Pseudosymmetry and cation ordering in heterophyllosilicates. 2. Refinement of the crystal structure of Ca-bearing mineral of the perraultite series

Object of Study. The symmetry and the choice of the space group (sp. gr.) for the structures of perraultite and related minerals – bafertisite-like Mn/Fe-Ti heterophyllosilicates with ordered alkali and alkaline earth cations is still under discussion. In the present work, the crystal structure of the Ca-containing member of the perraultite series is studied. Materials and Methods. The mineral perraultite used in this research was collected from the Oktyabrsky alkaline massif (Northern Azov Sea region). The crystal structure of the sample was studied using single crystal X-ray diffraction analysis. The parameters of the monoclinic unit cell are: a = 10.7230(3) Å, b = 13.8313(4) Å, c = 20.8178(7) Å, β = 95.0348(3)°, V = 3075.638(15) Å3. Results. The crystal structure of perraultite was refined within two space groups, C2 and C2/m, using data sets for 5120 and 2948 independent reflections with I > 3σ(I), respectively. The final R-factor values were 4.66 % for the sp. gr. C2 and 4.84 % for sp. gr. C2/m. For the structural model refined using the sp. gr. C2 the crystal chemical formula is (Z = 2): A1(Ba) A2(Ba 0.64 K 0.36 ) 2 A3(K 0.8 7Ba 0.13 ) B1(Na 0.70 Ca 0.30 ) B2(Na 0.70 Ca 0.30 ) B3(Na 0.90 Ca 0.10 ) 2 [M1(Mn 0.50 Fe 2+ 0.40 Zn 0.10 ) M2(Mn 0.60 Fe 2+ 0.30 Zn 0.10 ) M3(Mn 0.50 Fe 2+ 0.50 ) M4(Mn 0.70 Fe 2+ 0.30 ) M5(Mn 0.50 Fe 2+ 0.50 ) M6(Mn 0.60 Fe 2+ 0.40 ) M7(Mn 0.60 Fe 2+ 0.40 ) M8(Mn 0.70 Fe 2+ 0.30 ) (OH) 4 ] 2 [Ti1(Ti 0.91 Nb 0.09 ) Ti2(Ti 0.91 Nb 0.09 ) Ti3(Ti 0.77 Nb 0.13 Zr 0.10 ) Ti4(Ti 0.91 Nb 0.09 ) (Si 2 O 7 ) 4 O 4 F 2 ] 2 , where square brackets denote the main structural fragments. Conclusions. The sp. gr. C2 is proposed as more suitable for describing the perrotite-type structure, since it allows better identification of the features of the occupancy of positions within the HOH modules. The studied mineral from the Oktyabrsky massif is a F-dominant analogue of perraultite, differing from the latter also by the presence of calcium, a high iron content and a low niobium content.