DEHYDROXYLATION AND TRANSFORMATIONS OF THE 2:1 PHYLLOSILICATE PYROPHYLLITE AT ELEVATED TEMPERATURES: AN INFRARED SPECTROSCOPIC STUDY

The thermally-induced dehydroxylation and transformations of the 2:1 phyllosilicate pyrophyllite have been studied using infrared spectroscopy in the frequency range 350ˇ11000 cm ˇ1 and the temperature range 200ˇ15008C. The dehydroxylation of pyrophyllite to pyrophyllite dehydroxylate occurs between 500 and 9008C. It is characterized by a decrease in the intensity of the OH signals and phonon bands of pyrophyllite and the eventual disappearance of these features as well as the appearance of extra signals related to pyrophyllite dehydroxylate and an intermediate phase. Our results are consistent with previous observations that the SiO4 tetrahedral sheet structure still exists in pyrophyllite dehydroxylate, that the SiOAl linkages and 2:1 structure remain in the pyrophyllite dehydroxylate, and that AlO5 trigonal bipyramids form. Two extra OH bands at 3690 and 3702 cm ˇ1 and their overtones at 7208 and 7234 cm ˇ1 are observed, for the first time, in samples annealed at the temperature range 550ˇ9008C. Our results suggest that the formation and dehydroxylation of the extra OH species can be strongly affected by kinetic effects. The experimental evidence shows that the dehydroxylation of pyrophyllite is a two-stage process. The appearance of these additional OH bands is interpreted in terms of an unknown intermediate phase, and may be related to the second endothermic peak observed at high temperatures in DTA experiments. Pyrophyllite dehydroylate decomposes into a Si-rich amorphous phase and mullite in the temperature range 950ˇ11008C. Cristobalite is observed in the temperature range 1150ˇ15008C. Key Words—Cristobalite, Dehydroxylation, IR Spectroscopy, Intermediate Phase, Mullite, Pyrophyllite Dehydroxylate, Transformation.