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Regular version of the site

Article

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

Crystal Growth & Design. 2021. Vol. 21. No. 1. P. 65-74.
Gavryushkin P., Belonoshko Anatoly B., Sagatov N., Sagatova D., Zhitova E., Krzhizhanovskaya M. G., Recnik A., Alexandrov E. V., Medrish I. V., Popov Z., Litasov K.

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 …