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: comparison of monoclinic P 2 1 / c and Ia {\overline 3} d polymorph structures and inverse relationship between tetrahedral cation ( T = Si and X )—O bond distances and intertetrahedral T —O— T angles

The leucite tectosilicate mineral analogues K 2 X 2+ Si 5 O 12 ( X = Fe 2+ , Co, Zn) and Rb 2 X 2+ Si 5 O 12 ( 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 K 2 X 2+ Si 5 O 12 ( X = Fe 2+ , Co, Zn) samples shows that they crystallize in the monoclinic space group P 2 1 / c and have tetrahedral cations (Si and X ) ordered onto distinct framework sites [ cf. hydrothermal K 2 MgSi 5 O 12 ; Bell et al. (1994 a ), Acta Cryst. B 50 , 560–566]. Dry-synthesized K 2 X 2+ Si 5 O 12 ( X = Co, Zn) and Rb 2 X 2+ Si 5 O 12 ( 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 K 2 MgSi 5 O 12 . Both structure types have tetrahedrally coordinated SiO 4 and X O 4 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 (Fe 2+ —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—Fe 2+ ). For the compositions with both disordered and ordered tetrahedral cation structures (K 2 MgSi 5 O 12 , K 2 CoSi 5 O 12 , K 2 ZnSi 5 O 12 , Rb 2 MnSi 5 O 12 and Cs 2 CuSi 5 O 12 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.