The crystal structures of aluminosilicate-sodalites: X-ray diffraction studies and computer modelling

Abstract X-ray powder methods have been used to study the room-temperature structures of the synthetic sodalites: Li8(Al6Si6O24)Cl2, K7.6Na0.4(Al6Si6O24)Cl2, and Na8(Al6Si6O24)I2. Natural sodalite was also studied and the atomic coordinates show satisfactory agreement with those determined from the single-crystal data of Löns and Schulz (1967). The LiCl- and KCl- as well as the NaCl-sodalites refined in the expected sodalite space group P 3n, but the NaI-sodalite fitted 1 3m better. The resulting structural data reveal shortcomings in the previous computer models for sodalite structures and an improved computer modelling procedure is devised which successfully predicts atomic coordinates, starting from the experimental a value and an estimate of the cationanion distance. The method incorporates the experimental result that the average T-O distance (T = Al, Si) throughout the samples is ∼ 1.678 Å, and Si-O and Al-O are set at 1.618 and 1.738 Å, respectively. Although T-O remains little changed throughout the samples, the data confirm the inverse relationship between ∠ T-O-T and the tetrahedron tilt angle ϕ, in which ∠ T-O-T approaches ∼ 160° as ϕ → 0° and the sodalite cage becomes fully expanded.