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.
Bibliographical noteFunding Information:
This work was supported by: The Open University of Israel Research Fund, the CNRS/INSU (Geomateriaux Program), France, and The Belfer Center for Energy Research, Israel. This support is gratefully acknowledged. The assistance of Ms. Tatiana Krongaouz of the Open University of Israel is noted with appreciation.