THE CRYSTAL CHEMISTRY OF NEPHELINE

Abstract

MoK X-radiation. No superstructure reflections were observed. Viewed down [001], the aluminosilicate tetrahedra occur at the vertices of a 63 net in which there are two distinct types of six-membered rings of tetrahedra: one quarter are nearly regular hexagonal rings centered on (0, 0, z), whereas the other three-quarters are flattened hexagonal rings centered on 0 z. In both rings, the tetrahedra have the sequence ududud, and linkage up and down c forms a four-connected three-dimensional framework. The A site is located in the channel along 0 0 z and is [9]-coordinated by oxygen atoms between 2.97 and 3.04 Å; <A–O> = 2.999, 3.009 and 3.008 Å for samples 1, 2 and 3, respectively. The A site is fully occupied by K in nepheline (1), and it is partly occupied by K (0.80 and 0.88) in samples (2) and (3). The B site occurs in the channels at 0 z and is [8]-coordinated by oxygen atoms between 2.48 and 2.79 Å; <B–O> = 2.633, 2.632 and 2.627 Å, respectively. The B site is occupied mainly by Na [with minor Ca in nepheline (3)]. For all three samples, Si and Al are highly ordered over the T sites of the framework: T(1,4) = Al, <T(1)–O> = 1.731, 1.723 and 1.732, <T(4)–O> = 1.736, 1.728 and 1.734; T(2,3) = Si, <T(2)–O> = 1.606, 1.603 and 1.605, <T(3)–O> = 1.616, 1.615 and 1.616 Å. For the nepheline sample from the Khibina–Lovozero complex, the simplified chemical formula, K2 Na6 Si8