Theoretical Equilibrium Morphology of Gypsum (CaSO₄·2H₂O). 2. The Stepped Faces of the Main [001] Zone

Abstract

The athermal equilibrium shape (ES) of gypsum crystal is calculated, in a vacuum, in the zone interval of the stepped {1k0} forms (3 e k e 8). The surface profiles are obtained by applying either the Hartman-Perdok method of the periodic bond chains (PBC) or the method of systematic cuts (SC) generated under the only conditions of stoichiometry, electroneutrality, and annihilation of the dipole moment perpendicular to each d 1k0 slice. The specific surface energy values γ 1k0 have been calculated, both for ideal and relaxed surface profiles, using a semiempirical potential function proposed by Adam. From calculations, it follows that the ES in the [001] zone is characterized not only by the well-known {120} and {010} flat forms and by the stepped {100} pinacoid but also by the {140} and {180} stepped forms. Further, following the PBC method, the stepped {130} and {170} forms also should enter the ES. Finally, an unambiguous analogy arises between our results and the systematic presence of the {1k0} faces on the giant crystals found in the Naica mine, whose growth occurs very near the equilibrium.

Three study cases of growth morphology in minerals: Halite, calcite and gypsum

“…This is a potential source of confusion and mistakes, since the indexing of crystallographic surfaces and directions change with the reference frame chosen. Three main conventions have been used for gypsum (length of parameters in Å) [32]: In this paper we will use the reference frame by de Jong and Bouman, since it is the most suitable to work with the growth morphology of gypsum; it's not unintentional that this frame, along with the analogues by Cole-Lancucki and Heijnen-Hartman have been called as "morphological cell. "…”

The (100) Contact Twin of Gypsum

“…Successively, the energy minimization (here performed using the GULP program; 27 for computational details see Massaro et al 14,15 ) refines the value of the three components of the rigid body translation and determines the atomic movements in the interface t. The atomic displacements (t) occur in a small volume extending over several d 200 layers, defining the interface phase.…”

“…The surface energy γ b of the stable (100) surface, corresponding to the profile of the B/W interface (γ b = γ 100 = 705.7 erg cm À2 , corresponding to 0.7057 J m À2 ) 14,15 has been used for calculating β bw .…”

“…As in the calculation, the translation vectors parallel to the 100 plane are fixed and periodic boundary conditions are imposed, the 2D parameters at the surface are the same as in the bulk, and the energy converges to a minimum when the atomic coordinates are optimized. The potential function implemented in GULP have been determined by Adam 17 and used previously 14,15 to calculate the surface energy of gypsum. The good agreement between calculated equilibrium morphology of gypsum with that of Naica crystals grown in condition not too far from saturation indicates the good quality of the Adam's potential.…”

Preparation and growth mechanism of micro spherical ammonium dinitramide crystal based on ultrasound-assisted solvent-antisolvent method

“…The growth morphology method could predict the shape of the crystal more accurately than the BFDH method because it takes the energetics of the system into account based on the attachment energy (AE) [20] . The equilibrium morphology of the crystal only depends on the specific free energies of the different {hkl} forms of the crystal according to Gibbs-Wulff’s theorem [21] .…”