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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 <i>Pbm2</i>, <i>a</i> = 14.6127(7), <i>b</i> = 7.3383(4), <i>c</i> = 7.1148(3) &#197;, <i>V</i> = 762.94(6) &#197;³) 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 &#197; unit cell (<i>a</i> = 14.1755(7), <i>b</i> = 14.6306(9), <i>c</i> = 14.2896(7) &#197;, <i>V</i> = 2963.6(3) &#197;³ for the Ag-exchanged form of elpidite from Lovozero and <i>a</i> = 14.1411(5), <i>b</i> = 14.5948(4), <i>c</i> = 14.3035(5) &#197;, <i>V</i> = 2952.04(17) &#197;³ for the Ag-exchanged form of elpidite from Khan Bogdo) and space group <i>Cmce</i>. 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.