Refinement of cronstedtite-1M

The structure of cronstedtite-1M was refined using a crystal from Eisleben (Saxony-Anhalt, Germany). The space group is Cm, a = 5.5033 (3), b = 9.5289 (6), c = 7.3328 (5) Å, β = 104.493 (7)°, composition (Fe(2+)2.461Fe(3+)0.539)(Si1.461Fe(3+)0.539)O5(OH)4, Z = 2. Despite diffusely streaked characteristic polytype reflections, the refinement converged to Robs = 0.0242 for 1117 independent reflections. Separate Robs values of 0.0183 and 0.0553 were obtained for 415 subfamily and 702 characteristic reflections, respectively. The structure is built of one edge-sharing octahedral (Oc) and one corner-sharing tetrahedral (Tet) sheet forming the 1:1 layer by sharing apical corners of the Tet sheet. Consecutive layers are shifted by a/3 of the hexagonal cell, the polytype belongs to the subfamily (Bailey's group) A. There are two octahedral sites M1 (at m), M2 (in a general position) both fully occupied by Fe. Since the M2 octahedron is larger than M1, the Oc sheet is meso-octahedral. The Tet sheet contains one tetrahedral position, T1, occupied by Si and Fe in the ratio 0.731:0.269 (9). The ring of tetrahedra is strongly ditrigonalized, α = +12.7°; Franzini type A. Of three H atoms localized, two are involved in hydrogen bonds, linking OH groups of the Oc sheet with basal O atoms of the Tet sheet. The basal O atoms (acceptors) of the Tet sheet are rotated close to OH groups (donors) of the Oc sheet of an adjacent layer in order to achieve reasonable lengths and geometries of hydrogen bonds. The structure is interpreted as an example of the order-disorder (OD) structure of more than one kind of layer with a very low degree of desymmetrization.