High temperature dehydroxylation of muscovite-2M1: a kinetic study by in situ XRPD |
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| Authors: | E Mazzucato G Artioli A Gualtieri |
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| Institution: | (1) Dipartimento di Scienze della Terra, Università di Milano, Via Botticelli 23, I-20133 Milano, Italy Fax: +39-2-70638681 e-mail: artioli@iummix.terra.unimi.it, IT;(2) Dipartimento di Scienze della Terra, Università di Modena, Via S.Eufemia 19, I-41100 Modena, Italy, IT |
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| Abstract: | Muscovite-2M1 shows a major phase transition at about 800°C, which is generally attributed in the literature to the structural dehydroxylation
process, although a number of structural models have been proposed for the dehydroxylated phase, and different transformation
mechanisms have also been put forward. The observed first order transformation involves an increase in the cell volume, and
it is not clear to date how the cell expansion is related to the loss of hydroxyl groups. The phase change has been re-investigated
here by in situ high temperature powder diffraction, both in non-isothermal and isothermal modes, to combine for the first
time the structural and the kinetic interpretation of the transformation. The results unequivocally confirm that the reaction
taking place in the temperature range 700–1000°C is truly a dehydroxylation process, involving the nucleation and growth of
the high temperature dehydroxylated phase, having Al in 5-fold coordination. Structural simulations of the basal peaks of
the powder diffraction patterns indicate that the model originally proposed by Udagawa et al. (1974) for the dehydroxylated
phase correctly describes the high temperature phase. The kinetic analysis of the isothermal data using an Avrami-type model
yields values for the reaction order compatible with a reaction mechanism limited by a monodimensional diffusion step. Apparent
activation energy of the process in vacuum is about 251 kJ/mol. Experiments carried out at temperatures much higher than the
onset temperature of the reaction show that the dehydroxylation reaction overlaps with the reaction of formation of mullite,
the final product in the reaction pathway.
Received: 24 April 1998 / Revised, accepted: 12 October 1998 |
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| Keywords: | Muscovite-2M1 In situ powder diffraction Dehydroxylation Kinetics Structure simulation |
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