PREPUBLICATION: From structure topology to chemical composition. XXIII. Revision of the crystal structure and chemical formula of zvyaginite, a seidozerite-supergroup mineral from the Lovozero alkaline massif, Kola peninsula, Russia

• Published in: Mineralogical Magazine
• Main Authors: Sokolova, E., Genovese, Alessandro, Falqui, Andrea, Hawthorne, F. C., Cámara, F.
• Other Authors: Biological and Environmental Sciences and Engineering (BESE) Division, Bioscience Program, Electron Microscopy, Department of Geological Sciences, University of Manitoba, 125 Dysart Road, Winnipeg, MB, R3T 2N2 Canada, Dipartimento di Scienze della Terra “Ardito Desio”, Università degli Studi di Milano, Via Mangiagalli 34, 20133, Milano, Italy
• Format: Article in Journal/Newspaper
• Language: unknown
• Published: Mineralogical Society 2018
• Subjects: Chemical formula; Crystal-structure refinement; EMP analysis; Electron diffraction; Epistolite; HRTEM; Lamprophyllite group; Seidozerite supergroup; TS-block minerals; Zvyaginite; kola peninsula
• Online Access: http://hdl.handle.net/10754/623295; https://doi.org/10.1180/minmag.2017.081.015

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 C⎯1, 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)]:(Na0.75Ca0.09K0.04��1.12)Σ2 (Na1.12Zn0.88Mn0.17Fe2+0.04��0.79)Σ3(Nb1.68Ti1.25Al0.07)Σ3 (Si4.03O14)O2 [(OH)1.11F0.89]Σ2(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]MO(1,4,5) sites are occupied mainly by Ti, Zn and Na and the [6]MO(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 Si2O7 groups constitute the H sheet. The ideal structural formula is Na��Nb2NaZn��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 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. We are grateful to an anonymous reviewer and Associate Editor Peter Leverett for useful comments. We thank ...