Publication Date:
2014-12-01
Description:
The structure of cronstedtite-1Mwas refined using a crystal from Eisleben (Saxony-Anhalt, Germany). The space group isCm,a= 5.5033 (3),b= 9.5289 (6),c= 7.3328 (5) Å, β = 104.493 (7)°, composition (Fe2+2.461Fe3+0.539)(Si1.461Fe3+0.539)O5(OH)4,Z= 2. Despite diffusely streaked characteristic polytype reflections, the refinement converged toRobs= 0.0242 for 1117 independent reflections. SeparateRobsvalues 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 theTetsheet. Consecutive layers are shifted bya/3 of the hexagonal cell, the polytype belongs to the subfamily (Bailey's group) A. There are two octahedral sitesM1 (atm),M2 (in a general position) both fully occupied by Fe. Since theM2 octahedron is larger thanM1, theOcsheet ismeso-octahedral. TheTetsheet 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 theOcsheet with basal O atoms of theTetsheet. The basal O atoms (acceptors) of theTetsheet are rotated close to OH groups (donors) of theOcsheet 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.
Print ISSN:
2052-5192
Electronic ISSN:
2052-5206
Topics:
Chemistry and Pharmacology
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Geosciences
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Physics
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