Crystal Structure of Chabazite, a Molecular Sieve

THE ‘molecular-sieve’ properties of chabazite, a zeolite occurring in basaltic rocks, have been studied extensively. Molecules such as argon (diameter 3.84 A.) and methane (4.25 A.) are rapidly sorbed in the dehydrated zeolite; propane (width 4.9 A.) and n-butane are slowly sorbed, while iso-butane (5.6 A.) is excluded. A crystal structure was proposed by Wyart1 in 1933, which consists of a three-dimensional framework of four- and six-membered rings of (Si,Al)O4 tetrahedra (Fig. 1a). The rings form two cages per unit cell, the cages sharing six-membered rings which provide apertures of maximum dimension 2.9 A. if the usual Si—O and Al—O distances are used, and 3.1 A. if the somewhat unusual distances in Wyart's structure are adopted. Although Kington and Laing2 have interpreted the sorptive properties of chabazite in terms of Wyart's structure, the agreement can scarcely be regarded as satisfactory when a molecule of minimum dimension 4.25 A. passes easily through an aperture of diameter 2.9–3.1 A. As the structure of chabazite was proposed in the early days of crystal structure determination further investigation appeared desirable.