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Ab Initio Thermodynamics of Hydrated Calcium Carbonates and Calcium Analogues of Magnesium Carbonates: Implications for Carbonate Crystallization Pathways

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journal contribution
posted on 16.01.2018, 00:00 by Anne M. Chaka
Formation of calcium carbonate and its hydrates are important for a wide variety of geological, biological, and technological concerns. Recent studies have determined that formation of anhydrous crystalline calcite, aragonite, and vaterite can involve a complex series of nonclassical pathways in which the hydrated polymorphs monohydrocalcite (CaCO3·H2O), ikaite (CaCO3·6H2O), and amorphous calcium carbonate (ACC) play key roles and in some instances are stable or metastable endproducts. The stages of nucleation and crystallization along these pathways are not well understood, nor is how what is learned in an aqueous environment transfers to CO2-rich conditions. In this work ab initio thermodynamics based on density-functional theory and experimental chemical potentials for H2O-rich and CO2-rich systems are used to determine the stability of calcium carbonate polymorphs as a function of environmental conditions. In water-saturated supercritical CO2, formation of ikaite and monohydrocalcite are both highly exothermic, yet metastable to calcite, and are therefore likely intermediates upon carbonation of CaO and Ca­(OH)2 according to the Ostwald step rule. Hence low energy nonclassical crystallization pathways that utilize these intermediates are available for calcite formation in CO2-rich environments as well as aqueous systems, particularly in water-saturated systems even though water is less than only 1% by mass. Formation free energies calculated for Ca analogues of nesquehonite (MgCO3·3H2O), lansfordite (MgCO3·5H2O), hydromagnesite (Mg5(CO3)4(OH)2·4H2O), and pokrovskite (Mg2CO3(OH)2) are exothermic in both aqueous and water-saturated scCO2 from 273 to 373 K, but they are always metastable with respect to the observed Ca minerals. Hence they may form prenucleation clusters, transient intermediates, or localized coordination arrangements trapped in hydrated ACC, but will never be observed in nature. The arrangement of CaCO3·6H2O complexes in ikaite is proposed as the structure of prenucleation clusters.