Structure and Dynamics of Water at Step Edges on the Calcite {101̅4} Surface

The behavior of liquid water around obtuse and acute steps parallel to on the {101̅4} cleavage surface of calcite has been investigated by means of classical molecular dynamics simulations performed with a force-field fitted against thermodynamic properties. Water density maps, radial distribution functions, and water average residence times have been investigated. The structure and dynamics of the first two ordered hydration layers, which have been previously observed for the basal surface of calcite, are found to be disrupted by the presence of the steps over a range of five molecular rows either side of the step edge. Calcium sites along the step top edge can coordinate up to three water molecules, as compared with just the single water that can be adsorbed per calcium ion on the flat surface. Water residence times at calcium sites in the vicinity of the step span greater than 2 orders of magnitude, from tenths to several tens of ns, as compared to 2 and 0.2 ns for the flat surface and a calcium ion in aqueous solution, respectively. The occurrence of waters with long residence times at the step corners points toward the possible role of step dehydration as a rate-limiting factor in calcite crystal growth. Indeed, the different distributions of slow and fast waters along the obtuse and acute steps appear to correlate with the different rates of growth observed for the two types of steps

Uncovering the Atomistic Mechanism for Calcite Step Growth.

Determining a complete atomic-level picture of how minerals grow from aqueous solution remains a challenge as macroscopic rates can be a convolution of many reactions. For the case of calcite (CaCO3 ) in water, computer simulations have been used to map the complex thermodynamic landscape leading to growth of the two distinct steps, acute and obtuse, on the basal surface. The carbonate ion is found to preferentially adsorb at the upper edge of acute steps while Ca(2+) only adsorbs after CO3(2-) . In contrast to the conventional picture, ion pairs prefer to bind at the upper edge of the step with only one ion, at most, coordinated to the step and lower terrace. Migration of the first carbonate ion to a growth site is found to be rate-limiting for kink nucleation, with this process having a lower activation energy on the obtuse step

DBNet, a tool to convert Dynamic Fault Trees into Dynamic Bayesian Networks

The unreliability evaluation of a system including dependencies involving the state of components or the failure events, can be performed by modelling the system as a Dynamic Fault Tree (DFT). The combinatorial technique used to solve standard Fault Trees is not suitable for the analysis of a DFT. The conversion into a Dynamic Bayesian Network (DBN) is a way to analyze a DFT. This paper presents a software tool allowing the automatic analysis of a DFTexploiting its conversion to a DBN. First, the architecture of the tool is described, together with the rules implemented in the tool, to convert dynamic gates in DBNs. Then, the tool is tested on a case of system: its DFT model and the corresponding DBN are provided and analyzed by means of the tool. The obtained unreliability results are compared with those returned by other tools, in order to verify their correctness. Moreover, the use of DBNs allows to compute further results on the model, such as diagnostic and sensitivity indices

Probing the Multiple Structures of Vaterite through Combined Computational and Experimental Raman Spectroscopy

First-principles Raman spectra have been computed for several new vaterite structural models that have been recently proposed, and compared with spectra recorded on a set of biogenic, geological and synthetic samples. This set includes new measurements collected on Herdamania momus spicules (Great Barrier Reef, Queensland, Australia), which are known to have purity and crystallinity that are higher than for other biogenic samples. Overall, due to the close structural connection between the various models, the computed Raman spectra are found to be broadly similar. However, the spectra obtained for the two most stable models (monoclinic C2 and trigonal P3221, corresponding to two different polytypes of vaterite) exhibit features that are in excellent agreement with the experimental spectra, whereas the other theoretical structures show minor peaks that are not observed experimentally. When comparing the spectra for the two lowest energy structural models (C2 and P3221), the differences are too small to discriminate between these candidates. The Raman spectrum of Herdamania momus is of higher quality with respect to spectra obtained in previous studies on other biogenic samples. However, there is no significant and systematic difference with respect to samples of geological and synthetic origin