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Crystal structures of K 2 [ X Si 5 O 12 ] ( X = Fe 2+ , Co, Zn) and Rb 2 [ X Si 5 O 12 ] ( X = Mn) leucites

DOI: 10.1107/S2052520618004092 DOI Help

Authors: Anthony M. T. Bell (Sheffield Hallam University) , C. Michael B. Henderson (University of Manchester; ASTeC, Science and Technology Facilities Council)
Co-authored by industrial partner: No

Type: Journal Paper
Journal: Acta Crystallographica Section B Structural Science, Crystal Engineering And Materials , VOL 74 , PAGES 274 - 286

State: Published (Approved)
Published: June 2018

Abstract: The leucite tectosilicate mineral analogues K2X2+Si5O12 (X = Fe2+, Co, Zn) and Rb2X2+Si5O12 (X = Mn) have been synthesized at elevated temperatures both dry at atmospheric pressure and at controlled water vapour pressure; for X = Co and Zn both dry and hydro­thermally synthesized samples are available. Rietveld refinement of X-ray data for hydro­thermal K2X2+Si5O12 (X = Fe2+, Co, Zn) samples shows that they crystallize in the monoclinic space group P21/c and have tetrahedral cations (Si and X) ordered onto distinct framework sites [cf. hydro­thermal K2MgSi5O12; Bell et al. (1994a), Acta Cryst. B50, 560–566]. Dry-synthesized K2X2+Si5O12 (X = Co, Zn) and Rb2X2+Si5O12 (X = Mn) samples crystallize in the cubic space group Ia{\overline 3}d and with Si and X cations disordered in the tetrahedral framework sites as typified by dry K2MgSi5O12. Both structure types have tetrahedrally coordinated SiO4 and XO4 sharing corners to form a partially substituted silicate framework. Extraframework K+ and Rb+ cations occupy large channels in the framework. Structural data for the ordered samples show that mean tetrahedral Si—O and X—O bond lengths cover the ranges 1.60 Å (Si—O) to 2.24 Å (Fe2+—O) and show an inverse relationship with the intertetrahedral angles (T—O—T) which range from 144.7° (Si—O—Si) to 124.6° (Si—O—Fe2+). For the compositions with both disordered and ordered tetrahedral cation structures (K2MgSi5O12, K2CoSi5O12, K2ZnSi5O12, Rb2MnSi5O12 and Cs2CuSi5O12 leucites) the disordered polymorphs always have larger unit-cell volumes, larger intertetrahedral T—O—T angles and smaller mean T—O distances than their isochemical ordered polymorphs. The ordered samples clearly have more flexible frameworks than the disordered structures which allow the former to undergo a greater degree of tetrahedral collapse around the interframework cavity cations. Multivariant linear regression has been used to develop equations to predict intertetrahedral T—O—T angle variation depending on the independent variables Si—O and X—O bond lengths, cavity cation ideal radius, intratetrahedral (O—T—O) angle variance, and X cation electronegativity.

Journal Keywords: X-ray powder diffraction; Rietveld refinement; leucite-group minerals; silicate framework structures

Subject Areas: Materials, Earth Science, Chemistry

Instruments: I11-High Resolution Powder Diffraction