THE CRYSTAL CHEMISTRY OF NEPHELINE

Abstract

The crystal structures of three nepheline samples, (1) K1.91 Na6.04 Si8.15 Al7.72 Fe3+0.10 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 Fe3+0.04 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 | > 4F) 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 ½ 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 ½ 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.