Thermal expansion and dehydroxylation of phengite micas |
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Authors: | Mauro Gemmi Marco Merlini Alessandro Pavese Nadia Curetti |
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Institution: | (1) Dipartimento di Scienze della Terra “A. Desio”, Università degli Studi di Milano, via Botticelli, 23, 20133 Milan, Italy;(2) ESRF, 6 rue Jules Horowitz, BP220, 38043 Grenoble, France;(3) National Research Council, IDPA, Section of Milan, Via Botticelli 23, 20133 Milan, Italy;(4) Dipartimento Scienze Mineralogiche e Petrologiche, Università degli Studi di Torino, Via Valperga Caluso 35, 10025 Torino, Italy |
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Abstract: | Phengite samples (2M
1 and 3T politypes) and a synthetic end-member muscovite specimen were studied by in situ high-temperature synchrotron radiation X-ray
diffraction. The measured volume thermal expansion of 2M
1 phengite (<α
V> ≈ 36.6 × 10−6 K−1) was systematically greater than <α
V> of the 3T polytype (≈33.3 × 10−6 K−1). A positive linear correlation between the average thermal expansion on (001) plane and the mean tetrahedral rotation angle
at ambient condition is proposed on the ground of new measurements and literature data. Dehydroxylation processes were observed
in 2M
1, starting at 1,000 K in 3T at 800 and 945 K in synthetic muscovite. Rietveld refinements allowed a determination of structural variations upon heating
of phengite samples and their dehydroxylate phases. The phengite structure expands by regularizing the tetrahedral sheet and
by reducing the bond length differences between the outer and inner coordination shell of the interlayer site. The dehydroxylate
phase derived from 2M
1 is characterized by fivefold polyhedra in the low temperature form as a consequence of two OH groups reacting to form H2O + O (residual). The dehydroxylate exhibits an increase of the cation–cation distances along the M–Or–M bonds with respect
to low-temperature phengite structures. For the 3T phase, we were unable to achieve completion of dehydroxylation. The refined structural model of the dehydroxylate phase shows
two hydroxyl sites, but at a short distance from one another. This result suggests that the dehydroxylation reaction did not
proceed to completion.
Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users. |
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Keywords: | Mica Phengite Thermal expansion Dehydroxylation Metamorphism |
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