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Direct formation of the -CaSO₄ phase in dehydration process of gypsum: In situ FTIR study

¹ National Geophysical Research Institute, Hyderabad 500 007, India
² School of Physics, University of Hyderabad, Hyderabad 500 046, India

Correspondence: ^* E-mail: psrprasad{at}ngri.res.in

The dehydration mechanism of natural single crystals of gypsum was investigated in the temperature range 300–430 K by in situ infrared (FTIR) spectroscopy. The thermal evolution of the second-order modes of H₂O and SO₄ groups in gypsum, in the wavenumber range 4850–5450 cm^–1 and 2050–2300 cm^–1 respectively, were used to probe the dehydration and rehydration sequence. A total disappearance of the combination modes of H₂O and the replacement of four SO₄^–2 bands (2245, 2200, 2133, and 2117 cm^–1) observed at room temperature by three bands (2236, 2163, and 2131 cm^–1) observed at 390 K indicates the direct formation of -CaSO₄ upon heating. Upon cooling water re-enters into the -CaSO₄ structure at around 363 K to form bassanite. This observation, that the dehydration of gypsum directly yields -CaSO₄ (anhydrite) without the intermediate formation of hemi-hydrate (bassanite), is further corroborated by the dehydration behavior of bassanite. The second-order SO₄ modes of bassanite observed around 2218, 2136, and 2096 cm^–1 were replaced with the bands of -CaSO₄ at about 378 K upon heating.

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