The crystallization kinetics of gamma-anhydrite was investigated in the temperature and pressure ranges of 373-473 K and 1094-1903 MPa using a hydrothermal diamond anvil cell (HDAC) and Raman spectroscopy. A calcium sulfate solution was formed by dissolving gypsum in water at high pressure, and gamma-anhydrite crystallized due to the increasing temperature. The relationship among the reaction rate, k, the temperature, T, and the pressure, P was established as k = -1.75 x 10(-3) + 1.83 x 10(-6)P + 357 x 10(-7)T, with an R-2 value of 0.943, of which the applicable range is 373 K <= T <= 473 K and 1196 MPa <= P <= 1903 MPa. An elevation of T or P will accelerate the crystallization rate of gamma-anhydrite. The time exponent n obtained between 0.96 and 1.29 indicates that the process of crystallization should be controlled by instant nucleation on the grain boundary and diffusion controlled growth. In the process of crystallization, the volume is reduced because of the decrease of pressure, and the volume change Delta V is equal to -6.05 x 10(-6) m(3)/mol. The calculated activation energy E-a is 10.7 kJ/mal, and the pre-exponential factor A is 2.27 x 10(-2) s(-1). (C) 2014 Elsevier B.V. All rights reserved