From Structure Topology to Chemical Composition. XXX. Refinement of the Crystal Structure and Chemical Formula of Shkatulkalite, Na2Nb2Na3Ti(Si2O7)2O2(FO)(H2O)4(H2O)3, a Lamprophyllite-Group (Seidozerite Supergroup) TS-Block Mineral from the Lovozero Massif, Kola Peninsula, Russia

Abstract

Shkatulkalite, ideally Na₅TiNb₂(Si₂O₇)₂O₃F(H₂O)₇, is a lamprophyllite-group (seidozerite supergroup) TS-block mineral from the Lovozero massif, Kola Peninsula, Russia. The crystal structure of shkatulkalite was refined as triclinic, space group P ⁠, a 5.464(1), b 7.161(1), c 15.573(1) Å, α 90.00(3), β 95.75(3), γ 90.00(3)°, V 606.3(4) ų, R₁ = 7.26%. The previously given chemical analysis has been modified to better fit the crystal structure: Nb₂O₅ 24.15, TiO₂ 11.35, SiO₂ 27.22, BaO 1.15, SrO 2.20, MnO 1.68, CaO 0.46, K₂O 0.29, Na₂O 14.78, H₂O 15.27, F 1.61, O = F −0.68, sum 99.48 wt.%; H₂O was determined from structure-refinement results. The empirical formula was calculated on 25.27 (O + F) apfu (in accord with the crystal structure): (Na_1.40Sr_0.19Ba_0.07K_0.05)_Σ1.71(Na_2.86Mn_0.10Ca_0.07)_Σ3.03(Nb_1.62Ti_1.27Mn_0.11)_Σ3Si_4.03O_24.52H_15.11F_0.75, Z = 1. The ideal structural formula is as follows: Na₂Nb₂Na₃Ti(Si₂O₇)₂O₂(FO)(H₂O)₄(H₂O)₃. The crystal structure of shkatulkalite [basic structure type B5(LG)] is a combination of a TS (titanium-silicate) block and an I (intermediate) block. The TS block consists of HOH sheets (H-heteropolyhedral, O-octahedral). The TS block exhibits linkage and stereochemistry typical for the lamprophyllite group where Ti (+ Nb + Fe³⁺ + Mg) = 3 apfu. The O sheet is composed of Ti-dominant M^O(1) and Na-dominant M^O(2,3) octahedra. In the H sheet in shkatulkalite, Si₂O₇ groups link to Nb-dominant M^H octahedra. The A^P site occurs in the plane of the H sheet and splits into A^P(1) and A^P(2) sites, occupied by Na at 70% and Sr (less Ba and K) at 11%. The I block consists of H₂O groups. The I block of shkatulkalite is topologically identical to those in the derivative structures of kazanskyite and nechelyustovite. The structure of the lamprophyllite-group mineral shkatulkalite has a counterpart structure in the murmanite group (Ti = 4 apfu): kolskyite, ideally Na₂CaTi₄(Si₂O₇)₂O₄(H₂O)₇ [basic structure type B7(MG)]: the two structures have TS blocks of different topology and similar I blocks, mainly occupied by H₂O groups.