THE CRYSTAL CHEMISTRY OF NEPHELINE

Abstract

The crystal structures of three nepheline samples, (1) K 1.91 Na 6.04 Si 8.15 Al 7.72 Fe 3+ 0.10 O 32 , a 9.9995(6), c 8.384(2) Å, P6 3 , Z = 1, D x = 2.673 g cm -3 , from the Khibina-Lovozero complex, Kola Peninsula, Russia, (2) K 1.60 Na 6.21 Ca 0.08 Si 8.44 Al 7.43 O 32 , a 9.985(1), c 8.372(1) Å, P6 3 , Z = 1, D x 2.660 g/cm 3 , from Bancroft, Ontario, Canada, and (3) K 1.76 Na 5.86 Ca 0.13 Si 8.12 Al 7.84 Fe 3+ 0.04 O 32 , a 9.9979(6), c 8.3852(2), P6 3 , Z = 1, D x 2.654 g/cm 3 , from Monte Somma, Italy, have been refined to R 1 indices of 1.8, 2.7 and 1.7% using 1273, 1191 and 1364 unique observed (| F o | > 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 6 3 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; <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 ½ 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, K 2 Na 6 Si 8 Al 8 O 32 , 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 29 Si MAS NMR results.Our results suggest that nepheline exhibits maximal Al-Si order irrespective of temperature of origin of the nepheline.