New Insights into the Transition Temperature of Gypsum to Anhydrite Based on DFT Calculations of Thermodynamic Properties

Understanding the thermodynamic properties of minerals is crucial for elucidating the physicochemical conditions of their formation and mutual conversion, which support geological exploration and mineral resource development. The dehydration of gypsum to form anhydrite apparently occurs readily in nature. There have been a number of studies of the equilibrium temperature of the dehydration reaction, suggesting that the transition temperature of gypsum to anhydrite is mostly between 40 and 60 °C. The phase transformation temperature of gypsum to anhydrite was investigated by means of density functional theory (DFT) calculations of thermodynamic properties in this paper. The calculated lattice parameters of gypsum and anhydrite are in good agreement with the experimentally reported data. The thermodynamic properties of gypsum and anhydrite, including isobaric heat capacity, entropy, enthalpy, and Gibbs free energy functions, were then computed by performing phonon calculations as a function of temperature in the range of 0-1000 K. The results showed that (1) the computed volume exhibits minimal error, approximately 0.49% for gypsum and 0.27% for anhydrite, compared to experimental data; (2) the calculated isobaric heat capacity function of gypsum (DH) and anhydrite (AH) are <i>C</i> _p(<i>T</i>, DH) = 191.17 + 0.38<i>T</i> + 3.76 × 10⁴ <i>T</i> ^-2 - 1.38 × 10³ <i>T</i> ^-0.5 - 2.13 × 10^-4 <i>T</i> ² and <i>C</i> _p(<i>T</i>, AH) = 107.90 + 0.21<i>T</i> + 2.10 × 10⁴ <i>T</i> ^-2 - 0.77 × 10³ <i>T</i> ^-0.5 - 1.24 × 10^-4 <i>T</i> ², respectively; (3) at 298.15K, the calculated values for the isobaric specific heat and entropy of gypsum were 206.21 and 206.32 J·mol^-1·K^-1, respectively; (4) at 298.15 K, the calculated values for the isobaric specific heat and entropy of anhydrite were 114.53 and 116.29 J·mol^-1·K^-1, respectively; and (5) the calculated value for the free energy of formation of gypsum and anhydrite at 298.15K are -1849.72 KJ·mol^-1 and -1350.79 KJ·mol^-1, respectively. The Gibbs energy variation equation with temperature of the dehydration process of gypsum to anhydrite was obtained by calculations: Δ_RG-(<i>T</i>) = -17622.08 + 282.41<i>T</i> - 67.53T ln T⁺0.085<i>T</i> ² + 0.83 × 10⁴ <i>T</i> ^-1 + 2.44 × 10³ <i>T</i> ^0.5 - 1.50 × 10^-5 <i>T</i> ³. It is shown that careful assessment of Gibbs energy of the dehydration process based on DFT calculations yields a transition temperature of 46.58 °C. This research provides valuable insights into the thermodynamic properties and phase transition of gypsum to anhydrite, highlighting the efficacy of theoretical methods as predictive tools for analogous cases.