Calcite crystals were nucleated and grown from supersaturated aqueous solutions in the presence of variable concentrations of lithium. The diagram of supersaturation vs [Li+]/[Ca2+] concentration ratio (“morphodrome”) shows a continuous habit variation, from the dominant {011̄1} rhombohedron (at low [Li+]/[Ca2+] ratio) to the dominant {0001} form (at high [Li+]/[Ca2+] ratio). The morphological change is interpreted in terms of two-dimensional layers having the structure of the monoclinic Li2CO3 crystal which are epitaxially adsorbed on the restructured {0001} form of calcite. Hence, even if {0001} is a K form (in the sense of Hartman-Perdok), the corresponding surface behaves like a F form, growing layer by layer from low to high supersaturation values.

The theoretical and equilibrium morphology of gypsum (CaSO4·2H2O) is reassessed, starting from the historical papers by Simon and Bienfait (1965) and by Heijnen and Hartman (1991). The surface profiles of the most frequently observed crystal forms have been determined following two ways: in the first one we used the Hartman−Perdok method based on the periodic bond chain (PBC) analysis, while in the second one the profile of each face was obtained using the GDIS program. In both cases, the calculation of the specific surface energy values has been made using the general utility lattice program (GULP) code. From the synthesis of the two methods, a new and much more isotropic equilibrium shape is calculated, in the case of both unrelaxed and relaxed surfaces. Further, and for the first time, beyond the well-known and singular {010}, {120}, {01̅1}, and {1̅11} F-forms, two stepped, {100} and {1̅22}, and one kinked form, {1̅02}, are found to build the equilibrium shape of gypsum.

The “Four Twin Laws of Calcite”, expressed by their twin planes (10.4, 01.8, 01.2 and 0.01, hexagonal frame) have been investigated in order to assess the structure of the interfaces between twinned individuals. Athermal twinning and adhesion energies were evaluated by calculations using empirical potential functions: the relaxed interfaces were modeled to minimize the surface energy. To this end, different configurations of the twin interfaces have been considered when surface reconstructions were needed to cancel out the surface dipole moment, which is intrinsic to the ideal interfaces of both the (00.1) and (01.2) twins. Calculation shows that (i) the (00.1) twin law is the most likely to occur, and (ii) the minimum of the twinning energy is obtained if the twin operation is a diad axis perpendicular to the (00.1) interface. Finally, it has been demonstrated that the (00.1) twins are the only ones that can be generated by a 2D heterogeneous nucleation mechanism, while the 3D (either homogeneous or heterogeneous) nucleation is the dominating mechanism for the other twins.

This paper deals with several properties of calcite crystals. First, the solvated surface energies at 0 K of the {101̅ 4}, {101̅ 0}, {112̅ 0}, {011̅ 8}, {011̅ 2}, {0001}, and {213̅ 4} forms were determined by means of the COSMIC method. We find that the presence of water reduces by ∼7−14% the dry surface energy at 0 K, in a homogeneous way. The solvated and dry equilibrium shapes at 0 K result to be nearly homothetic: in both cases the {101̅ 4}, {101̅ 0}, {011̅ 2}, and {0001} forms are present. In a second part of the paper, the free energy at 300 K of the most widely studied {101̅ 4}/water interface has been determined (γ (101̅ 4)/w 300 K = 0.412 ± 0.020 J/m 2 ) by combining simple thermodynamic relations and contact angle measurements. Some considerations on the free energy of other interfaces:{101̅ 0}, {112̅ 0}, {011̅ 8}, {011̅ 2}, {0001}, and {213̅ 4} are also proposed. We show that our results are consistent with thermodynamic constraints and fit quite well the known dependence of the solubility of calcite on grain size experimentally determined. Furthermore, as a side result and for the first time, on the basis of our estimate of γ (101̅ 4)/w 300 K and calculated (101̅ 4) solvated surface energy, the latter affected by the limits of the COSMIC method, a rough estimate of the vibrational energy and entropy (γ (101̅ 4)/w 300 K,vib ) and configurational entropy (γ (101̅ 4)/w 300 K,config ) of the (101̅ 4)/water interface is obtained: (γ (101̅ 4)/w 300 K,vib + γ (101̅ 4)/w 300 K,config ) = −0.066 ± 0.038 J/m 2 . Finally, by applying the classical nucleation theory and our estimate of γ (101̅ 4)/w 300 K we calculated the activation energy for homogeneous and heterogeneous nucleation at room temperature. We found extremely high values for such thermodynamical quantity, suggesting that the direct formation of calcite should be prevented. This strongly supports the view that calcite formation should proceed throughout the early formation of an amorphous calcium carbonate phase (ACC).