Effect of Water Activity on Reaction Kinetics and Intergranular Transport: Insights from the Ca(OH) 2 + MgCO 3 → CaCO 3 + Mg(OH) 2 Reaction at 1·8 GPa

The kinetics of the irreversible reaction Ca(OH)2 + MgCO3 → CaCO3 + Mg(OH)2 were investigated at high pressures and temperatures relevant to metamorphic petrology, using both in situ synchrotron X-ray diffraction and post-mortem analysis of reaction rim growth on recovered samples. Reaction kinetics are found to strongly depend on water content; comparable bulk-reaction kinetics are obtained under water-saturated (excess water, c. 10 wt %) and under intermediate (0·1–1 wt % water) conditions when temperature is increased by c. 300 K. In contrast, similar reaction kinetics were observed at ∼673 K and 823 K between intermediate and dry experiments, respectively, where dry refers to a set of experiments with water activity below 1·0 (no free water), as buffered by the CaO–Ca(OH)2 assemblage. Given the activation energies at play, this gap—corresponding to the loss of no more than 1 wt % of water by the assemblage—leads to a difference of several orders of magnitude in reaction kinetics at a given temperature. Further analysis, at the microscopic scale, of the intermediate and dry condition samples, shows that intergranular transport of calcium controls the reaction progress. Grain boundary diffusivities could be retrieved from the classic treatment of reaction rim growth rate. In turn, once modeled, this rate was used to fit the bulk kinetic data derived from X-ray powder diffraction, offering an alternative means to derive calcium diffusivity data. Based on a comparison with effective grain boundary data for Ca and Mg from the literature, it is inferred that both dry and intermediate datasets are consistent with a water-saturated intergranular medium with different levels of connectivity. The very high diffusivity of Ca in the CaCO3 + Mg(OH)2 rims, in comparison with that of Mg in enstatite rims found by earlier workers, emphasizes the prominent role of the interactions between diffusing species and mineral surfaces in diffusion kinetics. Furthermore, we show that the addition of water is likely to change the relative diffusivity of Mg and Ca in carbonate aggregates. From a qualitative point of view, we confirm, in a carbonate-bearing system, that small water content variations within the 0–1 wt % range have tremendous effects on both intergranular transport mechanisms and kinetics. We also propose that the water content dependent diffusivity of major species (Mg, Ca) in low-porosity metamorphic rocks is strongly dependent on the interaction between diffusing species and mineral surfaces. This parameter, which will vary from one rock-type to another, needs also to considered when extrapolating (P, T, t, xH2O) laboratory diffusion data to metamorphic processes

Evidence of a Louse-Borne Outbreak Involving Typhus in Douai, 1710-1712 during the War of Spanish Succession

Background: The new field of paleomicrobiology allows past outbreaks to be identified by testing dental pulp of human remains with PCR. Methods: We identified a mass grave in Douai, France dating from the early XVIII th century. This city was besieged during the European war of Spanish succession. We tested dental pulp from 1192 teeth (including 40 from Douai) by quantitative PCR (qPCR) for R. prowazekii and B. quintana. We also used ultra-sensitive suicide PCR to detect R. prowazekii and genotyped positive samples. Results and Discussion: In the Douai remains, we identified one case of B. quintana infection (by qPCR) and R. prowazekii (by suicide PCR) in 6/21 individuals (29%). The R. prowazekii was genotype B, a genotype previously found in a Spanish isolate obtained in the first part of the XX th century. Conclusion: Louse-borne outbreaks were raging during the XVIII th century; our results support the hypothesis that typhus was imported into Europe by Spanish soldiers from America

Recoupling flow and chemistry in variably saturated reactive transport modelling - An algorithm to accurately couple the feedback of chemistry on water consumption, variable porosity and flow

International audienceMost reactive transport codes and algorithms decouple the flow from the reactive transport calculations. In some cases, geochemical reactions lead to significant modifications of porosity or water-content, which can have an impact on the flow. The flow problem is based on the continuity equation and is described in terms of pressure. However, most reactive transport codes do not model the pressure-evolution through mineral reactions. The aim of this study is to recouple the reactive transport and the flow, by providing an accurate description of the evolution of both the porosity and the water in the reactive system. We discuss a formulation of the geochemical solver, based on a mole-conservation, which allows an accurate computation of the volume and masses of all phases. This allows for a water and pore volume computation at the scale of the REV which can impact the fluid-pressure, hence the flow. Additionally, solving the geochemical equilibrium in terms of moles instead of concentrations is more accurate for problems involving important mineral reactions. Finally, this method is suited to saturated, unsaturated and two-phase flow. This method is easy to implement and can be used in different reactive transport simulators, regardless of their numerical approaches. We also test the numerical efficiency of this approach and apply it to fully-coupled problems involving variable porosity, variable saturation, water production/consumption

Evidence for glutamatergic tectotectal neurons in the cat superior colliculus: a comparison with GABAergic tectotectal neurons.

