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American Mineralogist

Journal of Earth and Planetary Materials

Ed. by Putirka, Keith / Swainson, Ian


IMPACT FACTOR 2015: 1.918
5-year IMPACT FACTOR: 2.196
Rank 9 out of 29 in category Mineralogy and 37 out of 81 in category Geochemistry & Geophysics in the 2015 Thomson Reuters Journal Citation Report/Science Edition

SCImago Journal Rank (SJR) 2015: 1.185
Source Normalized Impact per Paper (SNIP) 2015: 0.979
Impact per Publication (IPP) 2015: 1.929

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ISSN
1945-3027
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Thermal decomposition of calcite: Mechanisms of formation and textural evolution of CaO nanocrystals

1 / Encarnacion Ruiz-Agudo1 / Ana Luque1 / Alejandro B. Rodriguez-Navarro1 / Miguel Ortega-Huertas1

1Departamento de Mineralogia y Petrologia, Universidad de Granada, Fuentenueva s/n, 18002, Granada, Spain

Citation Information: American Mineralogist. Volume 94, Issue 4, Pages 578–593, ISSN (Online) 1945-3027, ISSN (Print) 0003-004X, DOI: https://doi.org/10.2138/am.2009.3021, April 2015

Publication History

Received:
2008-05-16
Accepted:
2008-11-20
Published Online:
2015-04-01

Abstract

Field emission scanning electron microscopy (FESEM), two-dimensional X-ray diffraction (2DXRD), and transmission electron microscopy coupled with selected area electron diffraction (TEMSAED) 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 (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 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.

Keywords : Calcite; lime; thermal decomposition; CaO nanocrystals; TEM-SAED; oriented aggregation; kinetics; topotactic

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