The kinetics and mechanisms of amorphous calcium carbonate (ACC) crystallization to calcite, viavaterite.

TLDR

The study presents a model to calculate the degree of calcite crystallization from ACC at environmentally relevant temperatures (7.5–40 °C). Using synchrotron‑based in situ ED‑XRD, high‑resolution electron microscopy, ex‑situ XRD, and IR spectroscopy at 7.5–25 °C, the authors show that ACC crystallizes to calcite via a two‑stage process: rapid dehydration to vaterite followed by a slower dissolution‑and‑reprecipitation conversion to calcite governed by calcite surface area. The conversion from vaterite to calcite is about ten times slower than the initial dehydration step, with activation energies of 73 ± 10 kJ mol⁻¹ for nucleation and 66 ± 2 kJ mol⁻¹ for crystallization.

Abstract

The kinetics and mechanisms of nanoparticulate amorphous calcium carbonate (ACC) crystallization to calcite, viavaterite, were studied at a range of environmentally relevant temperatures (7.5–25 °C) using synchrotron-based in situ time-resolved Energy Dispersive X-ray Diffraction (ED-XRD) in conjunction with high-resolution electron microscopy, ex situX-ray diffraction and infrared spectroscopy. The crystallization process occurs in two stages; firstly, the particles of ACC rapidly dehydrate and crystallize to form individual particles of vaterite; secondly, the vaterite transforms to calcitevia a dissolution and reprecipitation mechanism with the reaction rate controlled by the surface area of calcite. The second stage of the reaction is approximately 10 times slower than the first. Activation energies of calcite nucleation and crystallization are 73 ± 10 and 66 ± 2 kJ mol−1, respectively. A model to calculate the degree of calcite crystallization from ACC at environmentally relevant temperatures (7.5–40 °C) is also presented.