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

Abstract

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 (CaCO 3 •H 2 O), ikaite (CaCO 3 •6H 2 O), 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 CO 2 -rich conditions. In this work ab initio thermodynamics based on density-functional theory and experimental chemical potentials for H 2 O-rich and CO 2 -rich systems are used to determine the stability of calcium carbonate polymorphs as a function of environmental conditions. In water-saturated supercritical CO 2 , 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 CO 2 -rich environments as well as aqueous systems, particularly in watersaturated systems even though water is less than only 1% by mass. Formation free energies calculated for Ca analogues of nesquehonite (MgCO 3 •3H 2 O), lansfordite (MgCO 3 •5H 2 O), hydromagnesite (Mg 5 (CO 3 ) 4 (OH) 2 •4H 2 O), and pokrovskite (Mg 2 CO 3 (OH) 2 ) are exothermic in both aqueous and water-saturated scCO 2 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 CaCO 3 •6H 2 O complexes in ikaite is proposed as the structure of prenucleation clusters.

Integrative analysis of the mineralogical and chemical composition of modern microbialites from ten Mexican lakes: What do we learn about their formation?

“…Interestingly, cell surfaces or exopolymeric substances may as well occur at a lower (Ca 2+ )(CO3 2-) product than the ACC solubility line by . Following an Ostwald step rule, when conditions are thermodynamically favored, the most soluble phases precipitate first since they are kinetically favored (Chaka, 2018). Then they transform to less soluble, more thermodynamically stable phases, either by dissolution-precipitation or solid-state transformation.…”

Development and application of ReaxFF methodology for understanding the chemical dynamics of metal carbonates in aqueous solutions

“…The results of the CPMD simulations are in good agreement with the static density functional theory (DFT) calculations of the bicarbonate stability in aqueous solution and the interconversion of monodendate and bidendate coordination is facile. Chaka 15 performed molecular modeling and ab initio thermodynamics simulations to determine the structure and thermodynamics of the known hydrated CaCO 3 polymorphs and Ca analogues of hydrated MgCO 3 to determine their potential role in forming pre-nucleation clusters, transient intermediates, or local coordination arrangements of amorphous CaCO 3 in both aqueous and CO 2 rich environments. Ab-initio molecular dynamics (AIMD) simulations are computationally expensive limiting them to system sizes of a few nanometers and simulation times of tens to hundreds of picoseconds.…”

Investigation on the stability, electronic, optical, and mechanical properties of novel calcium carbonate hydrates via first‐principles calculations

“…value [17] 10.554 10.554 7.564 Exp. value [13] CaCO 3 ÁxH 2 O (x = 1/2, 1 and 6) are very flat, while the conduction band has some curvatures, indicating that the effective masses of holes are much larger than the electron masses. In order to grasp and describe the structural stability, the TDOS and PDOS are discussed here and shown in Figure 3B,D [8] The calculated charge densities of calcium carbonate hydrates are presented in (100) planes and shown in Figure 4.…”

“…It was also regarded as an adsorbent to remove phosphate from solution and the mechanism of phosphate on monohydrocalcite including ionic strengths, reaction times, temperatures, etc., which were discussed elaborately by Yagi et al In addition, the structures of both CaCO 3 ·6H 2 O and CaCO 3 ·1H 2 O had been studied by Demichelis et al through the PBE0 level of theory . Chaka had studied the thermodynamics of hydrated calcium carbonates and calcium analogues of magnesium carbonates by ab initio. In order to determine the stability of calcium carbonate polymorphs, the incorporation of ab initio thermodynamics based on density functional theory and experimental chemical potentials for H 2 O‐rich and CO 2 ‐rich systems were used.…”