Journal of Crystal Growth 310 (2008) 706–715 Theoretical surface morphology of {0 1 1 ¯ 2} acute rhombohedron of calcite A comparison with experiments and {1 0 1 ¯ 4} cleavage rhombohedron F.R. Massaro, L. Pastero, M. Rubbo, D. Aquilano à Dipartimento di Scienze Mineralogiche e Petrologiche, Universita` degli Studi, via V. Caluso 35, 10125 Torino, Italy Received 10 August 2007; received in revised form 13 November 2007; accepted 15 November 2007 Communicated by R. Kern Available online 22 November 2007 Abstract Theoretical surface profiles of the {011 ¯ 2} form of calcite crystal are drawn by applying the Hartman-Perdok method that allows to build non-polar outermost crystal layers without imposing arbitrary surface reconstructions. Profiles consistent with the bulk crystal symmetry are showed to correspond to minima of the surface energy for both the Ca 2+ and CO 3 2 terminated profiles. To take into account the interface vibration entropy and the energy of water adsorption is crucial to evaluate the relative variations of the surface free energy of {0 1 1 ¯ 2} and {1 0 1 ¯ 4} rhombohedra with temperature and to explain why both forms can make the equilibrium shape of the crystal. Furthermore, the calculation of the edge energies allows to account for the surface patterns observed on both rhombohedra. r 2007 Elsevier B.V. All rights reserved. PACS: 65.40.gp; 68.35.Md; 68.37.Hk; 81.10.Aj; 81.10.Dn; 82.70.Gg Keywords: A1. Crystal morphology; A1. Surface structure; A2. Growth from solution; B1. Calcium compounds 1. Introduction The cleavage {1 0 1 ¯ 4} and the steep {0 1 1 ¯ 2} rhombohe- dra, along with the flat {0 1 1 ¯ 8} and the acute {1 0 1 ¯ 1} ones, belong to the growth morphology of calcite crystals most frequently observed in nature, while the two first forms largely dominate the morphology of crystals obtained in laboratory both from aqueous solution or gel growth [1,2]. The studies on the surface growth properties of the {1 0 1 ¯ 4} form are innumerable owing to the uniqueness of its surface profile: as a matter of fact, the cleavage rhombohedron is the most compact among the flat forms of calcite, is not polar, is the most stable form having the lowest specific surface energy and is easily manageable when simulating important features for growth (relaxation, adsorption, surface defects, etc.). Moreover, the growth and dissolution kinetics of this form can be carefully determined, since AFM measurements became available, thanks to the uncommon quality of its surface patterns (spirals, 2D nuclei, etch pits) [3–5]. The studies of the {0 1 1 ¯ 2} form developed differently from the previous one. The works done on this form can be roughly divided in two main branches: the more ancient papers concern the growth of the form in inorganic systems and the more recent ones deal with the interaction between the (0 1 1 ¯ 2) calcite crystal face and organic materials (biomineralization). We must mention two main historical studies on the experimental morphology of calcite in which the steep rhombohedron plays a fundamental role. The first one is due to Kalb [6] who proposed a morphogenetic order of ARTICLE IN PRESS www.elsevier.com/locate/jcrysgro 0022-0248/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2007.11.086 à Corresponding author. Tel.: +39 011 6705125; fax: +39 011 6705128. E-mail address: dino.aquilano@unito.it (D. Aquilano).