Location of OH groups and oxidation processes in triclinic chloritoid

Abstract

Triclinic Fe2+ and Fe3+-chloritoid end members, as well as Fe2+-Fe3+-chloritoid solid solutions were synthesized in order to determine the location of the OH groups in the structure and to investigate the possibility of Fe3+ incorporation on the M1B site and the corresponding oxidation mechanism. The samples were analyzed using transmission electron microscopy, electron energy-loss and 57Fe Mossbauer spectroscopy, polarized single-crystal, KBr powder and in situ high-pressure infrared spectroscopy. The structure of a natural triclinic chloritoid was refined by single-crystal X-ray diffraction in order to locate the proton-accepting oxygens in the triclinic structure. The results of valence bond calculations, polarized single-crystal and in situ high-pressure IR spectroscopy revealed that the orientations of the OH dipoles are the same as those in monoclinic chloritoid. The annealing experiments in air show that the incorporation of Fe3+ on M1B increases with temperature. After 3 h at 530 °C all Fe2+ was oxidized to Fe3+ without decomposition of the structure. This also holds true for a heating duration of 24 h. The incorporation of Fe3+ at M1B sites is coupled with a dehydration process starting at about 350 °C during which the H1A protons leave first.