The structure of sodalite was first studied by
Linus Pauling in 1930. It is a cubic mineral of
space group P3n (
space group 218) which consists of an aluminosilicate cage network with Na+ cations and chloride anions in the interframework. (There may be small amounts of other cations and anions instead.) This framework forms a
zeolite cage structure. Each unit cell has two cavities, which have almost the same structure as the
borate cage found in the
zinc borate , the
beryllosilicate cage , and as in the similar mineral
tugtupite () (see Haüyne#Sodalite group). There is one cavity around each chloride ion. One chloride is located at the corners of the unit cell, and the other at the centre. Each cavity has
chiral tetrahedral symmetry, and the cavities around these two chloride locations are mirror images one of the other (a
glide plane or a four-fold
improper rotation takes one into the other). There are four sodium ions around each chloride ion (at one distance, and four more at a greater distance), surrounded by twelve tetrahedra and twelve tetrahedra. The silicon and aluminum atoms are located at the corners of a
truncated octahedron with the chloride and four sodium atoms inside.) Each oxygen atom links between an
tetrahedron and an tetrahedron. All the oxygen atoms are equivalent, but one half are in environments that are
enantiomorphic to the environments of the other half. The silicon atoms are at the location (0, 1/2, 1/4) and symmetry-equivalent positions, and the aluminum ions at the location (1/2, 0, 1/4)and symmetry-equivalent positions. The three silicon atoms and the three aluminum atoms listed above closest to a given corner of the unit cell form a six-membered ring of tetrahedra, and the four in any face of the unit cell form a four-membered ring of tetrahedra. The six-membered rings can serve as channels in which ions can diffuse through the crystal. The structure is a crumpled form of a structure in which the three-fold axes of each tetrahedron lie in planes parallel to the faces of the unit cell, thus putting half the oxygen atoms in the faces. As the temperature is raised the sodalite structure expands and uncrumples, becoming more like this structure. In this structure the two cavities are still chiral, because no
indirect isometry centred on the cavity (i.e. a reflexion, inversion, or improper rotation) can superimpose the silicon atoms onto silicon atoms and the aluminum atoms onto aluminum atoms, while also superimposing the sodium atoms on other sodium atoms. A discontinuity of the
thermal expansion coefficient occurs at a certain temperature when chloride is replaced by sulfate or iodide, and this is thought to happen when the framework becomes fully expanded or when the cation (sodium in natural sodalite) reaches the coordinates (1/4, 1/4, 1/4) (et cetera). == Properties ==