In Situ Observation of Gypsum-Anhydrite Transition at High Pressure and High Temperature

An in-situ Raman spectroscopic study of gypsum-anhydrite transition under a saturated water condition at high pressure and high temperature is performed using a hydrothermal diamond anvil cell (HDAC). The experimental results show that gypsum dissolvs in water at ambient temperature and above 496 MPa. With increasing temperature, the anhydrite (CaSO₄) phase precipitates at 250–320°C in the pressure range of 1.0–1.5 GPa, indicating that under a saturated water condition, both stable conditions of pressure and temperature and high levels of Ca and SO₄ ion concentrations in aqueous solution are essential for the formation of anhydrite. A linear relationship between the pressure and temperature for the precipitation of anhydrite is established as P(GPa)=0.0068T−0.7126 (250°C≤T≤320°C). Anhydrite remained stable during rapid cooling of the sample chamber, showing that the gypsum-anhydrite transition involving both dissolution and precipitation processes is irreversible at high pressure and high temperature.