X-ray powder diffraction and 57Fe-Mossbauer spectroscopy of synthetic trioctahedral micas {K}[Me3](TSi3)O10(OH)2, Me = Ni2+, Mg2+, Co2+, Fe2+; T = Al3+, Fe3+

Trioctahedral 1M micas have been synthesized along (pseudo) binary joins using hydrothermal techniques and controlled oxygen fugacities. Octahedrally coordinated iron in annite {K}[ Fe-3] (AlSi3)O-10(OH)(2) was successively replaced by Co2+, Mg2+ and Ni2+ and tetrahedrally coordinated aluminum by Fe3+. Unit cell parameters decrease almost linearly with decreasing average radius of the octahedral cation/average M-O bond length within the octahedral sheet. With increasing substitution of Fe2+ the octahedral sheet becomes more flattened, the ditrigonal distortion of the tetrahedral sheet increases up to a maximum value of approximate to 10 degrees for micas with tetrahedral sheet compositions close to (AlSi3) and up to approximate to 14 degrees for those containing a (FeSi3) tetrahedral sheet. All iron-bearing samples were studied by Fe-57 Mossbauer spectroscopy. With increasing substitution of iron by smaller divalent cations the quadrupole splitting distribution (QSD) evolves from a broad bimodal distribution in annite to a smaller unimodal distribution in Mg2+ and Ni2+-rich samples so that for high substitution rates more regular local environments are dominating. These results, however, can not be interpreted in terms of an octahedral cation ordering scheme. For none of the micas investigated reliable Fe2+ M2/M1 area ratios can be extracted. Moreover, the complete QSD is shifted towards higher quadrupole splitting values. Similar observations were obtained for substituting Fe2+ by Mg2+ and Ni2+ in tetra-ferri-annite free of octahedral coordinated trivalent cations. Unlike in the Al3+ bearing micas a third QSD component is missing which supports the claim that the appearance of this third QSD component is closely related to the presence of trivalent cations (Al3+, Fe3+) in octahedra coordination.