From Structure Topology to Chemical Composition. XXXI. Refinement of the Crystal Structure and Chemical Formula of Selivanovaite, NaFe3+Ti4(Si2O7)2O4(H2O)4, a Murmanite-Group (Seidozerite Supergroup) TS-Block Mineral from the Lovozero Massif, Kola Peninsula, Russia

Abstract

Selivanovaite, ideally NaFe³⁺Ti₄(Si₂O₇)₂O₄(H₂O)₄, is a murmanite-group (seidozerite supergroup) TS-block mineral from the Lovozero massif, Kola Peninsula, Russia. The crystal structure of selivanovaite was refined in space group C ⁠, a 10.524(6), b 13.815(6), c 12.213(14) Å, α 99.74(6), β 107.45(8), γ 90.15(10)°, V 1666.8(26) ų, R₁ = 21.40%. The previously given chemical analysis has been modified to better fit the crystal structure: Nb₂O₅ 8.51, ZrO₂ 2.94, TiO₂ 31.96, SiO₂ 30.62, Al₂O₃ 0.05, Fe₂O₃ 5.08, FeO 3.23, MnO 3.36, CaO 2.14, MgO 0.75, Na₂O 2.47, H₂O 8.88, sum 99.99 wt.%; H₂O was determined from structure-refinement results: H₂O = 3.34 pfu, OH = 1.05 pfu. The empirical formula calculated on 22 O apfu is: (Na_0.63Ca_0.30Mn_0.36)_Σ1.29(Fe³⁺_0.50Fe²⁺_0.35)_Σ0.85(Ti_3.14Nb_0.50Zr_0.19Mg_0.15Mn_0.01Al_0.01)_Σ4.00Si_3.99O_22.00H_7.73, Z = 4. The crystal structure of selivanovaite [basic structure type B1(MG)] 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 M^O(1,2) sites, Na-dominant M^O(3) site, and □-dominant M^O(4) sites give ideally Na□Ti₂pfu. In the H sheet, the Ti-dominant M^H(1,2) sites, Fe³⁺-dominant A^P(1) site, and vacant A^P(2) sites give ideally Fe³⁺□Ti₂pfu. The M^H and A^P(1) polyhedra and Si₂O₇ groups constitute the H sheet. The ideal structural formula of selivanovaite of the form A^P₂M^H₂M^O₄(Si₂O₇)₂(X^O_M,A)₄(X^P_M,A)₄ is Fe³⁺□Ti₂Na□Ti₂(Si₂O₇)₂O₄(H₂O)₄. Selivanovaite is a Fe³⁺-bearing and OH-poor analogue of vigrishinite, ideally Zn□Ti₂Na□Ti₂(Si₂O₇)₂O₂O(OH)(H₂O)₄. Vigrishinite and selivanovaite are related by the following substitution: ^O(OH)^–_vig + ^H(Zn²⁺)_vig ↔ ^O(O^2–)_sel + ^H(Fe³⁺)_sel. Selivanovaite is a Fe³⁺-bearing and Na-poor analogue of murmanite, ideally Na₂Ti₂Na₂Ti₂(Si₂O₇)₂O₄(H₂O)₄. Murmanite and selivanovaite are related by the following substitution: ^O(Na⁺)_mur + ^H(Na⁺₂)_mur ↔ ^O(□)_sel + ^H(Fe³⁺)_sel + ^H(□)_sel. The doubling of the t₁ and t₂ translations of selivanovaite compared to those of murmanite is due to the ordering of Fe³⁺ and □ in the H sheet and Na and □ in the O sheet of selivanovaite.