Revision of the crystal structure and chemical formula of vigrishinite

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Marco E. Ciriotti
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Revision of the crystal structure and chemical formula of vigrishinite

Messaggio da Marco E. Ciriotti » mer 09 ago, 2017 6:20

Prossima pubblicazione.

Referenza:
▪ Sokolova, E. & Hawthorne, F.C. (2017): From structure topology to chemical composition. XXIV. Revision of the crystal structure and chemical formula of vigrishinite, NaZnTi4(Si2O7)2O3(OH)(H2O)4, a seidozeritesupergroup mineral from the Lovozero alkaline massif, Kola peninsula, Russia. Mineralogical Magazine, 81, (in press).

Abstract:
The crystal structure of vigrishinite, ideally NaZnTi4(Si2O7)2O3(OH)(H2O)4, a murmanite-group mineral of the seidozerite supergroup from the type locality, Mt. Malyi Punkaruaiv, Lovozero alkaline massif, Kola Peninsula, Russia, was refined in space group C-1, a = 10.530(2), b = 13.833(3), c = 11.659(2) Å, α = 94.34(3), β = 98.30(3), γ = 89.80(3)°, V = 1675.5(2.1) Å3, R1 = 12.52%. Based on electron-microprobe analysis, the empirical formula calculated on 22 (O + F), with two constraints derived from structure refinement, OH + F = 1.96 pfu and H2O = 3.44 pfu, is (Na0.67Zn0.21Ca0.05o1.07)Σ2 (Zn0.86o1.14)Σ2(Zn0.14o0.36)Σ0.5(Ti2.60Nb0.62Mn0.30Fe2+0.23Mg0.10Zr0.06Zn0.05 Al0.03Ta0.01)Σ4(Si4.02O14)[O2.60(OH)1.21F0.19]Σ4 [(H2O)3.44(OH)0.56]Σ4{Zn0.24P0.03K0.03Ba0.02}, Z = 4. It seems unlikely that constituents in the {} belong to vigrishinite itself. The crystal structure of vigrishinite is an array of TS blocks (Titanium Silicate) connected via hydrogen bonds. The TS block consists of HOH sheets (H = heteropolyhedral, O = octahedral) parallel to (001). In the O sheet, the Ti-dominant MO(1,2) sites, Na-dominant MO(3) and o-dominant MO(4) sites give ideally NaoTi2 pfu. In the H sheet, the Ti-dominant MH(1,2) sites, Zn-dominant AP(1) and vacant AP(2) sites give ideally ZnoTi2 pfu. The MH and AP(1) polyhedra and Si2O7 groups constitute the H sheet. The ideal structural formula of vigrishinite of the form AP2MH2MO4(Si2O7)2(XOM)2(XOA)2(XPM,A)4 is ZnoTi2NaoTi2(Si2O7)2O2O(OH)(H2O)4. Vigrishinite is a Zn-bearing, Na-poor and OH-rich analogue of murmanite.

NdR: Questa revisione di formula avrebbe dovuto essere validata da IMA CNMNC. Il sodio (Na) non era presente nella formula precedente che è quella che appare nella lista delle valide specie IMA - Updated list of IMA-approved minerals (May 2017) : http://nrmima.nrm.se//
Marco E. Ciriotti

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