Crystal structures of K2[XSi5O12] (X = Fe2+, Co, Zn) and Rb2[XSi5O12] (X = Mn) leucites: comparison of monoclinic P21/c and Iad 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 X Si O (X = Fe , Co, Zn) and Rb X Si O (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 X Si O (X = Fe , Co, Zn) samples shows that they crystallize in the monoclinic space group P2 /c and have tetrahedral cations (Si and X) ordered onto distinct framework sites [cf. hydrothermal K MgSi O ; Bell et al. (1994a), Acta Cryst. B50, 560-566]. Dry-synthesized K X Si O (X = Co, Zn) and Rb X Si O (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 MgSi O . Both structure types have tetrahedrally coordinated SiO and XO 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 -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 ). For the compositions with both disordered and ordered tetrahedral cation structures (K MgSi O , K CoSi O , K ZnSi O , Rb MnSi O and Cs CuSi O 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.