Transformation kinetics of monohydrocalcite to aragonite in aqueous solutions

Abstract

Monohydrocalcite (CaCO₃·H₂O; MHC) is a rare mineral in geological settings. It is metastable with respect to calcite and aragonite. This metastability of MHC is considered to make it a rare mineral in geological settings. Alteration experiments of MHC in aqueous solutions in a closed system were conducted at temperatures between 10 and 50 °C in order to measure its metastability quantitatively. In the present study, monohydrocalcite transformed to aragonite with time. There are two rate-limiting steps in the transformation of monohydrocalcite to aragonite: the nucleation and crystal growth of aragonite. On the other hand, the dissolution of monohydrocalcite is a faster process than the nucleation and crystal growth of aragonite. The amounts of aragonite were calculated from the X-ray diffraction (XRD) intensity to evaluate the rate of both the processes at different temperatures. The induction times for the nucleation of aragonite were estimated to be 2.7 ± 0.9×10³, 5.4 ± 1.8×10³, 3.2 ± 0.4×10⁴, and 7.3 ± 0.4×10⁵ s at 50, 40, 25, and 10 °C, respectively. The conventional rate constants by assuming a zero-order reaction of aragonite crystal growth were estimated to be 1.0 ± 0.3×10⁻⁵, 6.1 ± 0.8×10⁻⁵, 1.0 ± 0.3×10⁻⁵, and 1.6 ± 0.3×10⁻⁶ mmol·s⁻¹ at 10, 25, 40, and 50 °C, respectively. From Arrhenius plots, the apparent activation energies were estimated to be 108.1 kJ·mol⁻¹ and 80.7 kJ·mol⁻¹ for the nucleation and crystal growth steps, respectively.