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 close-packed (cp) Ca layers. The two- and six-layered polytypes have hexagonal symmetry P6322 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, CO3 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  of aragonite being parallel to  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 CaCO3-II, -III, -IIIb, and -VI. The similarity of CaCO3-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 CaCO3 cation array at calcite to a post-aragonite transformation is described.
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