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Thermal decomposition of calcite: Mechanisms of formation and textural evolution of CaO nanocrystals

Departamento de Mineralogia y Petrologia, Universidad de Granada, Fuentenueva s/n, 18002, Granada, Spain

Correspondence: ^* E-mail: carlosrn{at}ugr.es

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 (104)_calcite display the following topotactic relationships: (1) (110)_calcite//(110)_CaO; (2) (104)_calcite (110)_CaO; (3) (018)_calcite//(110)_CaO; and (4) (014)_calcite(110)_CaO; with [841]_calcite//[10]_CaO in all four cases. At this stage, the reaction mechanism is independent of P_CO2 (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 = 176 ± 9 kJ/mol (for > 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.

Key Words: Calcite • lime • thermal decomposition • CaO nanocrystals • TEM-SAED • oriented aggregation • kinetics • topotactic