The Crystal Chemistry of Nepheline

The crystal structures of three nepheline samples, (1) K1.91 Na6.04 Si8.15 Al7.72 Fe0.10 O32, a 9.9995(6), c 8.384(2) Å, P63, Z = 1, Dx = 2.673 g cm–3, from the Khibina–Lovozero complex, Kola Peninsula, Russia, (2) K1.60 Na6.21 Ca0.08 Si8.44 Al7.43 O32, a 9.985(1), c 8.372(1) Å, P63, Z = 1, Dx 2.660 g/cm3, from Bancroft, Ontario, Canada, and (3) K1.76 Na5.86 Ca0.13 Si8.12 Al7.84 Fe0.04 O32, a 9.9979(6), c 8.3852(2), P63, Z = 1, Dx 2.654 g/cm3, from Monte Somma, Italy, have been refined to R1 indices of 1.8, 2.7 and 1.7% using 1273, 1191 and 1364 unique observed (| Fo | > 4 F) reflections collected with a single-crystal diffractometer and 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 1⁄2 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; = 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 1⁄2 z and is [8]-coordinated by oxygen atoms between 2.48 and 2.79 Å; = 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, = 1.731, 1.723 and 1.732, = 1.736, 1.728 and 1.734; T(2,3) = Si, = 1.606, 1.603 and 1.605, = 1.616, 1.615 and 1.616 Å. For the nepheline sample from the Khibina–Lovozero complex, the simplified chemical formula, K2 Na6 Si8 Al8 O32, corresponds to the ideal end-member formula of nepheline with the A site fully occupied by K, the B site fully occupied by Na, and Si and Al completely ordered at the T sites. This sample crystallized in a hyperagpaitic rock formed at the last stages of low-temperature pneumatolitic activity. Nepheline (2), also formed in low-temperature pegmatitic environment, shows very high Al–Si order at the T sites. The Monte Somma nepheline is of high-temperature origin. Previous refinements of the structure had indicated a fairly disordered Al–Si distribution for it. The present results indicate maximal Al–Si order, in accord with previous 29Si MAS NMR results. Our results suggest that nepheline exhibits maximal Al–Si order irrespective of temperature of origin of the nepheline.