Crystal structures of the Rb- and Sr-exchanged forms of ivanyukite-Na-T |
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| Authors: | D V Spiridonova S V Krivovichev V N Yakovenchuk Ya A Pakhomovsky |
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| Institution: | 1. St. Petersburg State University, Universitetskaya nab. 7/9, St. Petersburg, 199034, Russia
2. Geological Institute, Kola Science Center, Russian Academy of Sciences, ul. Fersmana 14, Apatity, 184209, Russia
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| Abstract: | 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 (Na0.10K0.07Ca0.15Sr0.05Rb1.81Ba0.02)Σ = 2.20(Ti3.65Nb0.19Fe0.05Mn0.01)Σ = 3.90O2.07/(OH)1.93(Si2.98Al0.02)Σ = 3.00 O12] · 3.61H2O; the Sr-exchanged form (K0.03Sr0.81Ca0.04Ba0.07)Σ = 0.95(Ti3.74Nb0.19Fe0.03)Σ = 3.96] O1.83/(OH)2.17](Si2.99Al0.01)Σ = 3.00O12) · 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. |
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