The study on the thermal transformation of illite was made by differential thermal analysis, thermo-gravimetric analysis and X-ray powder diffraction, and the transformation mechanism was discussed from structural point of view. The process of thermal transformation of illite can be summarized as follows:
Illite500°C Illite anhydroxylate 900°C Spinel-type oxide 1050°C Fe-bearing spinel
1100°C Mullite.
Futhermore, above about 850°C and immediately following the destruction of the dehydroxylated phase, a glass phase appears, and hematite starts to from at about the same temperature and melted into a glass phase above 1200°C.
An investigation of the transformation mechanism was made using an oriented specimen of illite. The following topotactic relations among the original illite and high temperature phases were maintained through the entire transformation process:
(001)I//(001)I·A//(111) Sp//(c-axis)M,
where the suffixes I, I. A, Sp and M signify illite, illite anhydroxylate, spinel-type oxide and mullite, respectively. The thermal transformation process of illite is characterized by a structural continuity reflected by the topotactic transformation mentioned above.