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Etude expérimentale et modélisation de la stabilité des phyllosilicates soumis à un fort gradient thermique. Test dans le contexte du site géothermique de Soultz-sous-Forêts

Abstract : Thermodynamic data of hydrated phyllosilicates, in particular clay minerals are not well known. The stability fields of these minerals, in particular versus temperature, are not well determined; following some authors they even do not exist, these phases being metastable. We have developped an experimental approach, in which a sequence of local equilibrium states between a fluid and produced minerals take place in a closed gold cell along a strong thermal gradient (Goffé et al., 1987). We obtain a sequence of secondary minerals produced near equilibrium as a function of temperature and we deduce the thermal stability fields of the corresponding mineral assemblages. The experiments were conducted in simple chemical systems : Mg-Al-Si-H2O (MASH), K-Al-Si-H2O (KASH), and K-Mg-Al-Si-H2O (KMASH). The sequences of crystallization observed along the thermal gradient are the same if one exchanges the position of the cells containing the initial reacting materials with respect to the thermal gradient end-members. This « reversibility » of our experiments shows that the crystallization sequences correspond to local equilibrium states. Following the temperature increase (from 200 to 350°C) one observes the following mineralogical sequences : dioctahedral smectite → trioctahedral smectite; kaolinite → donbassite → trioctahedral chlorite; smectite → illite → muscovite ; or even kaolinite → illite + smectite → donbassite ; theses mineralogical sequences are commonly observed in hydrothermal systems. They allow to develop a thermodynamic model for hydrated phyllosilicates, taking into account their hydration state, as a function of temperature. This model shows the stability fields of clay minerals between 200 and 350°C. The chemical and mineralogical dynamics showed in these experimental systems has been applied to predict the possible dissolutions and/or precipitations which may take place between the circulated hot fluid and the geothermal granitic reservoir in the Hot Fractured Rock geothermal system at Soultz-sous-Forêts. These processes may affect the duration of the geothermal reservoir, as a function of possible evolution in the morphology of the porosity related to the fracturation. Our experimental approach shows that feldspars and smectites are forming the major part of the total volume of silicates which may precipitate in addition to carbonates already described in previous studies and which were not considered here.
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Contributor : Armelle Baldeyrou-Bailly <>
Submitted on : Friday, August 26, 2005 - 3:43:28 PM
Last modification on : Thursday, April 23, 2020 - 2:26:28 PM
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  • HAL Id : tel-00009986, version 1



Armelle Baldeyrou-Bailly. Etude expérimentale et modélisation de la stabilité des phyllosilicates soumis à un fort gradient thermique. Test dans le contexte du site géothermique de Soultz-sous-Forêts. Géologie appliquée. Université Louis Pasteur - Strasbourg I, 2003. Français. ⟨tel-00009986⟩



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