The dehydration process of gypsum under high pressure |
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Authors: | P Comodi A Kurnosov S Nazzareni L Dubrovinsky |
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Institution: | (1) Dipartimento di Scienze della Terra, Universit? di Perugia, Perugia, Italy;(2) Bayerisches Geoinstitut, Bayreuth University, Bayreuth, Germany |
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Abstract: | The effects of pressure on the dehydration of gypsum materials were investigated up to 633 K and 25 GPa by using Raman spectroscopy
and synchrotron X-ray diffraction with an externally heated diamond anvil cell. At 2.5 GPa, gypsum starts to dehydrate around
428 K, by forming bassanite, CaSO4 hemihydrate, which completely dehydrates to γ-anhydrite at 488 K. All the sulphate modes decrease linearly between 293 and
427 K with temperature coefficients ranging from −0.119 to −0.021 cm−1 K−1, where an abrupt change in the ν3 mode and in the OH-stretching region indicates the beginning of dehydration. Increasing the temperature to 488 K, the OH-stretching
modes completely disappear, marking the complete dehydration and formation of γ-anhydrite. Moreover, the sample changes from
transparent to opaque to transparent again during the dehydration sequence gypsum-bassanite-γ-anhydrite, which irreversibly
transforms to β-anhydrite form at 593 K. These data compared with the dehydration temperature at room pressure indicate that
the dehydration temperature increases with pressure with a ΔP/ΔT slope equal to 230 bar/K. Synchrotron X-ray diffraction experiments show similar values of temperature and pressure for the
first appearance of bassanite. Evidence of phase transition from β-anhydrite structure to the monazite type was observed at
about 2 GPa under cold compression. On the other hand at the same pressure (2 GPa and 633 K), β-anhydrite was found, indicating
a positive Clausis-Clayperon slope of the transition. This transformation is completely reversible as showed by the Raman
spectra on the sample recovered after phase transition. |
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