Thermal decomposition of calcite; mechanisms of formation and textural evolution of CaO nanocrystals

• Published in: The American mineralogist Vol. 94; no. 4; pp. 578 - 593
• Main Authors: Rodriguez-Navarro, Carlos, Ruiz-Agudo, Encarnacion, Luque, Ana, Rodriguez-Navarro, Alejandro B, Ortega-Huertas, Miguel
• Format: Journal Article
• Language: English
• Published: Washington Mineralogical Society of America 01.04.2009; De Gruyter; Walter de Gruyter GmbH
• Subjects: Calcite; CaO nanocrystals; carbonates; Chemical reactions; Crystal structure; Crystals; Decomposition; Diffraction; heating; kinetics; lime; Mineralogy; Minerals; Nanocrystals; nanoparticles; nonsilicates; orientation; oriented aggregation; Porosity; Scanning electron microscopy; SEM data; TEM-SAED; textures; Thermal decomposition; thermal properties; topotactic; transformations; Transmission electron microscopy; X-ray diffraction; X-ray diffraction data
• ISSN: 0003-004X; 1945-3027
• DOI: 10.2138/am.2009.3021

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 (101̄4)calcite display the following topotactic relationships: (1) (12̄10)calcite//(110)CaO; (2) (1̄104)calciteperpendicular(110)CaO; (3) (1̄018)calcite//(110)CaO; and (4) (01̄14)calciteperpendicular(110)CaO; with [841]calcite//[11̄0]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 (approximately 5 nm in thickness) that form crystal bundles up to approximately 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 Eα=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.