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Origin of diffuse superstructure reflections in labuntsovite-group minerals

¹ Laboratorium für chemische und mineralogische Kristallographie, Universtät Bern, Freiestrasse 3, CH-3012 Bern, Switzerland
² Department of Crystallography, St. Petersburg State University, University Emb. 7/9, 199034 St. Petersburg, Russia
³ Laboratory of Crystallography, ETH, ETH-Zentrum, CH-8092 Zurich, Switzerland
⁴ Institut für Geologie, Universtät Bern, Baltzerstr. 1, CH-3012 Bern, Switzerland
⁵ Institute of Geology of Ore Deposits, Petrology, Mineralogy and Geochemistry, Russian Academy of Sciences, Staromonetny per. 35, 109017 Moscow, Russia
⁶ Geological Institute, Kola Science Center of the Russian Academy of Sciences, 184200 Apatity, Russia

Correspondence: ^* E-mail: thomas.armbruster{at}krist.unibe.ch

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(Si₄O₁₂)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, ß= 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 in-dependent (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 extraframework 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.

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