From structure topology to chemical composition. XXIV. Revision of the crystal structure and chemical formula of vigrishinite, NaZnTi₄(Si₂O₇)₂O₃(OH)(H₂O)₄, a seidozerite-supergroup mineral from the Lovozero alkaline massif, Kola peninsula, Russia

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$\bar 1$, 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 and 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: (Na0.67Zn0.21Ca0.05□1.07)Σ2 (Zn0.86□1.14)Σ2(Zn0.14□0.36)Σ0.5(Ti2.60Nb0.62Mn0.30${\rm Fe}_{{\rm 0}{\rm. 23}}^{{\rm 2 +}} $Mg0.10Zr0.06Zn0.05Al0.03Ta0.01)Σ4(Si4.02O14) [O2.60(OH)1.21F0.19]Σ4[(H2O)3.44(OH)0.56]Σ4{Zn0.24P0.03K0.03Ba0.02} with 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 and O = octahedral) parallel to (001). In the O sheet, the Ti-dominant MO(1,2) sites, Na-dominant MO(3) and □-dominant MO(4) sites give ideally Na□Ti2 pfu. In the H sheet, the Ti-dominant MH(1,2) sites, Zn-dominant AP(1) and vacant AP(2) sites give ideally Zn□Ti2 pfu. The MH and AP(1) polyhedra and Si2O7 groups constitute the H sheet. The ideal structural formula of vigrishinite of the form ${\rm A}_{\rm 2}^{P} {\rm M}_{\rm 2}^{\rm H} {\rm M}_{\rm 4}^{\rm O} $(Si2O7)2(${\rm X}_{\rm M}^{\rm O} $)2(${\rm X}_{\rm A}^{\rm O} $)2(${\rm X}_{{\rm M,A}}^{P} $)4 is Zn□Ti2Na□Ti2(Si2O7)2O2O(OH)(H2O)4. Vigrishinite is a Zn-bearing, Na-poor and OH-rich analogue of murmanite, ideally Na2Ti2Na2Ti2(Si2O7)2O2O2(H2O)4. Murmanite and vigrishinite are related by the following substitution: H(${\rm Na}_{\rm 2}^{\rm +} $)mur + O(Na+)mur + O(O2–)mur ↔ H(Zn2+)vig + H(□)vig + O(□)vig + O[(OH)–]vig. The doubling of the t1 and t2 translations of vigrishinite compared to those of murmanite is due to the order of Zn and □ in the H sheet and Na and □ in the O sheet of vigrishinite.

From structure topology to chemical composition. XXVI. Crystal structure and chemical composition of a possible new mineral of the murmanite group (seidozerite supergroup), ideally Na2CaTi4(Si2O7)2O4(H2O)4, from the Lovozero alkaline massif, Kola Peninsula, Russia

“…This pattern is analogous to that in murmanite-group minerals: murmanite, originally proposed by Khalilov (1989) and described in detail (including positions of H atoms) by Cámara et al . (2008); in vigrishinite, NaZnTi 4 (Si 2 O 7 ) 2 O 3 (OH)(H 2 O) 4 (Sokolova and Hawthorne, 2018); and in the lamprophyllite-group minerals: epistolite, Na 4 TiNb 2 (Si 2 O 7 ) 2 O 2 (OH) 2 (H 2 O) 4 (Sokolova and Hawthorne, 2004) and zvyaginite, Na 2 ZnTiNb 2 (Si 2 O 7 ) 2 O 2 (OH) 2 (H 2 O) 4 (Sokolova et al ., 2017). In these structures, H 2 O groups form a ribbon which extends along a ( t 1 ) (Fig.…”

“…The crystal structure of the MRM is topologically identical to the structures of murmanite (Cámara et al ., 2008) and calciomurmanite (Lykova et al ., 2016) and is related to vigrishinite (Sokolova and Hawthorne, 2018). The main structural unit in the crystal structure of MRM is a TS block that consists of HOH sheets.…”

“…We conclude that: (1) the general topology of the crystal structure of MRM described above is in accord with the topology of murmanite (Khalilov, 1989; Cámara et al ., 2008) and calciomurmanite (Lykova et al ., 2016); and (2) the stereochemistry of Ca and Na in the TS block is different from that reported for calciomurmanite (Lykova et al ., 2016): disorder of Ca and □ in the O sheet of MRM versus disorder of Ca and □ in the H sheet of calciomurmanite (Table 1). A similar stereochemistry of Zn and □ was described for two TS-block minerals: the order of Zn and □ in the O sheet of zvyaginite (Pekov et al ., 2014; Sokolova et al ., 2017) and the order of Zn and □ in the H sheet of vigrishinite (Pekov et al ., 2013; Sokolova and Hawthorne, 2018).…”

“…Following our previous work on murmanite, ideally Na 4 Ti 4 (Si 2 O 7 ) 2 O 4 (H 2 O) 4 (Cámara et al ., 2008) and a murmanite-related mineral vigrishinite, ideally NaZnTi 4 (Si 2 O 7 ) 2 O 3 (OH)(H 2 O) 4 (Sokolova and Hawthorne, 2018), we wanted to understand the details of the bond topology in the structure of calciomurmanite, especially the pattern of hydrogen bonding. In February of 2017 at the Tucson Gem and Mineral Show, we purchased a ‘calciomurmanite’ sample from Mt.…”