Fe-rich trioctahedral micas hydrothermally synthesized are characterized by infrared spectroscopy, the spectra are collected from powder suspensions. In the lattice vibrational range, the vibrations Si–Onb, Si–Ob, Al–Onb, Al–O–Si, Si–O–Si and delta OH are all characterized by the presence of doublets in the annite KBr-absorption spectra. In agreement with observations in the vibrational range of the OH-groups, this feature is interpreted to reflect the chemical heterogeneity of the octahedral and tetrahedral layers, imposed by crystallochemical constraints. With increasing Al content of the micas along the annite-siderophyllite join, the evolution of the bands shows that Al and Si become more ordered in the tetrahedral layer. In the interlayer vibrational range, the OH-annite end-member shows clearly five of the six predicted vibrations. The bands occuring at 66 cm-1 and at 120–130 cm-1 are related to vibrations involving interlayer cation, whereas the band observed at 152 cm-1 is assigned to basal oxygen vibrations around the interlayer cations. The Tschermak substitution (starting from the annite end-member) increases the misfit between the octahedral and tetrahedral layers. However, variations of fO2 do not affect significantly the band frequencies resulting from motions related to the interlayer cations, suggesting that the geometry of the interlayer site is not significantly disturbed by variation of the Fe3+/Fe2+ ratio in annite.
