Cluster Models for Calcite Surfaces:  Ab Initio Quantum Chemical Studies

Abstract

Cluster models for the calcite (CaCO 3 ) (001) surface were calculated by the ab initio Hartree-Fock method with the purpose of developing a model for studies of local surface reactions such as adsorption. The models were evaluated in terms of their density of states and charge distribution. The termination of a naked cluster model is important, and two methods of stabilizing the models were considered. An array of point charges, a method of stabilizing ionic models, was tested and found to work poorly. An alternative method was then tested in which cluster models of semi-ionic surfaces were stabilized by surrounding the cluster with water molecules. The water molecules around the cluster increased the stability of the model substantially and also made the model more realistic. An estimate for the energy of adsorption of water to the (001) surface of calcite is presented.

Onset of Amorphous Structure in CaCO3: Geometric and Electronic Structures of (CaCO3) n (n = 2–7) Clusters by Ab Initio Calculations

“…In the dimers and trimers, smaller peaks appear centered at about 1,000 cm -1 -due to pyramidalization of the CO 3 2-group-and 400 cm -1 (CaÁÁÁO stretching). Although these peaks show increased intensity in the tetramers and larger clusters, one should not overinterpret theoretical spectra of this level of theory.…”

“…Navrotsky discussed mainly thermodynamic and kinetic phenomena that nanoparticles could explain, but the structure of the nanoparticles was not treated. Previously, Ruuska et al [3] studied (CaCO 3 ) n (n = 4, 10,13,22,25) clusters as models for calcite surfaces at the HF/3-21G level of theory. As they were interested in surface models for adsorption processes, they did not optimize the structures of the clusters.…”

“…In addition, the observation of the appearance of an amorphous calcium carbonate (ACC) phase upon deposition of CaCO 3 interacting with a polypeptide [7], named polymer-induced liquid precursor, raises the possibility of its use as an easily-mouldable precursor before the onset of crystallization. Molecular dynamics calculations in water by Quigley and Rodger [8], show that nanoparticles of several hundred units of CaCO 3 show amorphous structure, without the intervention of any polymers to induce aggregation. Given that generating an amorphous structure is much less sterically stringent than fitting in a crystal lattice, we think the smallest clusters of calcium carbonate can participate in aggregation-based growth of ACC.…”

Atomistic models of carbonate minerals: Bulk and surface structures, defects, and diffusion

“…Application of the carbonate potential model to surface structures and surface energy calculations, and in defect formation and migration energy analysis provides a sound theoretical basis for evaluating numerous geochemical and materials processes. Although recent quantum mechanical studies [8,56,57] may provide a more accurate description of the electronic structure of carbonate solids, the computational cost of Hartree -Fock or density functional methods currently prohibit their routine use in modeling complex bulk or surface configurations involving many hundreds of atoms where many states may have to be examined.…”

Isotopic fractionations associated with phosphoric acid digestion of carbonate minerals: Insights from first-principles theoretical modeling and clumped isotope measurements

“…We have tried to develop an understanding of these second-order effects by constructing a 'cluster model' that describes the dissociation of H 2 CO 3 attached to XCO 3 clusters, which simulates the situation where H 2 CO 3 is influenced by bonds on the surface of adjacent, un-reacted carbonate. Similar ab initio cluster models have been used previously to investigate local properties and reactions of carbonate surfaces, such as hydration (Mao and Siders, 1997) and adsorption (Ruuska et al, 1999). In this study, we limit our model to small clusters comprising only two XCO 3 units, i.e.…”