Theoretical Equilibrium Morphology of Gypsum (CaSO₄·2H₂O). 1. A Syncretic Strategy to Calculate the Morphology of Crystals

Abstract

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.

Growth morphology of gypsum in the presence of copolymers

“…Theoretical unrelaxed and athermal equilibrium shape of gypsum used as reference in this work, as obtained in a recent paper [35].…”

“…A detailed description of these profiles and of the methods we applied to obtain them has been proposed in a recent paper [35], where the theoretical and athermal equilibrium shape of gypsum has been predicted without considering the adsorption of water. This equilibrium shape is drawn in figure 7, as obtained by applying the Wulff's theorem to the surface energies of the crystal forms whose profiles are described in figures 4-6.…”

Morphology of gypsum: a case study

“…A comparison of figures 3 and 5 shows that the surface generated by the systematic cut has two molecules of water per unit mesh, more that the optimal surface obtained by the PBC method. The surface energy values are 543 (SC) and 561 erg/cm 2 (PBC), [6]. The energy gain of the surface SC is 2.1×10 -20 J per unit mesh (≅115 Å 2 ) and it is of the order of the energy of weak hydrogen bonds formed with the oxygen of the sulfate at the surface: the two shortest H---O distances are 1.89 Å and the H-O-S angle is 136°.…”

“…It is worth to appreciate that the richness of forms making the equilibrium morphology is due to the relaxation energy, affecting to an higher degree the less stable forms. This is discussed at length in our previous papers on gypsum [6,7]. In order to complete our discussion on the morphology of gypsum we report in figure 8, the growth morphology calculated using the energy of attachment associated to the most stable surface structures.…”

“…The computer programs we used to manipulate the crystal structure and performing the energy minimization are GDIS and GULP [4,5]. A description of the PBC analysis and of the equilibrium properties of the gypsum forms are given in [6,7]. In the following, the indexes of the faces are based on the structure of gypsum determined by [8].…”