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. (December 2017)

• Record Type: Journal Article
• Authors: Sokolova, E.; Genovese, A.; Falqui, A.; Hawthorne, F.C.; Cámara, F.
• Is Part Of: Mineralogical magazine. Volume 81:Number 6(2017)
• Page Start: 1533
• Page End: 1550
• Publication Date: 2017-12
• Subjects: zvyaginite, -- crystal-structure refinement, -- HRTEM, -- electron diffraction, -- EMP analysis, -- chemical formula, -- lamprophyllite group, -- seidozerite supergroup, -- TS-block minerals, -- epistolite; Mineralogy -- Periodicals; Mineralogy -- Great Britain -- Periodicals; 549.05
• DOI: 10.1180/minmag.2017.081.015
• Languages: English
• ISSNs: 0026-461X

Abstract

Abstract: The crystal structure and chemical formula of zvyaginite, ideally Na2 ZnTiNb2 (Si2 O7 )2 O2 (OH)2 (H2 O)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 structurewas 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) Å3, R1 = 9.23%. The electron-microprobe analysis gave the following empirical formula [calculated on 22 (O + F)]: (Na0.75 Ca0.09 K0.04 □1.12 )Σ2 (Na1.12 Zn0.88 Mn0.17 Fe 2+ 0.04 □0.79 )Σ3 (Nb1.68 Ti1.25 Al0.07 )Σ3 (Si4.03 O14 )O2 [(OH)1.11 F0.89 ]Σ2(H2 O)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 H2 O groups. The TS block consists of HOH sheets (H = heteropolyhedral, O = octahedral) parallel to (001). In the O sheet, the [6]MO(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] MH(1, 2) sites are occupied mainly by Nb and the [8] AP(1) and [8] AP(2) sites are occupied mainly by Na and □. The MH and AP polyhedra and Si2 O7 groups constitute the H sheet. The ideal structural formula is Na□Nb2 NaZn□Ti(Si2 O7 )2O2 (OH)2 (H2 O)4. Zvyaginite is a Zn-bearing and Na-poor analogue of epistolite, ideally (Na□)Nb2 Na3 Ti(Si2 O7 )2 O2 (OH)2 (H2 O)4 . Epistolite and zvyaginite are related by the following substitution in the O sheet of the TS-block: (Naþ 2 )epi↔Zn 2+ zvy +□zvy. The doubling of the t1 and t2 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.