Titre du document / Document title

Origin of diffuse superstructure reflections in labuntsovite-group minerals

Auteur(s) / Author(s)

ARMBRUSTER Thomas ; KRIVOVICHEV Sergey V. ; WEBER Thomas ; GNOS Edwin ; ORGANOVA Natalia N. ; YAKOVENCHUK Viktor N. ; SHLYUKOVA Zoya V. ;

Résumé / Abstract

The average crystal structures of two natural porous titanosilicates of the labuntsovite group, lemmleinite-Ba and lemmleinite-K, ideally Na⁴K⁴Ba²Ti⁸(Si⁴O¹²)⁴O⁴(OH)⁴.8H²O and Na⁴K⁴K²Ti⁸(Si⁴ O¹²)⁴O²(OH)⁶.8H²O, respectively, have been refined from single crystal X-ray diffraction data. Both samples represent an extensive solid solution with labuntsovite sensu strictu Na⁴K⁴D²Ti⁸(Si⁴O¹²)⁴O⁴(O H)⁴.10H²O where D = Mn, Fe, and Mg. In addition to the sharp Bragg reflections both crystals, space group C2/m, a = 14.3, b =13.8, c = 7.75 Å, β = 117°, exhibit diffuse layers at c*/2 intervals indicating faulty superstructures with c = 15.7 Å. The diffuse layers consist of two types of reflections. The dominant type is strongly diffuse and smeared along a* indicating an I-centered Bravais lattice. The other type is very sharp but also weak and is in agreement with a C-centered lattice. Models for both superstructures have been developed on the basis of crystal-chemical principles and their theoretical diffraction patterns have been calculated and compared with the observed diffuse layers yielding excellent qualitative agreement. X-ray structure refinements of the average structure at -160 and 22 °C indicate temperature independent (static) disorder of Ti within rather rigid TiO⁶ octahedra connected to chains that extend along a. This Ti disorder is interpreted in terms of long-range order of OH and O in the superstructures where these anions occupy the corner-connecting octahedral apices in an ordered fashion. An additional effect of OH, O order is an ordered arrangement of extraframe work Ba (K) that only bonds to O but not to OH sites exposed on the channel walls. Temperature dependent cell dimensions between -160 and +200 °C suggest a phase transition at ca. -80 °C. However, the structural data obtained from the average structures, refined at -160 and 22 °C, did not allow us to draw crystal-chemical conclusions about the nature of the phase transition. Dehydration of the investigated lemmleinite-Ba starts at ca. 150 °C leading to increasing extraframework disorder and decreasing crystal quality as evidenced by strong smearing of the originally sharp Bragg reflections.

Revue / Journal Title

The American mineralogist   ISSN 0003-004X   CODEN AMMIAY 

Source / Source

2004, vol. 89, n^o11-12, pp. 1655-1666 [12 page(s) (article)]

Langue / Language

Anglais

Editeur / Publisher

Mineralogical Society of America, Washington, DC, ETATS-UNIS  (1916) (Revue)

Localisation / Location

INIST-CNRS, Cote INIST : 503, 35400012113511.0100