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

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 CaCO3(s) -> CaO(s) + CO2(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 (10 (1) over bar4)(calcite) display the following topotactic relationships: (1) (1 (2) over bar 10)(calcite)//((CaO)); (2) ((1) over bar 104)(calcite perpendicular to)(110)(CaO) (3) ((1) over bar 018)(calcite)//(110)(CaO); and (4) (0 (1) over bar 14)(calcite perpendicular to)(110)(CaO); with [841 11 O]c (, 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 (similar to 5 nm in thickness) that form crystal bundles up to similar to 1 mu m in cross-section. Finally, sintering progresses up to the maximum T reached (1150 degrees C). Oriented aggregation and sintering (plus associated shrinking) reduce surface area and porosity (from 79.2 to 0.6 m(2)/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-alpha = 176 +/- 9 kJ/mol (for (alpha > 0.2), a value which approaches the equilibrium enthalpy for calcite thermal decomposition (177.8 kJ/mol). The overall good kinetic fit with the F, 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.