Origin of diffuse superstructure reflections in labuntsovite-group minerals

Abstract

The average crystal structures of two natural porous titanosilicates of the labuntsovite group, lemmleinite-Ba and lemmleinite-K, ideally Na 4 K 4 Ba 2 Ti 8 (Si 4 O 12 ) 4 O 4 (OH) 4 ·8H 2 O and Na 4 K 4 K 2 Ti 8 (Si 4 O 12 ) 4 O 2 (OH) 6 ·8H 2 O, respectively, have been refined from single crystal X-ray diffraction data. Both samples represent an extensive solid solution with labuntsovite sensu strictu Na 4 K 4 D 2 Ti 8 (Si 4 O 12 ) 4 O 4 (O H) 4 ·10H 2 O where D = Mn, Fe, and Mg. In addition to the sharp Bragg reflections both crystals, space group C 2/ m , a = 14.3, b =13.8, c = 7.75 A, β= 117°, exhibit diffuse layers at c */2 intervals indicating faulty superstructures with c = 15.7 A. 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 TiO 6 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.