered and Disordered Dolomite, MgCa(CO3)2: A Computational Study to Reveal the Growth Mechanism

Marco Bruno,Erica Bittarello

doi:10.3390/min8080323

Abstract

In this study, the stability of the (10.4) face of dolomite was systematically investigated. The surface energies at 0 K of the different (10.4) surfaces resulting from the cut of both ordered and disordered bulk structures were determined and compared, to establish how different atomic configurations (surface terminations) can affect the stability of the investigated face. To study the thermodynamic behavior of a surface, a 2D periodic slab model and the ab initio CRYSTAL code were adopted. The surface energies of the (10.4) faces of calcite and magnesite were also calculated in order to compare them with those of the different terminations of the (10.4) face of dolomite. Our calculations showed that the bulk of the dolomite crystal must have an ordered structure to reach the minimum of the energy, whereas the (10.4) surface is more stable when its structure is disordered. A growth model of the (10.4) face has been proposed: the peculiarity of this model consists in the existence of some disordered layers forming at the interface crystal/solution, which arrange in an ordered structure once covered by others disordered layers resulting by the spiral steps propagation.

Highlights

Dolomite, CaMg(CO3 )2, is one of the most abundant carbonate minerals in the Earth [1,2].The study and the analysis of the stability of carbonate mineral phases, in general, are significant in several research fields of Earth Science and industrial applications
A growth model of the (10.4) face has been proposed: the peculiarity of this model consists in the existence of some disordered layers forming at the interface crystal/solution, which arrange in an ordered structure once covered by others disordered layers resulting by the spiral steps propagation
In order to minimize this positive surface free energy, the surface will be it is possible that the disordered layers forming the interface have a different composition enriched by the constituent with the lowest surface free energy; see the recent paper by with respect to that of the bulk layers

Summary

Introduction

CaMg(CO3 ) , is one of the most abundant carbonate minerals in the Earth [1,2]. Zucchini et al [16] performed ab initio calculations at the athermal limit (T = 0 K) to study the effect of cation order/disorder on the stability of dolomite. They carried out the calculations by means of the Minerals 2018, 8, 323; doi:10.3390/min8080323 www.mdpi.com/journal/minerals. CRYSTAL code [20,21] and using the WC1LYP Hamiltonian [22] They took into account nine different bulk structures of dolomite (named in their work as SC1, SC8, SC278, SC279, SC4341, SC4895, SC5505, CC2 and CC3) having space group R3 or R3, and different degrees of cation disorder. (iii) characterization of the different (10.4) surface terminations resulting by the cut of the bulk structures; (iv) description of a new growth model for dolomite

Computational Details

Bulk Structures of Dolomite

A Growth Model for Dolomite

Conclusions