On the high temperature behaviour of monoclinic trinepheline

摘要

The crystal structure of monoclinic trinepheline can be explained as a stacking sequence of a conventional nepheline unit cell (composed of two layers) and one additional layer containing exclusively elliptical rings, similar to those observed in a related hexagonal trinepheline structure. When heated to 373(5) K, monoclinic trinepheline transforms to hexagonal symmetry (space group P6_1, a = 9.969(1) A, c = 25.056(2) A, K=2156.4(4) A~3), i.e., the tripling of the c lattice constant is retained. However, the superlattice reflections caused by the tripling are very weak. The average structure of this phase corresponds to that of a conventional pure sodium nepheline with P6_3-symmetry. The tripling of the c lattice parameter of the high-temperature modification is due to ordering phenomena of one apical oxygen and one sodium ion situated in a ditrigonal ring close to the 6_1 -screw axis. Layers containing exclusively elliptical rings cannot be found any longer. Upon further heating the ordering is averaged and at 473(5) K the crystal structure corresponds to that of conventional nepheline (space group P6_3, a = 9.977(2), c = 8.354(2) A, V= 720.2(2) A~3). This structure is retained until 873(5) K where the following unit cell parameters are observed: a= 10.057(2), c = 8.380(2) A, V= 734.0(2) A3.