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

Abstract

AbstractThe crystal structure of a murmanite-related mineral (MRM) of the murmanite group (seidozerite supergroup), ideally Na2CaTi4(Si2O7)2O4(H2O)4, from Mt. Pyalkimpor, the Lovozero alkaline massif, Kola Peninsula, Russia, was refined in space group P$ {\bar 1} $ with a = 5.363(2), b = 7.071(2), c = 12.176(5) Å, α = 92.724(3), β = 107.542(7), γ = 90.13(2)°, V = 439.7(4) Å3 and R1 = 5.72%. On the basis of electron-microprobe analysis, the empirical formula calculated on 22 (O + F), with two constraints derived from structure refinement, OH = 0.11 per formula unit (pfu) and H2O = 3.89 pfu, is (Na2.12K0.07Sr0.01)Σ2.20Ca0.85(Ti3.01Nb0.39Mn0.20Fe2+0.19Mg0.17Zr0.01Al0.01)Σ3.98(Si4.20O14)[O3.90F0.10]Σ4[(H2O)3.89(OH)0.11]Σ4{P0.03}, with Z = 1. It seems unlikely that {P0.03} belongs to MRM itself. The crystal structure of MRM 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 MO1 site and Ca-dominant MO2 site give ideally (Ca□)Ti2 pfu. In the H sheet, the Ti-dominant MH site and Na-dominant AP site give ideally Na2Ti2 pfu. The MH and AP polyhedra and Si2O7 groups constitute the H sheet. The ideal structural formula of MRM of the form AP2MH2MO4(Si2O7)2(XOM,A)4(XOA)2(XPM,A)4 is Na2Ti2(Ca□)Ti2(Si2O7)2O4(H2O)4. MRM is a Ca-rich and Na-poor analogue of murmanite, ideally Na2Ti2Na2Ti2(Si2O7)2O4(H2O)4 and a Na-rich and (OH)-poor analogue of calciomurmanite, ideally (Ca□)Ti2(Na□)Ti2(Si2O7)2O2[O(OH)](H2O)4. MRM and (murmanite and calciomurmanite) are related by the following substitutions: O(Ca2+□)MRM ↔ O(Na+2)mur and O(Ca2+□)MRM + H(Na+2)MRM + O(O2–)MRM ↔ O(Na+□)cal + H(Ca2+□)cal + O[(OH)–]cal. MRM is a possible new mineral of the murmanite group (seidozerite supergroup) where Ti + Mn + Mg = 4 apfu.