Origin of diffuse superstructure reflections in labuntsovite-group minerals
-
Thomas Armbruster
,
Sergey V. Krivovichev
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(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(OH)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 -60 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 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.
© 2015 by Walter de Gruyter Berlin/Boston
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Articles in the same Issue
- First-principle study of polytype structures of 1:1 dioctahedral phyllosilicates
- Calorimetry of liquids in the system Na2O-Fe2O3-SiO2
- A Mössbauer study of the oxidation state of Fe in silicate melts
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- The most reduced rock from the moon, Apollo 14 basalt 14053: Its unique features and their origin
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- V oxidation state and coordination number in silicate glasses by XAS
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- Isosymmetric structural phase transition of orthoenstatite: Molecular dynamics simulation
- Crystal nucleation in hydrous rhyolite: Experimental data applied to classical theory
- Investigation of polytypes in lepidolite using electron back-scattered diffraction
- Iron partitioning between basaltic melts and clinopyroxene as a function of oxygen fugacity
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- High-pressure viscometry of polymerized silicate melts and limitations of the Eyring equation
- Biological control on calcite crystallization: AFM investigation of coccolith polysaccharide function
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- Arsenate sorption on schwertmannite
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- Site distribution of Fe2+and Fe3+in the axinite mineral group: New crystal-chemical formula
- Epidote-rich talc-kyanite-phengite eclogites, Sulu terrane, eastern China: P-T-fO₂estimates and the significance of the epidote-talc assemblage in eclogite
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