Pseudosymmetry and cation ordering in heterophyllosilicates. 1. Refinement of the crystal structure of schüllerite Ba2Na(Mn,Ca)(Fe3+,Mg,Fe2+)2Ti2(Si2O7)2(O,F)4

Research subject. Currently, the symmetry of schüllerite and the choice of space group (sp. gr.) in its structure are subjects of debate. In this work, a re-examination of the holotype sample of schüllerite was conducted. Materials and Methods. The mineral schüllerite was found in the Löhley basalt quarry (Eifel volcanic area, Germany). The crystal structure was studied using single-crystal X-ray analysis. Results. We refined the crystal structure of schüllerite within two space groups – acentric P1 and centrosymmetric P1 using data sets for 2496 and 1683 independent reflections with I > 3σ(I), respectively. The final R-factor values were 4.42 % in sp. gr. P1 and 4.51 % in sp. gr. P1 . The parameters of the triclinic unit cell are: a = 5.4055(3) Å, b = 7.0558(3) Å, c = 10.1945(6) Å, α = 99.838(4)°, β = 99.715(5)°, γ = 90.065(4)°, V = 377.43(4) Å3. The idealized formula is Ba 2 Na(Mn,Ca)(Fe 3+ ,Mg,Fe 2+ )2Ti 2 (Si 2 O 7 ) 2 (O,F) 4 . Conclusions. The acentric space group P1 is proposed as more suitable for describing the structure of schüllerite, as it allows for the identification of more existing differences in site occupancies and cation-anion bond lengths in HOH modules.