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High Pressure Research
An International Journal
Volume 17, 2000 - Issue 1
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Original Articles

Pressure-induced phase transitions in gypsum

, , &
Pages 57-75 | Received 05 Apr 1999, Accepted 06 Aug 1999, Published online: 19 Aug 2006
 

Abstract

We report high-pressure Raman scattering spectroscopy and energy dispersive X-ray diffraction investigations on gypsum, CaSO4 · 2H2O, at room temperature in a diamond cell. With increasing pressure, measurements indicate that CaSO4 · 2H2O undergoes two stages of crystalline-state phase transitions at 5 and 9 GPa, and then converts to a disordered phase above 11 GPa. The structures of the three high-pressure phases of gypsum have not been determined yet. These phases are tentatively named as “post-gypsum-I” (PG-I), “post-gypsum-II” (PG-II) and “disordered” according to the sequence of their appearance with pressure.

Gypsum shows anisotropic compressibility along three crystallographic axes with b > c > a below 5 GPa. The difference in the behavior of the two OH stretching modes in gypsum is attributed to the different reduction rate in the hydrogen bonding distances by the anisotropic axial compressibility.

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