A new approach to kinetics study of the anhydrite crystallization at 373 K using a diamond anvil cell with Raman spectroscopy.

A new approach to the kinetics study of anhydrite (CaSO4) crystallization has been performed in situ using a hydrothermal diamond anvil cell with Raman spectroscopy in the pressure range 896-1322 MPa and a constant temperature of 373 K. Transformed volume fraction X(t) was determined from Raman peak intensity of the sulfate ion in aqueous solution. The transformation-time plots display a sigmoidal shape with time, which indicates that the reaction rate is different at each stage of anhydrite crystallization. At 373 K, the rate constant k increases from 1.14 × 10(-4) s(-1) to 1.86 × 10(-3) s(-1), demonstrating a positive effect of pressure on the overall rate at isothermal condition. We first achieved the molar volume change (ΔVm) equal to -1.82 × 10(-5) m(3)∕mol in the course of anhydrite crystallization through Avrami kinetic theory, showing a process of reduction in volume at high pressure and high temperature. According to the exponent n derived from our experiments, a grain-boundary nucleation and diffusion-controlled growth kinetically dominates the crystallization of anhydrite.