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

Abstract

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.