Crystal chemistry and infrared spectroscopy in the OH-stretching region of synthetic staurolites

Fe-staurolite synthesized in the pressure range 5 to 25 kbar at 680 °C was analysed by microprobe and by a titration method (for water-contents) which uses the "Karl-Fischer" reaction and also characterized by FT-infrared powder spectroscopy in the OH-stretching region (3000-4000 cm^-1). The study shows that the stoichiometry of the synthetic Fe-staurolites depends on the pressure of equilibration, with Fe pfu decreasing with increasing pressure, according to the heterovalent substitution of Fe²⁺ by Al³⁺ in the T2 site. This charge-increasing substitution is combined with charge-reducing substitutions in the "kyanite"-layer and by vacancies in the proton site H1. The incorporation of Al in T2 is supported by peculiarities of the IR-spectra in the range of the OH-valence vibrations. Mg- and Zn-staurolite were synthesized at 25 kbar and 700 °C and investigated by infrared spectroscopy. The comparison of the infrared spectra of synthetic Fe-, Mg- and Zn-staurolites shows that there is a significant amount of Fe²⁺, Mg and Zn in the respective M2 sites.