The thermal behaviour of an Fe-rich illite

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Abstract
The phase changes that took place upon heating an Fe-rich illite (OECD #5) to 1300°C in an oxidizing atmosphere were studied by a variety of mineralogical techniques. Infrared spectra, showing the stepwise dehydroxylation of the illite, showed good agreement with variations in sample colour and Mössbauer spectra. Dehydroxylation did not lead to noticeable variations in X-ray powder diffraction patterns until the structural breakdown of illite and formation of new phases at about 900°C Mössbauer spectroscopy proved to be very sensitive to all changes induced by heating, showing the disappearance of Fe2+ at 250°C, the gradual dehydroxylation between about 350 and 900°C, and characteristic features of the products formed at higher temperatures, e.g. the formation of hematite as the illite structure breaks down and the subsequent disappearance of hematite due to the incorporation of Fe in glass above 1200°C. The formation of hematite in clusters large enough to order magnetically at room temperature was first observed in the sample heated to 900°C, whereas at 4.2 K, significant proportions of a magnetically ordered phase could already be identified in the sample heated to 650°C.