Thermic transformation of sillimanite single crystals to 3:2 mullite plus melt: investigations by polarized IR-reflection micro spectroscopy

Oriented and polished high quality sillimanite single crystals were thermically treated for 4, 24 and 96 h at 1600°C. The products were quenched to room temperature and investigated using polarized IR microspectroscopy in reflection mode (600–4000 cm−1). The spectra of the 24 and 96 h thermically treated sample indicates the transformation of sillimanite to 3:2 mullite plus melt of eutectic SiO2/Al2O3 composition. The anisotropy of the spectra show a high degree of orientation of crystals according to a topotaxic reaction. The high wavenumber mullite lattice bands between 1100 and 1200 cm−1 are assigned to modes of [SiO4] units interconnected within the mullite specific (Al∗SiAl–Oc∗] 3-clusters, and Si–Oc–Si- and Si–Oc–Al-bonds. It is shown that the sillimanite to mullite transformation occurs by an increased damping of all modes of the sillimanite lattice due to an irreversible increase of anharmonicity preserved as a typical property of mullite.