Direct Observation of Microparticle Porosity Changes in Solid-State Vaterite to Calcite Transformation by Coherent X‑ray Diffraction Imaging

The simplest route to synthesize porous calcium carbonate in large quantity is to mix concentrated aqueous solutions containing Ca²⁺ and CO₃^2– ions. The formed vaterite microspheres have a porous structure, but are not thermodynamically stable. Heating above 350 °C induces a solid-state transformation of vaterite into the most stable phase, calcite, while maintaining an unusual spheroidal morphology. Here, by using three-dimensional coherent X-ray diffraction imaging, the morphological evolution associated with the thermally induced phase transition is studied. We observe that despite an overall similar pore volume, the pore geometry differs markedly before and after annealing. Before annealing, the microspheres display elongated and nanometer sized pores, while after annealing they exhibit large and open pores. During transition, the specific surface area decreases from 7 m²/g for vaterite to 3 m²/g for calcite. The general trend resulting from 3D observations is that the solid state phase transition is not only governed by the decrease of the Gibbs bulk free energy change (ΔG_bulk ≈ −3 kJ/mol) but is largely influenced by the surface energy change (ΔG_surf ≈ −0.1 kJ/mol per m²/g lost). The porous calcite microspheres produced by this facile two-step process may have potential use as low-density filler in paint, paper, pharmaceutical, and plastic industries.