Variable stoichiometry in tectosilicates having the leucite/pollucite-type structure with particular emphasis on modelling the interframework cavity cation environment

• Published in: Journal of solid state chemistry Vol. 251; pp. 90 - 104
• Main Authors: Henderson, C.M.B., Bell, A.M.T., Knight, K.S.
• Format: Journal Article
• Language: English
• Published: Elsevier Inc 01.07.2017
• Subjects: ((K,Rb)0.75□0.25)A[Al0.75Si2.25O6]T leucite/pollucite tetragonal solid solutions; Aristotype Ia-3d cubic pseudocell; K, Rb, Cs,Tl leucite/pollucite mineral topology; KAlSi3O8-RbAlSi3O8 system feldspar˗leucite phase stabilities; Mean A – O distance dependence vs V1/3 and mean T radius (r); Multivariant linear least-squares regression, pseudocell parameters; Mean A – O distance dependence vs V1/3 and mean T radius (r); Aristotype Ia-3d cubic pseudocell; K, Rb, Cs,Tl leucite/pollucite mineral topology; ((K,Rb)0.75□0.25)A[Al0.75Si2.25O6]T leucite/pollucite tetragonal solid solutions; KAlSi3O8-RbAlSi3O8 system feldspar˗leucite phase stabilities; Multivariant linear least-squares regression, pseudocell parameters
• ISSN: 0022-4596; 1095-726X
• DOI: 10.1016/j.jssc.2017.04.013

Unit cell data for synthetic samples of I41/a or Ia-3d AAlSi2O6 (A=K,Rb,Tl,Cs) leucite/pollucites are compared to those for feldspar-stoichiometry leucite analogues (Rb,Tl,Cs)0.75□0.25Al0.75Si2.25O6. I41/a solid solutions between KAlSi2O6 and KAlSi3O8 (orthoclase feldspar, Or) and between K(Al0.85Fe0.15)Si2O6 and Or both show increasing volumes and decreasing c/a with increasing Or. Phase relations are given for KAlSi3O8-RbAlSi3O8; monoclinic feldspars occur at low temperatures and I41/a leucites at higher temperatures; volumes of leucite solid solutions increase with increasing Rb. The cell volume of Rb0.75□0.25Al0.75Si2.25O6 leucite is larger than that of RbAlSi2O6 leucite. Published leucite/pollucite structures belonging to different Space Group symmetries are treated as having the cubic Ia-3d topology with a ‘pseudocell’ edge V1/3 (V = unit cell volume); inequivalent site bond lengths and angles provide mean structural parameters for the cubic ‘pseudocells’. Multivariant linear regression equations using mean A – O distances as the independent variable were obtained using other structural parameters as dependent variables. Only two independent variables (V and mean tetrahedral ionic radius) are required to account for >90% of the variation. These regression equations can be used to predict the sizes of the cavity cation sites in leucite/pollucite-type phases of any stoichiometry. [Display omitted] •Leucite topology allows wide range of compositions and space group symmetries.•Cations: Interframework K,Rb,Tl,Cs; framework Al,B,Si,Fe3+,Mg,Cu,Zn etc.•Monoclinic feldspars: Tetragonal and cubic leucites in system (K-Rb)AlSi3O8.•Published structures provide averaged Ia-3d ‘pseudocell’ edge (V1/3), A – O, T – O, T – O – T.•Multivariant regression shows (A – O) depends mainly on V1/3 and ideal mean T radii.