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

Summary

Trioctahedral 1 M micas have been synthesized along (pseudo)binary joins using hydrothermal techniques and controlled oxygen fugacities. Octahedrally coordinated iron in annite {K}[Fe₃]$\langle$AlSi₃$\rangle$O₁₀(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 ≈10° for micas with tetrahedral sheet compositions close to $\langle$AlSi₃$\rangle$ and up to ≈14° for those containing a $\langle$FeSi₃$\rangle$ tetrahedral sheet. All iron-bearing samples were studied by ⁵⁷Fe Mössbauer 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. fMoreover, 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.