Field emission scanning electron microscopy (FESEM), two-dimensional X-ray diffraction (2D-XRD), and transmission electron microscopy coupled with selected area electron diffraction (TEM-SAED) analyses of the reactant/product textural relationship show that the thermal decomposition of Iceland spar single crystals according to the reaction CaCO_3(s) → CaO_(s) + CO_2(g) is pseudomorphic and topotactic. This reaction begins with the formation of a mesoporous structure made up of up to four sets of oriented rod-shaped CaO nanocrystals on each rhombohedral cleavage face of the calcite pseudomorph. The four sets formed on (101̅4)_calcite display the following topotactic relationships: (1) (12̅10)_calcite//(110)_CaO; (2) (1̅104)_calcite^⊥ (110)_CaO; (3) (1̅018)_calcite//(110)_CaO; and (4) (01̅14)_calcite^⊥(110)_CaO; with [841]_calcite//[11̅0]_CaO in all four cases. At this stage, the reaction mechanism is independent of PCO₂ (i.e., air or high vacuum). Strain accumulation leads to the collapse of the mesoporous structure, resulting in the oriented aggregation of metastable CaO nanocrystals (~5 nm in thickness) that form crystal bundles up to ~1 μm in cross-section. Finally, sintering progresses up to the maximum T reached (1150 °C). Oriented aggregation and sintering (plus associated shrinking) reduce surface area and porosity (from 79.2 to 0.6 m²/g and from 53 to 47%, respectively) by loss of mesopores and growth of micrometer-sized pores. An isoconversional kinetic analysis of non-isothermal thermogravimetric data of the decomposition of calcite in air yields an overall effective activation energy E_α = 176 ± 9 kJ/mol (for α > 0.2), a value which approaches the equilibrium enthalpy for calcite thermal decomposition (177.8 kJ/mol). The overall good kinetic fit with the F₁ model (chemical reaction, first order) is in agreement with a homogeneous transformation. These analytical and kinetic results enable us to propose a novel model for the thermal decomposition of calcite that explains how decarbonation occurs at the atomic scale via a topotactic mechanism, which is independent of the experimental conditions. This new mechanistic model may help reinterpret previous results on the calcite/CaO transformation, having important geological and technological implications.

Amorphous calcium carbonate (ACC) is a key precursor of crystalline CaCO₃ biominerals and biomimetic materials. Despite recent extensive research, its formation and amorphous-to-crystalline transformation are not, however, fully understood. Here we show that hydrated ACC nanoparticles form after spinodal liquid–liquid phase separation and transform via dissolution/(re)precipitation into poorly hydrated and anhydrous ACC nanoparticles that aggregate, forming a range of 1D, 2D and 3D structures. The formation of these structures appears to be achieved by oriented attachment (OA), facilitated by the calcite medium-range order of ACC nanoparticles. Both electron irradiation processes in the TEM and under humid air exposure at room temperature of the latter ACC structures result in pseudomorphs of single crystalline mesostructured calcite. While the high-vacuum/e-beam heating leads to solid-state transformation, the transformation in air occurs via an interface-coupled dissolution/precipitation mechanism. Our results differ significantly from the currently accepted model, which considers that the low T ACC-to-calcite transformation in air and during biomineralization is a solid-state process. These results may help to better understand how calcite biominerals form after ACC and offer the possibility of biomimetically preparing single crystalline calcite structures after ACC by tuning pH₂O at room temperature.

The influence of mineral substrate composition and structure on bacterial calcium carbonate productivity and polymorph selection was studied. Bacterial calcium carbonate precipitation occurred on calcitic (Iceland spar single crystals, marble, and porous limestone) and silicate (glass coverslips, porous sintered glass, and quartz sandstone) substrates following culturing in liquid medium (M-3P) inoculated with different types of bacteria (Myxococcus xanthus,Brevundimonas diminuta, and a carbonatogenic bacterial community isolated from porous calcarenite stone in a historical building) and direct application of sterile M-3P medium to limestone and sandstone with their own bacterial communities. Field emission scanning electron microscopy (FESEM), atomic force microscopy (AFM), powder X-ray diffraction (XRD), and 2-dimensional XRD (2D-XRD) analyses revealed that abundant highly oriented calcite crystals formed homoepitaxially on the calcitic substrates, irrespective of the bacterial type. Conversely, scattered spheroidal vaterite entombing bacterial cells formed on the silicate substrates. These results show that carbonate phase selection is not strain specific and that under equal culture conditions, the substrate type is the overruling factor for calcium carbonate polymorph selection. Furthermore, carbonate productivity is strongly dependent on the mineralogy of the substrate. Calcitic substrates offer a higher affinity for bacterial attachment than silicate substrates, thereby fostering bacterial growth and metabolic activity, resulting in higher production of calcium carbonate cement. Bacterial calcite grows coherently over the calcitic substrate and is therefore more chemically and mechanically stable than metastable vaterite, which formed incoherently on the silicate substrates. The implications of these results for technological applications of bacterial carbonatogenesis, including building stone conservation, are discussed.