Growth twins and deformation twins of sodalite-type microporous compounds

Abstract

The tendency of sodalite-group minerals and synthetic materials with the sodalite-type structure to form growth twins has long been known. Judging by faceting, the twin components are connected by a 180-degree turn around the [111] axis of the cubic lattice. In this work, the structure of growth twins, including simple prismatic twins elongated in the [111] direction and V-shaped twins, has been studied in detail at the atomic level. It is shown for elongated twins that the boundary between the twin components passes in the (111) plane through the common vertices of TO4 tetrahedra (T = Al, Si). The coordinates of these vertices are connected by pseudosymmetry operators, each of which can be a twinning operator. This can be either a 180-degree turn around the [111] axis or a reflection in any of the planes (2‾ 11), (12‾ 1), or (112‾). Twinning by a plane is accompanied by smaller distortions of the motif at the twin boundary and, therefore, is energetically more favorable. V-shaped twins are regular intergrowths of two simple twins, the prism axes of which are directed along different ⟨111⟩ directions of the cubic lattice. In addition to growth twins, deformation twins of sodalite and related materials are discussed. Their symmetry is usually lower than cubic, and twinning is often combined with modulation of the crystal structure or the formation of a superstructure. Crystal twinning can be a consequence of a phase transition from a high-temperature cubic phase with a symmetry lowering. So far, no examples are known when growth twinning is combined with deformation twinning, but such a possibility is not ruled out.