Infrared emission spectroscopy study of the dehydroxylation of 10 Å halloysite from a Neogene cryptokarst of South Belgium

The dehydroxylation of 10 Å halloysite from the Wellin (Weillen) cryptokarst (southern Belgium) was studied in situ by applying infrared emission spectroscopy. Dehydroxylation is evidenced by the loss of the OH-stretching modes between 3600 and 3700 cm-1. A slow decrease of all intensities up to ± 500 °C is observed followed between 500° and 550 °C by a rapid decrease to almost zero. The IES bands at 920 and 938 cm-1, attributed to the accompanying inner and outer or inner sheet Al-OH libration modes, are removed on heating at the same rate and disappears at the same temperature as the OH-stretching bands. The low frequency bands that disappear upon heating are associated with OH-libration (920 and 938 cm-1) or OH-translation modes (798, 754 and 693 cm-1). At high temperatures only very broad bands around 800 cm-1, a weak shoulder around 900 cm-1 and two broad bands around 1000 and 1175 cm-1 are observed. The first two are associated with newly formed Al-O bonds whereas the other two are associated with Si-O bonds in the dehydroxylated halloysite, an X-ray amorphous mullite-like phase.