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

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.

Epitaxy: a methodological approach to the study of an old phenomenon

“…According to the terminology universally accepted concerning epitaxy, when the lattice constants of phases A and B match, that is when (1 × 1)-A ≡ (1 × 1)-B, the interface is said to be coherent; when a relation such as (m × n)-A ≡ (k × s)-B exists, with m, n, k and s integers (and the supercell parameters are not too long on the lattice length scale), the interface is commensurate; otherwise it is incommensurate. Moreover, as widely discussed in some recent papers, [2][3][4][5][6][7][8][9] crystal faces (hkl) A and (h′k′l′) B can show several surface terminations (e.g., different structures for the same surface). If the number of the surface terminations is p and r for (hkl) A and (h′k′l′) B , respectively, the number of possible interface configurations can be very high, p × r.…”

“…By examining Table 1, it is possible to highlight that: 2 than the surface energy of (10.4) Cc , γ (10.4) Cc being 0.507 J m −2 ; 2 (ii) 7 than the surface energy of (00.1) Cc , where γ (00.1) Cc = 0.711 J m −2 ; 7 (iii) (001) Ar shows a strong affinity with (1 ¯01) Za ; indeed, the (001) Ar /(1 ¯01) Za interface has an interfacial energy of…”

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

“…5−7 In particular, recent quantum mechanical and empirical calculations on dolomite nucleation and growth provided new insights into the formation of ordered dolomite. 8,9 These calculations have shown that while a dolomite bulk structure with a completely ordered cation arrangement is the configuration that is energetically most favorable, the dolomite (10.4) surface is energetically more favorable when Ca 2+ and Mg 2+ cations are randomly distributed, i.e., when such a surface shows cation disorder. 8,9 According to Bruno et al, 9 the cationic order in the dolomite bulk structure would increase progressively by the intracrystalline diffusion process.…”

“…In the last few decades, cation ordering arrangements in minerals have been studied using atomic computational methods. − In particular, recent quantum mechanical and empirical calculations on dolomite nucleation and growth provided new insights into the formation of ordered dolomite. , These calculations have shown that while a dolomite bulk structure with a completely ordered cation arrangement is the configuration that is energetically most favorable, the dolomite (10.4) surface is energetically more favorable when Ca 2+ and Mg 2+ cations are randomly distributed, i.e., when such a surface shows cation disorder. , According to Bruno et al, the cationic order in the dolomite bulk structure would increase progressively by the intracrystalline diffusion process. The sluggishness of such a process at ambient conditions is consistent with dolomite formation times of millions of years.…”

Impurity Effects on Habit Change and Polymorphic Transitions in the System: Aragonite–Calcite–Vaterite

“…All values are expressed in erg cm −2 . γ Cal10.4 and γ Zab 001 were calculated at B3LYP level by Bruno and Bittarello49 and Bruno and Prencipe,50 respectively.…”