Calcium carbonate is an abundant substance that can be created in several mineral forms by the reaction of dissolved carbon dioxide in water with calcium ions. Through biomineralization, organisms can harness and control this process to form various functional materials that can act as anything from shells through to lenses. The early stages of calcium carbonate formation have recently attracted attention as stable prenucleation clusters have been observed, contrary to classical models. Here we show, using computer simulations combined with the analysis of experimental data, that these mineral clusters are made of an ionic polymer, composed of alternating calcium and carbonate ions, with a dynamic topology consisting of chains, branches and rings. The existence of a disordered, flexible and strongly hydrated precursor provides a basis for explaining the formation of other liquid-like amorphous states of calcium carbonate, in addition to the non-classical behaviour during growth of amorphous calcium carbonate.

In recent years atomistic simulations have become increasingly important in providing molecular insight to complement experiments. Even for the seemingly simple case of ion-pair formation a detailed atomistic picture of the structure and relative stability of the contact, solvent-shared and solvent-separated ion-pairs can only be readily achieved by computer simulation. Here a new force field parameterization for the alkaline-earth carbonate interactions in water has been developed by fitting against experimental thermodynamic and structural data. We

Organic additives are well known to influence the nucleation and growth of minerals. A combination of experimental and theoretical methods has been used to probe how three simple additives, containing varying numbers of carboxylate groups, influence the early stages of the growth of calcium carbonate. Computationally, the free energy landscape has been examined for each additive binding to Ca 2+ , the calcium carbonate ion pair, the surface of an amorphous calcium carbonate nanoparticle, and the basal plane of calcite. The different influence of the three organic ligands on the early stages of growth of calcium carbonate observed experimentally can be rationalised in terms of the degree of association of each anion with the species present prior to, and immediately after nucleation.

ABSTRACT:The performance of eleven DFT functionals in describing the equilibrium structure and the vibrational spectra at the point of pyrope (Mg 3 Al 2 Si 3 O 12 ), forsterite (α-Mg 2 SiO 4 ), α-quartz (α-SiO 2 ) and corundum (α-Al 2 O 3 ) is discussed. The four systems, for which accurate experimental data are available, are here used as a representative sample of the large aluminosilicates family. Calculations were performed with the periodic ab initio CRYSTAL code by using all-electron Gaussian-type basis sets. All the functionals here considered provide reasonable structural predictions, the hybrid PBE0 giving the least deviation from the experimental unit cell volumes (from −0.3% to +0.6%). At the other extreme, SVWN and SPWLSD ( −3%) and PBE and PW91 ( +3%) provide the largest volume under-and over-estimation, respectively. Vibrational frequencies are more accurate when computed with hybrid functionals, with the best performance provided by B3LYP and WC1LYP (mean absolute differences with respect to experiments evaluated on a set of 134 vibrational frequencies,| |

CRYSTAL is a periodic ab initio code that uses a Gaussian-type basis set to express crystalline orbitals (i.e. Bloch functions). The use of atom-centred basis functions allows treating 3D (crystals), 2D (slabs), 1D (polymers) as well as 0D (molecules) systems on the same grounds. In turn, all-electron calculations are inherently permitted along with pseudopotential strategies. A variety of density functionals is implemented, including global and range-separated hybrids of various nature and, as an extreme case, Hartree-Fock (HF). The cost for HF or hybrids is only about 3-5 times larger than when using the local density approximation (LDA) or the generalized gradient approximation (GGA). Symmetry is fully exploited at all steps of the calculation. Many tools are available to modify the structure as given in input and simplify the construction of complicated objects, such as slabs, nanotubes, molecules, clusters. Many tensorial properties can be evaluated by using a single input keyword: elastic, piezoelectric, photoelastic, dielectric, as well as first and second hyperpolarizabilies, etc. The calculation of infrared and Raman spectra is available, and the intensities are computed analytically. Automated tools are available for the generation of the relevant configurations of solid solutions and/or disordered systems. Three versions of the code exist, serial, parallel and massive-parallel. In the second one the most relevant matrices are duplicated on each core, whereas in the third one the Fock matrix is distributed for diagonalization. All the relevant vectors are dynamically allocated and deallocated after use, making the code very agile. CRYSTAL can be used efficiently on high performance computing machines up to thousands of cores.

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For many years the nature of the disordered structure of vaterite, a calcium carbonate polymorph that plays a significant role in biomineralisation, has been debated. In the past year, two independent studies (Angew. Chem., Int. Ed., 51, 7041; CrystEngComm, 14, 44) have proposed new, yet different models on the basis of electron diffraction experiments and ab initio calculations, respectively. Here, it is shown that there are at least three effectively isoenergetic models that are equally likely to describe the vaterite structure at room temperature. These distinct models, each consisting of multiple structures, can explain the disorder of vaterite in terms of different orientations of the carbonate anions, multiple stacking sequences of the carbonate layers, and possible chiral forms. Hence, vaterite is not a single “disordered” structure but should instead be considered as a combination of different forms, each of which can exhibit rapid interchange between multiple structures that exhibit similar average properties.

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