The structure of some sodium and calcium aluminosilicates

Abstract

Two years ago it was pointed out(1) that the electrostatic valence rule requires that the aluminum ions in alkali aluminum silicates have the coordination number 4 rather than 6, oxygen ions in the crystal then being common to a silicon tetrahedron, an aluminum tetrahedron, and one or more alkali-ion polyhedra. Since that time examples of such structures have been found. Thus the cubic crystal sodalite,(2) Na4Al3Si3O12Cl, has a framework composed of tetrahedra, each of which shares its corners with four others, silicon and aluminum tetrahedra alternating. The three parameters determining the position of the oxygen atoms were evaluated from the x-ray data, and it was found that both the silicon and the aluminum tetrahedra are approximately regular, with the distances Si-O = 1.60 Å and Al-O = 1.74 Å. The sodium and chlorine ions are arranged in cavities in the tetrahedral framework.

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