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Modulated structure of nepheline

Title data

Friese, Karen ; Grzechnik, Andrzej ; Petricek, Vaclav ; van Smaalen, Sander ; Morgenroth, Wolfgang ; Schönleber, Andreas:
Modulated structure of nepheline.
In: Acta Crystallographica Section B. Vol. 67 (2011) Issue 1 . - pp. 18-29.
ISSN 2052-5206
DOI: https://doi.org/10.1107/S0108768110050822

Official URL: Volltext

Abstract in another language

The incommensurately modulated structure of a natural nepheline of composition K\sb 0.54Na\sb 3.24Ca\sb 0.03Al\sb 3.84Si\sb 4.16O\sb 16 has been determined in superspace. The compound crystallizes in the trigonal centered superspace group \it X3(00γ)0 with γ = 0.2048(10), \it X = (0, 0, 0, 0), (1/3, 2/3, 0, 2/3), (2/3, 1/3, 0, 1/3), \it a = 17.2889(8) and \it c = 8.3622(10)Å. The structure is characterized by a framework of corner-connected (Al,Si)O\sb 4 tetrahedra. The additional cations are incorporated in two different types of channels of the framework. All atoms in the structure are displacively modulated with amplitudes below 0.1Å. The modulation can be well described taking into account harmonics of first order only. Atomic positions in the smaller channels of the framework are fully occupied by Na\sp +. Cationic positions in the larger channel are occupationally modulated, yet the variation of electron density as a function of the internal coordinate \it t is very small and indicates that the incorporation of different types of cations (K\sp +, Na\sp +, Ca\sp 2+) and vacancies is realised in a highly disordered way. Average \it T—-O distances indicate a nearly complete Al/Si ordering in the tetrahedral framework. A large part of the O atoms are approximated by split-atom positions, which are additionally affected by occupational modulation resulting in a high degree of disorder in the modulated structure. Occupational probabilities for the split-atom positions are complementary. Occupational modulations of the cations in the larger channels and the O atoms of the tetrahedral framework are coupled and correlations between occupational and displacive modulations exist.

Further data

Item Type: Article in a journal
Refereed: Yes
Keywords: incommensurately modulated structure; occupational and displacive modulations; feldspathoids; rock-forming minerals
Institutions of the University: Faculties > Faculty of Mathematics, Physics und Computer Science > Group Material Sciences > Chair Crystallography > Chair Crystallography - Univ.-Prof. Dr. Sander van Smaalen
Faculties
Faculties > Faculty of Mathematics, Physics und Computer Science
Faculties > Faculty of Mathematics, Physics und Computer Science > Group Material Sciences
Faculties > Faculty of Mathematics, Physics und Computer Science > Group Material Sciences > Chair Crystallography
Result of work at the UBT: Yes
DDC Subjects: 500 Science > 530 Physics
Date Deposited: 31 Jul 2015 06:07
Last Modified: 31 Jul 2015 06:07
URI: https://eref.uni-bayreuth.de/id/eprint/17442