The role of clay surfaces in the heterogeneous nucleation of calcite: Molecular dynamics simulations of cluster formation and attachment

• Published in: Chemical geology Vol. 538; p. 119497
• Main Authors: Fodor, Melinda A., Ható, Zoltán, Kristóf, Tamás, Pósfai, Mihály
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 05.04.2020
• Subjects: Calcite; Heterogeneous nucleation; Ionic clusters; Kaolinite; Molecular dynamics; Montmorillonite; Molecular dynamics; Heterogeneous nucleation; Kaolinite; Ionic clusters; Montmorillonite; Calcite
• ISSN: 0009-2541; 1872-6836
• DOI: 10.1016/j.chemgeo.2020.119497

In many environments calcium carbonate minerals precipitate in the presence of clay minerals, and observations suggest that clays, particularly smectites, facilitate carbonate formation. In order to understand the interactions between clay surfaces and carbonate-precipitating solutions, we built model aqueous solutions of various compositions (containing Ca2+, Mg2+ and CO32– ions) between layers of clay minerals (montmorillonite and kaolinite), and performed extensive molecular dynamics simulations. The results were compared with simulations for bulk solutions. Contrary to intuition, ionic clusters formed preferentially in the interlayer solution (instead of on the clay surface). The clusters grew both by the association of individual ions and aggregation, and were adsorbed to the clay surfaces with distinctly different efficiencies in the various systems. Montmorillonite was found to be more efficient than kaolinite in capturing clusters from solution. However, the efficiency of anchoring ionic clusters to the clay surfaces strongly depended on the Na+ concentration of the solution, since Na+ appeared to strongly attach to the surface and thereby block it from clusters in the solution. Montmorillonite (and probably other smectite clays as well) may thus have an important role in certain, primarily freshwater, systems in the localization of ionic clusters on its surface, thereby promoting the nucleation and templated growth of crystalline calcium‑magnesium carbonate minerals. [Display omitted]