Traskite, ideal formula Ba9(Fe2+2Ti2)(SiO3)12(OH,Cl,F)6∙6H2O, is a heteropolyhedral zeolite-like mineral first discovered by Alfors et al. [1] and occurring in metamorphic sanbornite-quartz rocks. The polyhedral framework of traskite can be schematized as Si12O36 rings linked by clusters consisting of closely packed Si2O7 groups and (Ti,Fe)O6 octahedra. This polyhedral arrangement is the cause of one of the lowest framework density among the OPT (octahedral-pentahedral-tetrahedral) zeolite-related structures (i.e. FD = 11.75) [2], a feature which is common to another titanosilicate mineral, zorite, whose synthetic analog, ETS-4 [3], has found important applications in gas-adsorption processes where a tunable size-selectivity is required (e.g. N2/CH4, Ar/O2 and N2/O2) [4]. The presence of 12-membered tetrahedral rings in traskite is a peculiar feature, also found in ETS-10, which could be exploited for the incorporation of large cations, such as Cs, or other toxic elements. The crystal structure of traskite was solved by Malinovskii et al. [5] in the hexagonal P-6m2 space group. However, as pointed out by the same authors, traskite exhibits a faulted stacking sequence which doubles the c-axis. This structural disorder justifies the rather poor agreement factor (R = 0.12) of the single crystal refinement by Malinovskii and coworkers. It should be also pointed out that the choice of the merohedral P-6m2 space group does not guarantee a continuity of the structural framework. On the base of these latter observations, this work was aimed at reinvestigate the chemistry and the crystal structure of the traskite mineral. The best single crystal refinement at ambient condition was obtained in the holohedral P6/mmm space group with a final agreement factor R = 0.062. In addition to the better agreement factor, the choice of the holohedral hexagonal space group allows a framework topologically continuous. High-temperature single crystal diffraction experiments were also carried out to follow the structural modifications of traskite up to 500 °C. Despite to the large Si12O36 rings, the very low FD, and the faulted nature of its lattice, the traskite structure is thermally stable throughout the investigated temperature range. A strong discontinuity in the lattice evolution, associated to a decreasing of the unit-cell parameters, is observed at 400 °C. This latter fact could be related to a hexagonal to orthorhombic phase transition. References. [1] Alfors, J.T., Stinson, M.C., Matthews, R.A. (1965): American Mineralogist, 50, 314-340; [2] Chukanov, N.V., Pekov, I.V., Rastsvetaeva, R.K. (2004): Russian Chemical Reviews, 73, 205-223; [3] Cruciani, G., De Luca, P., Nastro, A., Pattison, P. (1998): Microporous Mesoporous Materials, 21, 143-153; [4] Kuznicki, S.M., Bell, V.A., Nair, S., Hillhouse, H.W., Jacubunas, R.M., Braunbarth, C.M., Toby, M.H., Tsapatsis, M. (2001): Nature, 412, 720-724; [5] Malinovskii, Y.A., Pobedimskaya, E.A., Belov, N.V. (1976): Soviet Physics Doklady, 21, 426-428.

Structure and thermal behaviour of traskite: a microporous heteropolyhedral mineral with 12 tetrahedral rings

ARDIT, Matteo;CRUCIANI, Giuseppe
2013

Abstract

Traskite, ideal formula Ba9(Fe2+2Ti2)(SiO3)12(OH,Cl,F)6∙6H2O, is a heteropolyhedral zeolite-like mineral first discovered by Alfors et al. [1] and occurring in metamorphic sanbornite-quartz rocks. The polyhedral framework of traskite can be schematized as Si12O36 rings linked by clusters consisting of closely packed Si2O7 groups and (Ti,Fe)O6 octahedra. This polyhedral arrangement is the cause of one of the lowest framework density among the OPT (octahedral-pentahedral-tetrahedral) zeolite-related structures (i.e. FD = 11.75) [2], a feature which is common to another titanosilicate mineral, zorite, whose synthetic analog, ETS-4 [3], has found important applications in gas-adsorption processes where a tunable size-selectivity is required (e.g. N2/CH4, Ar/O2 and N2/O2) [4]. The presence of 12-membered tetrahedral rings in traskite is a peculiar feature, also found in ETS-10, which could be exploited for the incorporation of large cations, such as Cs, or other toxic elements. The crystal structure of traskite was solved by Malinovskii et al. [5] in the hexagonal P-6m2 space group. However, as pointed out by the same authors, traskite exhibits a faulted stacking sequence which doubles the c-axis. This structural disorder justifies the rather poor agreement factor (R = 0.12) of the single crystal refinement by Malinovskii and coworkers. It should be also pointed out that the choice of the merohedral P-6m2 space group does not guarantee a continuity of the structural framework. On the base of these latter observations, this work was aimed at reinvestigate the chemistry and the crystal structure of the traskite mineral. The best single crystal refinement at ambient condition was obtained in the holohedral P6/mmm space group with a final agreement factor R = 0.062. In addition to the better agreement factor, the choice of the holohedral hexagonal space group allows a framework topologically continuous. High-temperature single crystal diffraction experiments were also carried out to follow the structural modifications of traskite up to 500 °C. Despite to the large Si12O36 rings, the very low FD, and the faulted nature of its lattice, the traskite structure is thermally stable throughout the investigated temperature range. A strong discontinuity in the lattice evolution, associated to a decreasing of the unit-cell parameters, is observed at 400 °C. This latter fact could be related to a hexagonal to orthorhombic phase transition. References. [1] Alfors, J.T., Stinson, M.C., Matthews, R.A. (1965): American Mineralogist, 50, 314-340; [2] Chukanov, N.V., Pekov, I.V., Rastsvetaeva, R.K. (2004): Russian Chemical Reviews, 73, 205-223; [3] Cruciani, G., De Luca, P., Nastro, A., Pattison, P. (1998): Microporous Mesoporous Materials, 21, 143-153; [4] Kuznicki, S.M., Bell, V.A., Nair, S., Hillhouse, H.W., Jacubunas, R.M., Braunbarth, C.M., Toby, M.H., Tsapatsis, M. (2001): Nature, 412, 720-724; [5] Malinovskii, Y.A., Pobedimskaya, E.A., Belov, N.V. (1976): Soviet Physics Doklady, 21, 426-428.
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Utilizza questo identificativo per citare o creare un link a questo documento: https://hdl.handle.net/11392/1860713
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