Positive {hk.l} and negative {hk.} forms of calcite (CaCO3) crystal. New open questions from the evaluation of their surface energies

Abstract

The surface profiles of the usually neglected negative {hk.} forms of calcite are investigated, for the first time, and compared to those of the corresponding and well known positive ones: {10.4}, {01.2}, {01.8} and {21.4}. The approach combines the periodic bond chain (PBC) analysis by Hartman–Perdok (HP) that allows the optimal surfaces for a given crystal structure to be built followed by the calculation of the surface energy. The athermal equilibrium shape (at T′= 0 K), calculated for the relaxed surfaces, shows that the cleavage {10.4} rhombohedron is the most stable form of the crystal. Among the negative forms, only the steep rhombohedron {01.} has a surface energy close to that of {01.2} and could be the crystal equilibrium shape in particular environments. The growth shape calculated by the Bravais–Friedel–Donnay–Harker's (BFDH) reticular approach does not account for the properties of crystal surfaces.