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.

The crystal structure of vigrishinite, an epistolite-group heterophyllosilicate with essential Zn, has been reinvestigated; the ideal end-member formula is revised to Zn 2 Ti 4-x (Si 2 O 7) 2 O 2 (OH,F,O) 2 (H 2 O,OH,&) 4 with x,1. Structure models of Zn-exchanged forms of murmanite after 5 and 24 hour experiments with 1N ZnSO 4 solution at 90 C have been obtained from single-crystal X-ray diffraction data. The structural formulae are [B1] Ca 0.04 {Na 1.22 (Ti 1.19 Mn 0.60 Nb 0.21)}{ [A1,A2] Zn 1.03 (Ti 1.64 Nb 0.36)[Si 2 O 7 ] 2 }O 2 (O,OH) 2 (H 2 O) 4 for vigrishinite and {Na 1.14 (Ti 1.45 Mn 0.50 Nb 0.05)}{ [A1] Ca 0.77 [A2] Zn 0.13 (Ti 1.85 Nb 0.15)[Si 2 O 7 ] 2 }O 2 (OH,O) 2 (H 2 O) 4 and [B1] Zn 0.15 [B2] Ca 0.18 {Na 1.06 (Ti 1.32 Mn 0.60 Nb 0.08)}{ [A1] Zn 0.70 [A2] Ca 0.12 (Ti 1.74 Nb 0.26)[Si 2 O 7 ] 2 }O 2 (OH,O) 2 (H 2 O) 4 for the 5 and 24 hour Zn-exchanged forms of murmanite, respectively (braces give successively the contents of the octahedral O and heteropolyhedral H sheets). The triclinic (PÀ1) unit-cell parameters are respectively:

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