Bernard Guy

Experimental study of dissolution rates of hedenbergitic clinopyroxene at high temperatures: dissolution in water from 25 °C to 374 °C

Steady-state pyroxene dissolution rates in aqueous solutions have been measured at temperatures from 25 to 374 °C at a pressure of 23 MPa and at neutral pH. The pyroxene is hedenbergitic clinopyroxene, of composition Na 0.04 Ca 0.95 Mg 0.3 Fe 2+ 0.64 Fe 3+ 0.06 Al 0.04 Si 1.97 O 6 . All experiments were performed at conditions far from equilibrium in Ti-alloy mixed-flow reactors. In most runs, the reactive solutions were undersaturated with respect to pyroxene and secondary minerals were rarely found at the reacted surface. The dissolution is non-stoichiometric in most cases, while the different chemical elements of the pyroxene are released at different rates. Stoichiometric steady-state dissolution was obtained in neutral solution at 100 °C. The release rates of the different elements vary with temperature and solution chemistry. The dissolution rates (r Si ) in neutral pH conditions increase with temperature from 25 to 300 °C, reach a maximum at 300 °C, and then decrease with continued temperature increase. At a given temperature, the rates decrease significantly with increasing pH of the reactive fluid and are also affected by the activities of Ca, Mg, Fe in the solution. At neutral pH, the dependence of the pyroxene dissolution rates on activities of Ca, Mg, Fe and H + in the fluid can be expressed by the relation: log r + ( T , a i ) = log ( A - E a / ( 2.303 R T ) + α log ( a H + ) Z i / a M i Z i + ) where r + is the far-from-equilibrium dissolution rate, R the gas constant, T the absolute temperature, Z i valence of metal M i and a i represents the activity of the subscript aqueous species. E a equals 22.667 kJ/mole/K and A = 2.011 × 10 −7 mole/cm 2 /s; α is the empirical reaction rate order, which can be derived from the experimental results. At temperatures below 300 °C, the exchange reactions 2H + ↔M i 2+ , where M i 2+ refers to divalent cations Mg 2+ , Fe 2+ or Ca 2+ , dominate in the dissolution. The following evolution of the dissolution with temperature is proposed: at 300 °C, the tetrahedral Si–O bonds break after the M i 2+ –O bonds in adjacent octahedral positions have been removed by proton exchange reaction, whereas, above 300 °C, the breaking of the octahedral M i 2+ –O bonds occurs after adjacent tetrahedral Si–O bonds have been broken.

Rôle de la déformation métamorphique dans la cristallinité du graphite : l'exemple des schistes graphiteux de la vallée de la Lotru (Carpathes, Roumanie)

Role of metamorphic strain in the cristallinity of graphite: the example of the graphitic schists from the Lotru valley (Carpathians, Romania). The study of the schists from Lotru valley (Carpathians mountains, Romania) shows the role of stress in graphite crystallinity. The carbonaceous material was separated and studied by X-ray diffraction and Raman microspectrometry. There exists a clear contrast between the crystallization degree of graphite found in the quartz- muscovite schists (good) and that found in the quartzites (poor). This observation is in conformity with a clear differentiation of fabric. The graphite suffered an anisotropic deformation in the graphitic schists, whereas it was protected from deformation by quartz in the quartzites. In this last case, the development of graphite from the organic material took place with a volume decrease, protected by the surrounding quartz. To cite this article: S.-C. Barzoi, B. Guy, C. R. Geoscience 334 (2002) 89-95.

Réflexions sur la formation des bandes de Forbes : l'instabilité de la fusion de la glace sale

Reflections on the formation of Forbes ogives: the instability of fusion of dirty ice. Forbes ogives show alternations of dark (ice + mineral dust) and light bands at the surface of certain glaciers. We propose to understand their origin by the influence of the content of mineral matter on the lowering of the temperature and pressure of ice fusion and upon the increase of fusion velocity. We are then in an unstable situation: a local increase in the mineral content being induced by the fusion process, which in turn increases; this creates a dark band. The movement of the glacier cannot keep up with the fusion: pressure is lowered again below the fusion point, and a white band is formed. To cite this article: B. Guy et al., C. R.

Use of thermodynamic chemical potential diagrams (µCaO, µCO 2 ) to understand the weathering of cement by a slightly carbonated water

Cement is a ubiquitous material that may suffer hazardous weathering. The chemical weathering of cement in natural environment is mostly characterized by the leaching of CaO and the addition of CO2. The different weathering zones that develop at the expense of the cement may be predicted by the help of chemical potential phase diagrams; these diagrams simulate the behaviour of systems open to some chemical elements. Some components have a so-called inert status, that is to say the system is closed for these components, their amount in the system remains constant; some other components have a mobile status, that is to say these components can be exchanged with the outside of the system, their amount can vary from one sample zone to another. The mobile components are represented in the model by their chemical potentials (linked to their concentrations) that are variable in the external environment. The main features of the weathering of a cement system open to CaO and CO2 are predicted in a phase diagram with µCaO et µCO2 as diagram axes. From core to rim, one observes the disappearance of portlandite, ettringite and calcium monosulfoaluminate, the precipitation of calcite and amorphous silica, the modification of the composition of the CSH minerals (hydrated calcium silicates) that see a decrease of their c/s ratio (CaO/SiO2) from the core to the rim of the sample. For the CSH minerals, we have separated their continuous solid solution into three compositions defined by different CaO/SiO2 ratios and called phases 1, 2 and 3: CaO = 0.8, 1.1, 1.8 respectively for one mole of SiO2 knowing that H2O varies in the three compositions.Cement is a ubiquitous material that may suffer hazardous weathering. The chemical weathering of cement in natural environment is mostly characterized by the leaching of CaO and the addition of CO 2 . The different weathering zones that develop at the expense of the cement may be predicted by the help of chemical potential phase diagrams; these diagrams simulate the behaviour of systems open to some chemical elements. Some components have a so-called inert status, that is to say the system is closed for these components, their amount in the system remains constant; some other components have a mobile status, that is to say these components can be exchanged with the outside of the system, their amount can vary from one sample zone to another. The mobile components are represented in the model by their chemical potentials (linked to their concentrations) that are variable in the external environment. The main features of the weathering of a cement system open to CaO and CO 2 are predicted in a phase diagram with µCaO et µCO 2 as diagram axes. From core to rim, one observes the disappearance of portlandite, ettringite and calcium monosulfoaluminate, the precipitation of calcite and amorphous silica, the modification of the composition of the CSH minerals (hydrated calcium silicates) that see a decrease of their c/s ratio (CaO/SiO 2 ) from the core to the rim of the sample. For the CSH minerals, we have separated their continuous solid solution into three compositions defined by different CaO/SiO 2 ratios and called phases 1, 2 and 3: CaO = 0.8, 1.1, 1.8 respectively for one mole of SiO 2 knowing that H 2 O varies in the three compositions.