CRYSTAL STRUCTURE OF ALLORIITE

Abstract

The cancrinite–sodalite group includes about 30 minerals, which are being studied in depth owing to their adsorption and ion-exchange properties. The minerals consist of a framework of Si and Al tetrahedra combined into six-membered rings. The rings of the same level along z are linked not to each other but to rings of neighbor levels shifted by 1/3 along x and y. The succession of alternation of rings is designated by letters A, B, and C , where A designates the rings around the axis [2/3 1/3 z]; B, around [1/3 2/3 z]; and C, around [0 0 z]. Minerals with 2 to 28 layers, which correspond to parameters with c ~ 5–74 Å, are known in this group. The channels and cages within the framework are occupied by large cations and anions (mono- and/or multiatom), while some minerals also include the water molecules. As is accepted for zeolites, the cages and channels can be represented as polyhedra formed by centers of Si and Al tetrahedra. Two-layer (AB) minerals of the cancrinite group (cancrinite and its analogs with diverse additional anions) have a wide channel and compact cancrinite cages consisting of five and six four-membered rings. In the multilayer minerals, the wide channel is missing, but cancrinite cages are supplemented with larger cages and their combination. In particular, the liottite cage found in the liottite structure, which is composed of 17 six-membered and 6 fourmembered rings, can be combined with a cancrinite cage.