In-situ Raman spectroscopic study of phase transition at high temperature and high pressure.

In-situ Raman spectroscopy is employed to investigate the dehydration of gypsum in presence of water under hydrothermal diamond anvil cell ( HDAC). At high temperature and high pressure, dehydration of gypsum shows no existence of intermediate phases of bassanite, and this is controversial to the generally accepted gypsum -> bassanite -> anhydrite transition sequence at ambient pressure. Additionally, the phase transition (gypsum -> anhydrite) equation of Pressure-Temperature can be determined to be P-(MPa) 19.56 (.) T(degrees C) - 2926. 5