Eudialyte-related structures: A quantitative analysis

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Marco E. Ciriotti
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Iscritto il: ven 25 giu, 2004 11:31
Località: via San Pietro, 55 I-10073 Devesi/Cirié TO - Italy
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Eudialyte-related structures: A quantitative analysis

Messaggio da Marco E. Ciriotti » lun 27 set, 2021 11:07

Referenza:
▪ Aksenov, S.M., Kabanova, N.A., Chukanov, N.V., Panikorovskii, T.L., Blatov, V., Krivovichev, S.V. (2021): The role of local heteropolyhedral substitutions in the stoichiometry, topological characteristics, and ion-migration paths in the eudialyte-related structures: A quantitative analysis. Acta Crystallographica, B77, (in press).

Abstract:
Eudialyte-group minerals (EGMs) are of a scientific and industrial interest as important concentrators of rare and strategic elements (mainly, Zr and REE) in agpaitic alkaline rocks. The crystal structure of EGMs is based on a microporous heteropolyhedral framework, which makes these minerals similar to zeolite-like materials and molecular sieves. Topological analysis of the heteropolyhedral MT-framework (where M and T – octahedral and tetrahedral cations, respectively) in the eudialyte-type structure and its derivatives was performed based on a natural tiling analysis of the 3D cation nets using the ToposPro software using both the standard representation and 0-1-2-free representation. To analyze the migration paths of sodium cations in these structures, the Voronoi method was used. The parental eudialyte-type MT-framework is formed by isolated ZO6 octahedra, six-membered [M(1)6O24] rings of edge-sharing M(1)O6 octahedra, and two kinds of rings of tetrahedra, [Si3O9] and [Si9O27]. Different occupancies of additional M(2), M(3), and M(4) sites with variable coordination numbers by Q, T*, and M* cations, respectively, result in 12 different types of the MT-framework. Corresponding point symbols for the cationic 3D-nets of the MT-frameworks as well as tile sequences have been calculated. Based on the results of natural tilings calculations as well as theoretical analysis of migration paths, it was found that Na+ ions can migrate through six- and seven-membered rings, while all other rings are too small for the migration. In eight types of the MT-frameworks, Na+-ion migration and diffusion is possible at ambient temperature and pressure, while in four other types cages are connected by narrow windows and, as a result, the Na+ diffusion in them is complicated at ambient conditions but may be possible either at higher temperatures or under mild geological conditions during long times. This conclusion is in a good agreement with numerous examples of the transformation of initial EGMs into their hydrated Na-deficient counterparts as a result of natural processes of sodium leaching and hydrolysis under hydrothermal conditions.
Marco E. Ciriotti

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