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 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 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. Crystal structures for synthetic analogues of the tectosilicate mineral leucite (KAlSi2O6) with stoichiometries of K2X2+Si5O12 (X = Fe2+, Co, Zn) and Rb2X2+Si5O12 (X = Mn) were refined from X‐ray powder diffraction data. Structures refined in P21/c have ordered Si and X cations on the tetrahedral sites, whereas structures refined in have disordered Si and X cations.