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

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.

Crystal Chemistry of the Microporous Zirconosilicate Na6Zr3[Si9O27], a Product of High-Temperature Transformation of Catapleiite, and Its Ag-Exchanged Form

“…Silver cations replace sodium; however, calcium cations remain practically unreplaced. As well as in the recently reported results of Ag exchange in another zirconosilicate elpidite, ideally Na2ZrSi6O15⋅3H2O [21], the incorporation of Ag+ in the structure of Na6Zr3[Si9O27] causes a significant distortion of the heteropolyhedral framework resulting in the doubling of a parameter of the hexagonal unit cell and the change of the space group from centrosymmetric P63/mcm (Na6Zr3[Si9O27]) to acentric P63cm (its Ag-exchanged form); in elpidite, the cation exchange accompanied by a significant distortion of the framework results in the doubling of the parameters of the orthorhombic unit cell and the change in symmetry from space groups Pma2 or Pbcm (characteristic to the initial elpidite samples from different localities) to Cmce. Funding: This work was supported by the Russian Foundation for Basic Research, grants nos.…”

Rietveld Analysis of Elpidite Framework Flexibility Using in Situ Powder XRD Data of Thermally Treated Samples

“…In this study, twist-, tiltand azimuth-angles have also been measured for the crystal structures determined by single crystal X-ray diffraction of the initial unprocessed elpidite samples from Lovozero (Pma2) [4], Khan Bogdo (Pbcm) [7], and Mont Saint-Hilaire (Pbcm) [9], as well as for all reliable crystal structure refinements performed here. Later in this work, they will be used to track the structural evolution and to evaluate the framework flexibility of these materials occurring upon heating.…”

“…The crystal structure of elpidite was first solved in 1963 by Neronova and Belov [2] in the space group Pbmm on a sample from the Lovozero (L) alkaline complex, Kola Peninsula, Russia. Subsequently, the structure of this mineral from the Lovozero complex was re-refined by different authors, in some cases in the acentric space group Pbm2 [3,4] or in Pbcm with one of the unit-cell parameters doubled [5]. Rietveld powder diffraction analysis has been applied in the latter case.…”

“…In 1973, Cannillo et al [8] and in 2016, Cametti et al [9] determined the elpidite crystal structure on samples from Mont Saint-Hilaire (MSH), Québec, Canada in space group (S.G.) Pbcm. Despite the observed differences in the space groups of different samples belonging to one mineral species and taken from the same or various localities, all studied compounds retain an equal structural topology that can be presented as a heteropolyhedral framework consisting of double Si 6 O 15 chains (ribbons) connected by isolated ZrO 6 octahedra [4].…”

“…This approach has recently been used in an attempt to evaluate the structural evolution and framework flexibility exhibited by elpidite samples from the three above-mentioned locations when subjected to various laboratory treatments [10]. For this purpose, the previously reported single crystal structural data on natural elpidites and their modifications-K-and Rb-exchanged forms (KB) [7]; Ag-exchanged forms (L, KB) [4]; and dehydrated elpidite (MSH) [9]-were processed. A set of geometric parameters was introduced in order to evaluate the framework flexibility.…”