Micro-Raman and FT-IR spectroscopy study of the thermal transformations of St. Claire dickite

The thermal transformations of St. Claire dickite up to 1300 °C were studied by micro-Raman and FT-IR spectroscopy methods. Spectra recorded after the thermal treatment at 700 °C show drastic changes from those recorded at 600 °C, which are compatible with the dehydroxylation and formation of a pseudo-amorphous phase of meta-dickite. Some silica is expelled together with the beginning of Al/Si ordering at 1000 °C, as observed by FT-IR spectroscopy through the splitting of the combined Al-O and Si-O deformation band into two modes at 553 and 465 cm^-1. A separate phase of amorphous silica is detected by micro-Raman spectroscopy at 1100 °C, as observed by the appearance of characteristic Raman bands at 432, 489 and 601 cm^-1. Presence of minor amounts of corundum is detected through luminescence spectroscopy at 1100 °C, and is indicated by the appearance of R-lines of Cr⁺³ impurity at 692 and 694 nm. Crystallization of some mullite is observed at 1200-1300 °C by the appearance of characteristic Raman bands at 308, 413, 960, 1035 and 1127 cm^-1 and by the appearance of IR band at 815 and 906 cm^-1.