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Modulated structure of nepheline.

Karen Friese1, Andrzej Grzechnik, Vaclav Petříček

  • 1Departamento Física Materia Condensada, Universidad del País Vasco, Apdo 644, 48080 Bilbao, Spain. karen.friese@ehu.es

Acta Crystallographica. Section B, Structural Science
|January 20, 2011
PubMed
Summary

The incommensurately modulated structure of natural nepheline was determined using superspace methods. This revealed a disordered incorporation of cations within the tetrahedral framework, impacting atomic positions and electron density variations.

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Area of Science:

  • Mineralogy and crystallography
  • Solid-state chemistry
  • Materials science

Background:

  • Nepheline, a feldspathoid mineral, exhibits complex structural chemistry.
  • Understanding its modulated structure is key to explaining cation incorporation and disorder.
  • Previous studies have hinted at deviations from ideal structures.

Purpose of the Study:

  • To determine the incommensurately modulated structure of a natural nepheline sample.
  • To elucidate the cation distribution and disorder within the tetrahedral framework.
  • To analyze the nature and extent of atomic modulations.

Main Methods:

  • Superspace crystallography was employed to solve the modulated structure.
  • High-resolution X-ray diffraction data were analyzed.
  • Displacive and occupational modulations were modeled using harmonics.

Main Results:

  • The natural nepheline structure is incommensurately modulated in superspace group X3(00γ)0.
  • Cations (Na+, K+, Ca2+) and vacancies are disordered in larger channels, with small electron density variations.
  • Atomic positions exhibit displacive modulations below 0.1 Å, primarily first-order harmonics.
  • Al/Si ordering is nearly complete in the tetrahedral framework.
  • Oxygen atoms show split-atom positions with occupational modulation, indicating high disorder.

Conclusions:

  • The structure of natural nepheline is characterized by significant displacive and occupational modulations.
  • Cation incorporation in the larger channels is highly disordered.
  • Coupled occupational modulations between cations and oxygen atoms suggest complex structural interactions.