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Dehydroxylation and Transformations of the 2:1 Phyllosilicate Pyrophyllite at Elevated Temperatures: An Infrared Spectroscopic Study

Published online by Cambridge University Press:  01 January 2024

Ling Wang ,
Ming Zhang ,
Simon A. T. Redfern  and
Zhenyu Zhang
Ling Wang*
Affiliation:
Changsha Institute of Geotectonics, Chinese Academy of Sciences, Changsha, 410013, P.R. China Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge CB2 3EQ, UK
Ming Zhang
Affiliation:
Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge CB2 3EQ, UK
Simon A. T. Redfern
Affiliation:
Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge CB2 3EQ, UK
Zhenyu Zhang
Affiliation:
Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, P. R. China
*
*E-mail address of corresponding author: [email protected]
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Abstract

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The thermally-induced dehydroxylation and transformations of the 2:1 phyllosilicate pyrophyllite have been studied using infrared spectroscopy in the frequency range 350–11000 cm−1 and the temperature range 200–1500°C. The dehydroxylation of pyrophyllite to pyrophyllite dehydroxylate occurs between 500 and 900°C. 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 Si-O-Al 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−1 and their overtones at 7208 and 7234 cm−1 are observed, for the first time, in samples annealed at the temperature range 550–900°C. 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 950–1100°C. Cristobalite is observed in the temperature range 1150–1500°C.

Keywords

CristobaliteDehydroxylationIR SpectroscopyIntermediate PhaseMullitePyrophyllite DehydroxylateTransformation
Type
Research Article
Information
Clays and Clay Minerals , Volume 50 , Issue 2 , April 2002 , pp. 272 - 283
Copyright
Copyright © 2002, The Clay Minerals Society

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