From structure topology to chemical composition. XXIII. Revision of the crystal structure and chemical formula of zvyaginite, Na₂ZnTiNb₂(Si₂O₇)₂O₂(OH)₂(H₂O)₄, a seidozerite-supergroup mineral from the Lovozero alkaline massif, Kola peninsula, Russia

The crystal structure and chemical formula of zvyaginite, ideally Na₂ZnTiNb₂(Si₂O₇)₂O₂(OH)₂(H₂O)₄, a lamprophyllite-group mineral of the seidozerite supergroup from the type locality, Mt. Malyi Punkaruaiv, Lovozero alkaline massif, Kola Peninsula, Russia have been revised. The crystal structure was refined with a new origin in space group C1, a = 10.769(2), b = 14.276(3), c = 12.101(2) Å, α = 105.45(3), β = 95.17(3), γ = 90.04(3)°, V = 1785.3(3.2) ų, R ₁ = 9.23%. The electron-microprobe analysis gave the following empirical formula [calculated on 22 (O + F)]: (Na_0.75Ca_0.09K_{0.04 1.12})_∑2 (Na_1.12Zn_0.88Mn_0.17Fe²⁺ _{0.04 0.79})_∑3 (Nb_1.68Ti_1.25Al_0.07)_∑3 (Si_4.03O₁₄)O₂ [(OH)_1.11F_0.89]_∑2 (H₂O)₄, Z = 4. Electron-diffraction patterns have prominent streaking along c* and HRTEM images show an intergrowth of crystalline zvyaginite with two distinct phases, both of which are partially amorphous. The crystal structure of zvyaginite is an array of TS (Titanium-Silicate) blocks connected via hydrogen bonds between H₂O groups. The TS block consists of HOH sheets (H = heteropolyhedral, O = octahedral) parallel to (001). In the O sheet, the ^[6] M ^O(1,4,5) sites are occupied mainly by Ti, Zn and Na and the ^[6] M ^O(2,3) sites are occupied by Na at less than 50%. In the H sheet, the ^[6] M ^H(1,2) sites are occupied mainly by Nb and the ^[8] A^P (1) and ^[8] A^P (2) sites are occupied mainly by Na and . The M^H and A ^P polyhedra and Si₂O₇ groups constitute the H sheet. The ideal structural formula is Na Nb₂ NaZn Ti(Si₂O₇)₂O₂(OH)₂(H₂O)₄. Zvyaginite is a Zn-bearing and Na-poor analogue of epistolite, ideally (Na)Nb₂Na₃Ti(Si₂O₇)₂O₂(OH)₂(H₂O)₄. Epistolite and zvyaginite are related by the following substitution in the O sheet of the TS-block: (Na⁺ ₂)_epi ↔ Zn²⁺ _zvy + _zvy. The doubling of the t ₁ and t ₂ translations of zvyaginite relative to those of epistolite is due to the order of Zn and Na along a (t₁ ) and b (t₂ ) in the O sheet of zvyaginite.