A partially disordered natrolite: relationships between cell parameters and silicon-aluminum distribution

Natrolite, Na16Al16Si24O80.16H2O, is orthorhombic, space group Fdd2, with a 18.354(10), b 18.587(10), and c 6.608(4) Å. The structure shows a partial disorder in the (Si,Al) distribution of 30% in all the tetrahedral sites (R = 5.4, Rw = 5.2% for 463 reflections). The relation between cell parameters and the degree of disorder are discussed in relation to the present findings and to those available in the literature. There is an increase of 0.027 Å in the c parameter in going from the ordered to the disordered phase; the b - a difference, which is zero in the disordered phase, increases to 0.35 Å in the ordered one. The rotation and twisting of the tetrahedral chain, under the influence of Na, are responsible for the b - a difference.