Crystal structures of the Rb- and Sr-exchanged forms of ivanyukite-Na-T

The Rb- and Sr-exchanged forms of ivanyukite have been obtained and structurally characterized. The chemical formulas derived from the electron microprobe data are as follows: the Rb-exchanged form (Na_0.10K_0.07Ca_0.15Sr_0.05Rb_1.81Ba_0.02)_{Σ = 2.20}[(Ti_3.65Nb_0.19Fe_0.05Mn_0.01)_{Σ = 3.90}O_2.07/(OH)_1.93(Si_2.98Al_0.02)_{Σ = 3.00} O₁₂] · 3.61H₂O; the Sr-exchanged form (K_0.03Sr_0.81Ca_0.04Ba_0.07)_{Σ = 0.95}[(Ti_3.74Nb_0.19Fe_0.03)_{Σ = 3.96}] [O_1.83/(OH)_2.17](Si_2.99Al_0.01)_{Σ = 3.00}O₁₂) · 7H₂O. The structures of the Rb- and Sr-exchanged forms of ivanyukite have been solved and refined using the least squares method. The structures are based on a mixed three-dimensional octahedral-tetrahedral framework of the pharmacosiderite type with channels occupied by Rb⁺ and Sr²⁺ cations and water molecules. The Rb⁺ cations in the Rb-exchanged form are 12-coordinated, whereas the Sr²⁺ cations in the Sr-exchanged form are 9- or 7-coordinated. The statistical investigation of the geometric parameters of the pharmacosiderite-type titanosilicates showed that symmetry changes are associated with the interactions of extraframework cations with the O atom of the Ti₄(O,OH)₄ clusters of the titanosilicate framework. The relationship between the unit-cell parameters in titanosilicates of the pharmacosiderite type and the structural geometric parameters of the titanosilicate framework has been proved by the use of multiple regression equations.