The tectotectal commissural pathway is commonly regarded as responsible for the reciprocal inhibition that takes place between the two superior colliculi (SC). Although this hypothesis has received strong support from electrophysiological studies, more recent investigations have suggested that some collicular cells, e.g. fixation neurons, may establish excitatory connections with cells in the contralateral SC through the collicular commissure. The goal of the present study was to seek immunohistochemical evidence for glutamatergic tectotectal cells in the cat SC by using a double-labelling technique. Tectotectal cells were retrogradely labelled with wheat germ agglutinin (WGA) -horseradish peroxidase (HRP) coupled to colloidal gold injected in the contralateral SC, and neurons containing glutamate or gamma-aminobutyric acid (GABA) were then identified with immunohistochemical techniques. The present study provides evidence that, in the cat SC, equal numbers of tectotectal cells are immunopositive to glutamate and GABA, suggesting that the tectotectal pathway may consist of two distinct functional components. The finding that an equal number of tectotectal cells are GABAergic and glutamatergic is somewhat surprising as electrophysiological studies have invariantly indicated that the inhibitory component of the tectotectal projection predominates. Another striking feature of the GABAergic and glutamatergic tectotectal cell populations is their identical topographic distribution in the SC. These results suggest that not only cells in the rostral fixation zone establish excitatory connections with the contralateral SC. Tectotectal projections could be potentially important to shape the spatial pattern of saccade-related activity that may occur simultaneously in the two SC during vertical and oblique orienting movements

Modélisation chimio-mécanique du gonflement et de la fissuration de matériaux cimentaires soumis à une attaque sulfatique externe modérée

International audienceDurability of concrete exposed to an external sulfate attack is a great concern for long-time reliability of nuclear waste containment. Concrete can be subjected to moderate external sulfate attacks, leading to the precipitation of gypsum and ettringite in the porous media. The formation of these minerals generates internal swelling pressures, leading to cracking and expansion of the material, reducing its containment properties. This paper presents experimental results obtained on cement paste samples subjected to a low concentration (30.10-3 mol/l) external sulfate attack during 6 months in realistic service conditions. Samples were characterized using multiple experimental characterization tools such as XRD, SEM-EDS, and nanoindentation to follow the evolution of chemical, mineralogical, microstructural and mechanical properties of the material. Cement paste samples with different C3A ratios were used to boost either ettringite or gypsum formation during the chemical attack in order to assess the impact of each mineral on the degradation. Results showed apparition of cracks parallel to the attacked surface, located in the gypsum formation and portlandite dissolution area. This observation suggested an active participation of gypsum formation in the expansion mechanism. Numerical simulations of the chemical degradation were performed using the reactive transport code HYTEC in order to enrich the interpretation of experimental results. Based on simulation results, an analytical homogenization scheme was applied to estimate Young Modulus values in the degraded area. Depth values of degradation fronts and calculated elastic properties were in good agreement with experimental results

Modélisation chimio-mécanique du gonflement et de la fissuration de matériaux cimentaires soumis à une attaque sulfatique externe modérée

International audienceDurability of concrete exposed to an external sulfate attack is a great concern for long-time reliability of nuclear waste containment. Concrete can be subjected to moderate external sulfate attacks, leading to the precipitation of gypsum and ettringite in the porous media. The formation of these minerals generates internal swelling pressures, leading to cracking and expansion of the material, reducing its containment properties. This paper presents experimental results obtained on cement paste samples subjected to a low concentration (30.10-3 mol/l) external sulfate attack during 6 months in realistic service conditions. Samples were characterized using multiple experimental characterization tools such as XRD, SEM-EDS, and nanoindentation to follow the evolution of chemical, mineralogical, microstructural and mechanical properties of the material. Cement paste samples with different C3A ratios were used to boost either ettringite or gypsum formation during the chemical attack in order to assess the impact of each mineral on the degradation. Results showed apparition of cracks parallel to the attacked surface, located in the gypsum formation and portlandite dissolution area. This observation suggested an active participation of gypsum formation in the expansion mechanism. Numerical simulations of the chemical degradation were performed using the reactive transport code HYTEC in order to enrich the interpretation of experimental results. Based on simulation results, an analytical homogenization scheme was applied to estimate Young Modulus values in the degraded area. Depth values of degradation fronts and calculated elastic properties were in good agreement with experimental results