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 465cm−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.