Abstract
About 80 different synthetic trioctahedral micas {K}[Fe2+ 3−x Me x ]〈Si,Al,Fe〉4O10(OH)2 with Me = Ni2+, Mg2+, Co2+, Al3+ and Fe3+ have been synthesized by hydrothermal methods and subsequently investigated by 57Fe Mossbauer spectroscopy. Mossbauer spectra were refined in terms of a quadrupole splitting distribution (QSD) with at least 2 components for Fe2+ and additional components for Fe3+ on octahedral and tetrahedral positions. Three Fe2+ components have to be used in all samples containing distinct amounts of trivalent cations (Fe3+, Al3+). A rough positive correlation between the intensity of this third Fe2+ QSD component and the content of trivalent octahedral cations has been found. Substitution of Fe2+ by smaller divalent cations causes a distinct increase of ferrous quadrupole splitting. This suggests the remaining Fe2+O4(OH)2 octahedra to become more regular. This holds true for micas with 〈AlSi3〉 and with 〈FeSi3〉 composition of the tetrahedral sheet. In micas with Fe3+ in tetrahedral coordination there is a distinct increase of the quadrupole splitting of tetrahedral Fe3+ with decreasing size of the octahedral sheet. This can be explained by an increasing distortion of the second oxygen coordination sphere around tetrahedral Fe3+ due to increasing ditrigonal distortion of the tetrahedral sheet.
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