The (100) Contact Twin of Gypsum

Abstract

We investigated the structure of the interface between two gypsum crystals forming a (100) contact twin. The athermal twinning and adhesion free energies were calculated using empirical potential functions. By minimizing the twin energy, the optimal interface configuration was obtained and carefully described. The layer expansion and in plane deformation occurring at the twin interface is analyzed in terms of the atomic relaxation displacements. A comparison with previous studies on this twin has been made. A discussion on the formation of the (100) contact twin follows where we show that this twin can be formed by two-dimensional nucleation.

Surface structure, morphology and (110) twin of aragonite

“…2). The same relation between energy and structure was observed for the (0001) contact twin of calcite 34,35 and (100) 36,37 contact twin of gypsum (CaSO 4 •2H 2 O), where the following twinning energies were estimated: 0.001-0.047 and 0.014 J m −2 , respectively. From the energetic point of view, the lower the twinning energy, the higher the probability of obtaining a twinned crystal.…”

“…These very low values are due to the strong similarity among the equilibrium structure of the 110 twin and that of the normal crystal (see Figure 2). The same relation between energy and structure was observed for the (0001) contact twin of calcite 34,35 and (100) 36,37 contact twin of gypsum (CaSO 4 •2H 2 O),…”

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

“…molecular dynamics simulations). [21][22][23][24][25][26][27][28] In this paper, first, we will predict the theoretical GS of calcite by applying the BFDH approach and compare our results with the occurrence frequency of the natural calcite {hk.l} forms. Secondly, we will extend the investigation to the surface profile of the negative {hk.l ¯} forms of calcite, following the HP approach, and focus our attention on their specific surface energies.…”

Interfaces structure and stress of gypsum (CaSO4·2H2O) penetration twins

“…The knowledge of the stable surface profiles is what we need in order to generate the twins: the essential features of the five contact twins of gypsum were determined and, for each geometrical law, the character of the original composition plane (OCP) was attributed and the corresponding twin energy was calculated. [13][14][15] Thus, according to the magnitude of the twin energy, the five twins were grouped in three classes corresponding to the 100 law, to the 001 and 1 ¯01 laws and, finally, to the 201 ¯and 101 ones. The 100 law can be assumed as the touchstone for the other laws, [13][14][15] because the {100} surfaces are the only ones belonging to the Eq.Sh of the crystal; the twin energy (13.6 erg cm 22 ) is an order of magnitude lower than that of the laws 001 and 1 ¯01 (145 and 255 erg cm 22 , respectively).…”

“…[13][14][15] Thus, according to the magnitude of the twin energy, the five twins were grouped in three classes corresponding to the 100 law, to the 001 and 1 ¯01 laws and, finally, to the 201 ¯and 101 ones. The 100 law can be assumed as the touchstone for the other laws, [13][14][15] because the {100} surfaces are the only ones belonging to the Eq.Sh of the crystal; the twin energy (13.6 erg cm 22 ) is an order of magnitude lower than that of the laws 001 and 1 ¯01 (145 and 255 erg cm 22 , respectively).…”