Surface structure, morphology and (110) twin of aragonite

In order to better understand the growth mechanisms involving aragonite in both geological and biological systems, a detailed study of its surfaces was performed. By adopting a two dimensional slab model and performing empirical calculations, we determined the equilibrium geometry at 0 K of the (100), (010), (001), (110), (101), (011), (111), (102), (012), (021), (112), (121), (122), (031) and (130) surfaces, both in anhydrous and hydrated conditions. Furthermore, the equilibrium geometry at 0 K of the (110) twin boundary interface was also determined. The dry and solvated surface energies at 0 K of the crystal faces were also calculated, as well as the (110) twinning energy at 0 K, a key thermodynamical quantity for the determination of the equilibrium morphology of twins, and the estimation of their most probable mechanism of formation during growth. As concerns the (110) twin, the interface elastic energy was also evaluated. The solvated equilibrium shape (ES) of aragonite is drawn and compared to the dry one; a comparison with the previous dry and solvated ES calculated at 0 K is also performed. Furthermore, the surface structure modifications due to the presence of water is discussed. Our calculations explain the high occurrence of twinned crystals in case of nucleation and growth in inorganic environments. On the contrary, in living organisms the crystallization route and the crystal sizes and morphologies are deeply modified by adhesion on organic substrates and adsorption of organic molecules.