Crystal Chemistry and Properties of Elpidite and Its Ag-Exchanged Forms

Natalia V Zubkova,Olga N Kazheva,Dmitry Yu Pushcharovsky,Rositsa P Nikolova,Nadezhda A Chervonnaya,Dmitry A Varlamov,Gennadiy V Shilov,Tatiana S Larikova,Nikita V Chukanov,Igor V Pekov,Vladislav V Kostov-Kytin

doi:10.3390/min9070420

Abstract

Elpidite from the Lovozero alkaline complex, Kola Peninsula, Russia, and Ag-exchanged forms of elpidite from two different localities (Lovozero and Khan Bogdo, Mongolia) were studied by means of single-crystal X-ray diffraction, electron microprobe analysis, thermogravimetry and IR spectroscopy. All studied samples retain the heteropolyhedral framework consisting of double Si6O15 chains (ribbons) and isolated ZrO6 octahedra. Zeolitic cavities in the initial elpidite from Lovozero (space group Pbm2, a = 14.6127(7), b = 7.3383(4), c = 7.1148(3) Å, V = 762.94(6) Å3) are occupied by Na+ cations and H2O molecules. Both Ag-exchanged forms are characterized by evident distortions of the heteropolyhedral framework and a strongly disordered arrangement of extra-framework cations which results in the appearance of the 14-14-14 Å unit cell (a = 14.1755(7), b = 14.6306(9), c = 14.2896(7) Å, V = 2963.6(3) Å3 for the Ag-exchanged form of elpidite from Lovozero and a = 14.1411(5), b = 14.5948(4), c = 14.3035(5) Å, V = 2952.04(17) Å3 for the Ag-exchanged form of elpidite from Khan Bogdo) and space group Cmce. Elpidite from both localities demonstrates a high exchange capacity to Ag. Exchanged Ag+ cations preferably occupy the sites that are close to the Na sites in the initial elpidite. The paper also contains a review of crystal chemical data on elpidite and its laboratory-modified forms.

Highlights

Synthetic zeolite-like microporous zirconosilicates with heteropolyhedral frameworks attract much attention due to the prospects of their wide application in chemical technologies as ion-exchangers, high-selectivity sorbents, catalysts, ion conductors, etc. [1]
The crystal structure of elpidite was first solved by [5] in the space group Pbmm. The structure of this mineral was re-refined by different authors, in some cases in the acentric space group Pbm2 [6] or in the space group Pbcm with one unit-cell parameter doubled [7], see Table 1 In all cases, the crystal structure of elpidite is described by a heteropolyhedral framework consisting of double Si6 O15 chains and isolated ZrO6 octahedra
The cation exchange is accompanied by a significant distortion of the heteropolyhedral framework, which results in the doubling of the ~7 Å parameters of the unit cell and the change in symmetry from space groups Pma2 or Pbcm to Cmce

Summary

Introduction

Synthetic zeolite-like microporous zirconosilicates with heteropolyhedral frameworks (i.e., frameworks formed by SiO4 tetrahedra and ZrO6 octahedra with the general formula [Zrm Sin O3m+2n ]−2m ) attract much attention due to the prospects of their wide application in chemical technologies as ion-exchangers, high-selectivity sorbents, catalysts, ion conductors, etc. [1]. Ideally Na2 ZrSi6 O15 ·3H2 O, is one of the best examples of such minerals. This zirconosilicate is characterized by strong ion-exchange properties which occur both in laboratory. The structure of this mineral was re-refined by different authors, in some cases in the acentric space group Pbm2 [6] or in the space group Pbcm with one unit-cell parameter doubled [7], see Table 1 In all cases, the crystal structure of elpidite is described by a heteropolyhedral framework consisting of double Si6 O15 chains (ribbons) and isolated ZrO6 octahedra. Zeolite cavities inside the heteropolyhedral framework are occupied by Na+ cations

Methods

Results

Conclusion