High-temperature synchrotron X-ray powder diffraction study of Cs2XSi5O12(X = Cd, Cu, Zn) leucites

Synchrotron X-ray powder diffraction data have been collected on three synthetic leucite analogues with the general formula Cs2XSi5O12(X = Cd, Cu, Zn) between 295 and 1173 K. All three samples have the orthorhombic Pbca leucite structure at room temperature with ordered framework T-site cations. The sample with X = Cd retains the Pbca structure over the whole of the investigated temperature range. The sample with X = Cu also retains the Pbca structure, but there is a transition to a less distorted structure with a larger unit-cell volume at ∼333 K. The sample with X = Zn shows evidence for a transition to a previously unknown Pa cubic structure, with some T-site cation disorder, at 566 K, on heating. This transition is reversible on cooling to 633 K.

Revision of the structure of Cs2CuSi5O12 leucite as orthorhombic Pbca

The crystal structure of a hydrothermally synthesized leucite analogue Cs2CuSi5O12 has been determined and refined using the Rietveld method from high-resolution synchrotron X-ray and neutron powder diffraction data. This structure is based on the topology and cation-ordering scheme of the Pbca leucite structure of Cs2CdSi5O12, and exhibits five ordered Si sites and one ordered Cu tetrahedrally coordinated (T) site. This structure for Cs2CuSi5O12 is topologically identical to other known leucite structures and is different from that originally proposed by Heinrich & Baerlocher [(1991), Acta Cryst. C47, 237–241] in the tetragonal space group P4_12_12. The crystal structure of a dry-synthesized leucite analogue Cs2CuSi5O12 has also been refined; this has the Ia\bar 3d cubic pollucite structure with disordered T sites.

Crystal structures and cation ordering in Cs2MgSi5O12, Rb2MgSi5O12 and Cs2ZnSi5O12 leucites

The crystal structures of the leucite analogues Cs2MgSi5O12, Cs2ZnSi5O12 and Rb2MgSi5O12 have been determined by synchrotron X-ray powder diffraction using Rietveld refinement in conjunction with 29Si MAS NMR spectroscopy. These leucites are framework structures with distinct tetrahedral sites (T sites) occupied by Si and a divalent cation (either Mg or Zn in these samples); there is also a monovalent extra-framework cation (either Cs or Rb in these samples). The refined crystal structures were based on the Pbca leucite structure of Cs2CdSi5O12, thus a framework with five ordered Si T sites and one ordered Cd T site was used as the starting model for refinement. 29Si MAS NMR shows five distinct Si T sites for Cs2MgSi5O12 and Rb2MgSi5O12, but six Si T sites for Cs2ZnSi5O12. The refined structures for Cs2MgSi5O12 and Rb2MgSi5O12 were determined with complete T-site ordering, but the refined structure for Cs2ZnSi5O12 was determined with partial disorder of Mg and Si over two of the T sites.

Synthesis of thallium-leucite (TlAlSi2O6) pseudomorph after analcime

Thallium leucite, TlAlSi2O6, has been synthesized at 450°C for 7 days, under ambient conditions, by the transformation of dehydrated analcime NaAlSi2O6 in the presence of excess TlCl. This substitution of Tl for Na leads to confirmation of a thallium-leucite pseudomorph after analcime. Their optical properties, X-ray powder diffraction patterns, electron microprobe analysis, infrared spectra, and X-ray photoelectron spectroscopy have characterized the synthetic Tl-leucites. The IR spectra show that the mid-IR modes T-O stretching and T-O-T bending vibrations for TlAlSi2O6 are more resemblant of those for analcime than for leucite, KAlSi2O6. This resemblance implies that Tl cation enters the W-site rather than the S-site in the analcime structure: Na (S) + H2O (W) ⇋ □ + K (leucite) ⇋ □ + Tl (Tl- leucite), where □ represents an S-site vacancy. The mechanism of this substitution is supported by the crystal chemical constraints: inasmuch as the S-site is smaller than the W-site, Tl+ cations being larger than Na+ plainly prefer the latter site to the former. One inference from the binding energy for Tl+ by XPS is that Tl+ occupies the extra-framework site in synthetic leucite pseudomorph, rather than the smaller tetrahedral site. The difference in Al/Si disordering between analcime and leucite and the nonstoichiometry due to the solid solution of the □Si3O6 component into the leucite structure may provide a fundamental insight into understanding why TlAlSi2O6 deviates from the trend defined by K-, Rb- and CsAlSi2O6 leucite series on the a-c parameter diagram, inasmuch as these three cations in the leucite structure occupy the W-sites. Finally, synthesis of TlAlSi2O6 leucite has an implication for the existence of other polymorphs due to different degrees of Al/Si disordering, except for high- and low-temperature leucites already known: natural leucites crystallized directly through igneous processes are different from those formed by substitution of K for Na in analcimes.

Minor element chemistry of leucite and pseudoleucite

SummaryLeucite and pseudoleucite invariably have lower K/Rb ratios and higher K/Sr and K/Ba ratios than the groundmasses of the rocks in which they occur. The distribution of Rb, Sr, and Ba between K-rich minerals (e.g. alkali feldspars) and the liquids from which they crystallize is generally such that Rb is impoverished and Sr and Ba enriched, relative to K, in the crystallizing phase. Leucite shows the opposite relationship and this is attributed to the leucite structure being sensitive to the size and valency of the ions replacing K.