Crystal structure of cronstedtite-6T2, a non-MDO polytype

The crystal structure of cronstedtite-6T₂ was refined using a crystal from the new occurrence in Pohled, Czech Republic. The space group is P3₁, lattice parameters a = 5.4976(3), c = 42.601(1) Å, Z = 6, composition (Fe²⁺_2.515 Fe³⁺_0.485) (Si_1.515 Fe³⁺_0.485) O₅ (OH)₄, R_obs = 4.13% for 3244 independent reflections. The separate R_obs values 2.50% and 6.96% were obtained for 1033 subfamily and 2196 characteristic reflections, respectively. The structure is built of edge-sharing octahedral (Oc), and corner-sharing tetrahedral (Tet) sheets forming the 1:1 layers by sharing apical corners of Tet sheet. There are two independent 1:1 layers, where the odd one is shifted with respect of the even one by −(a₁ + a₂)/3 and raised by c/6 of the hexagonal cell. The sextuple multiplicity is achieved by mapping this pair of layers by 3₁ axis repeatedly to two other equivalent positions raised by c/3, 2c/3. The polytype belongs to the subfamily (Bailey’s group) A. There are two tetrahedral and three octahedral sites per 1:1 layer (T1, T2, M1, M2, M3 in even layers, T11, T12, M11, M12, M13 in odd layers), all in general positions. The M3, M13 octahedra are smaller than M1, M2, M11, M12, thus Oc sheets in both layers are meso-octahedral. In even layers, however, the M2 octahedron is somewhat smaller than M1, so the Oc sheet is “transitional” to a hetero-octahedral character. The occupancies of Si:Fe in T positions were refined to: T1, 0.96:0.04(1); T2, 0.63:0.37(1); T11, 0.55:0.45(1); T12, 0.89:0.11(1). Both rings of tetrahedra are strongly ditrigonalized, α values are +11.4(5)°, and +10.9(5)°, in even and odd layers, respectively. The Franzini type is A. Four hydrogen positions were localized in each layer (eight altogether). Three of them per layer (six altogether) are involved in hydrogen bonds linking the OH groups (donors) of the Oc sheet with basal oxygen atoms of the Tet sheet of the adjacent 1:1 layer. The structure is an example of OD structure of more than one kind of layers with a very low degree of desymmetrization. Cronstedtite-6T₂ is a non-MDO polytype, because it contains more than one kind of layer triplets. Comparison of stacking sequences with other polytypes of the A subfamily is presented.