THERMAL DECOMPOSITION OF FERRIAN CHAMOSITE: AN INFRARED EMISSION SPECTROSCOPIC STUDY

Abstract

The thermal behaviour of ripidolite, an iron-rich chlorite, has been studied in situ by infrared emission spectroscopy up to 800 °C. The more di,trioctahedral nature due to significant amounts of Fe3+ is reflected, in addition to the two bands around 3420 and 3560 cm-1, by an extra band around 3345 cm-1. This extra band is absent in pure dioctahedral chlorites without Fe3+. These bands have been assigned to (AlAl)O-OH, (SiAl)O-OH and (SiSi)O-OH stretching modes with increasing frequencies. The bands disappear upon dehydroxylation around 650 °C. A similar behaviour is observed for the corresponding libration modes around 716, 759 and 802 cm-1. The stretching and bending modes of the inner-OH of the octahedral sheet in the 2:1 clay-like layer are observed around 3645, 943 and 904 cm-1. Although the bands decrease in intensity, they remain present up to 800 °C as dehydroxylation of the octahedral sheet is not yet complete at this temperature. The presence of two bending modes is explained as being due to a differentiation between Mg-OH and Fe-OH modes. At 650 °C a new sharp band is observed around 502 cm-1 assigned to a (Fe,Mg)-O-Al bending mode caused by the formation of a spinel-like interlayer phase after dehydroxylation.