Metastable structures of CaCO₃ and their role in transformation of calcite to aragonite and postaragonite

Abstract

Using molecular dynamics simulation and evolutionary metadynamic calculations, a series of structures were revealed that possessed enthalpies and Gibbs energies lower than those of aragonite but higher than those of calcite. The structures are polytypes of calcite, differing in the stacking sequence of closepacked (cp) Ca layers. The two-and six-layered polytypes have hexagonal symmetry P6 3 22 and were named hexarag and hexite, respectively. Hexarag is similar to aragonite, but with all the triangles placed on the middle distance between the cp layers. On the basis of the structures found, a two-step mechanism for the transformation of aragonite to calcite is suggested. In the first step, CO 3 triangles migrate to halfway between the Ca layers with the formation of hexarag. In the second step, the two-layered cp (hcp) hexarag structure transforms into three-layered cp (fcc) calcite through a series of many-layered polytypes. The topotactic character of the transformation of aragonite to calcite, with [001] of aragonite being parallel to [0001] of calcite, is consistent with the suggested mechanism. High-temperature X-ray powder diffraction experiments did not reveal hexarag reflections. To assess the possibility of the formation of the polytypes found in nature or experiments, a TEM analysis of ground aragonite was performed. A grain was found that had six superstructure reflections in a direction perpendicular to the plane of the cp layer. This grain is believed to correspond to one of the predicted polytypes, with the diffuse character of the diffraction spots indicating a partial disordering of the cp layer stacking. A topological analysis was also performed, along with energy calculations, of the metastable high-pressure polymorphs CaCO 3 -II, -III, -IIIb, and -VI. The similarity of CaCO 3 -II, -II, and -IIIb to the calcite structure and the small energy difference explain the metastable formation of these polymorphs during the cold compression of calcite. On the basis of the performed analysis, the evolution of the CaCO 3 cation array at calcite to a post-aragonite transformation is described.

Calcite/Aragonite Epitaxy: A Computational Study for Understanding Mollusk Shell Formation

“…Surprisingly, the computationally predicted CaCO 3 polymorph spontaneously generates at the (001) Ar /(00.1) Cc interface and at low temperature (0 K), is the same as that already obtained by other researchers, when studying the high-temperature aragonite–calcite breakdown by means of both empirical , and quantum-mechanical simulations . In particular, Gavryushkin et al, through molecular dynamics simulation at the quantum-mechanical level and evolutionary metadynamic calculations, found that the aragonite structure was stable up to 700–800 K. At 800 K, the new phase with hexagonal symmetry was formed and was stable up to 1000 K; this hexagonal CaCO 3 polymorph was named hexarag (hAr). Instead, at an empirical level, depending on the potential used the phase transition from aragonite to hexarag has been fixed at 500 and 1706 K, respectively.…”

Novel Calcium sp3 Carbonate CaC2O5-I4̅2d May Be a Carbon Host in Earth’s Lower Mantle

“…On a pressure increase, calcite is known to transform into metastable high-pressure polymorphs (CaCO 3 -II, CaCO 3 -IIIb, CaCO 3 -III, and CaCO 3 -VI ,− ). CO 2 dry ice transforms into CO 2 -III at ∼11 GPa .…”

The role of local heteropolyhedral substitutions in the stoichiometry, topological characteristics and ion-migration paths in the eudialyte-related structures: a quantitative analysis

“…The simplification of the atomic nets in the crystal structures is actively used to classify various types of chemical compounds, both organic or metal-organic (O'Keeffe & Yaghi, 2012;Xie et al, 2019;Shevchenko et al, 2020), and inorganic (Kabanova et al, 2020;Gavryushkin et al, 2021). For example, the classification of zeolite frameworks (Baerlocher & McCusker, 2017) becomes more universal and comprehensive after applying the principles of topological classification based on underlying nets.…”