Effects of temperature, pH, and iron/clay and liquid/clay ratios on experimental conversion of dioctahedral smectite to berthierine, chlorite, vermiculite, or saponite

Régine Mosser-Ruck (1) , Michel Cathelineau (1) , D. Guillaume (2) , Delphine Charpentier (3) , D. Rousset (1) , Odile Barres (4) , Nicolas Michau (5)

1 G2R - Géologie et gestion des ressources minérales et énergétiques
2 LMTG - Laboratoire des Mécanismes et Transfert en Géologie
3 LCE - Laboratoire Chrono-environnement (UMR 6249)
4 LEM - Laboratoire Environnement et Minéralurgie
5 ANDRA - Agence Nationale pour la Gestion des Déchets Radioactifs

In deep geological repositories for high-level nuclear wastes, interactions between steel canisters and clay-rich materials may lead to mineralogical transformations with a loss of the confining properties of the clays. Experiments simulating the conversion of smectite to Fe-rich clay phases in contact with Fe metal have been carried out to evaluate such a possibility by taking into account the effects of a series of critical parameters, including temperature, pH, and Fe/clay (Fe/C) and liquid/clay (L/C) ratios. The mineralogical and chemical transformations observed in these experiments have been compared with data from the literature, and subsequently used to propose a conceptual model for the main mineralogical transformations which can be expected in clay formations surrounding high-level nuclear waste repositories. In the presence of Fe metal and under low oxygen fugacity (<1040) the main mineralogical sequences are as follows: (1) up to 150ºC, under neutral pH, and L/C > 5: dioctahedral smectite (di-sm) ? 7 A˚ Fe-rich phase (berthierine, odinitecronstedtite) for large Fe/C ratios (>0.5), or di-sm ? Fe-rich di-sm + Fe-rich trioctahedral smectite (tri-sm) for small Fe/C ratios (0.1); (2) up to 150ºC, under alkaline pH (1012), and L/C > 5: di-sm ? Fe di-sm (Npalygorskite) for a small Fe/C ratio (0.1); (3) at 300ºC, Fe/C = 0.1, and L/C > 5: di-sm ? Fe-rich saponite ? trioctahedral chlorite + feldspar + zeolite (near-neutral pH); di-sm ? Fe-rich vermiculite + mordenite (pH 1012). Low temperatures (<150ºC) and large L/C and Fe/C ratios seem to favor the crystallization of the serpentine group minerals instead of Fe-rich trioctahedral smectites or chlorites, the latter being favored by higher temperatures. The role of L/C and Fe/C ratios and the competition between them at different temperatures is a crucial point in understanding the transformation of smectite in contact with Fe metal.

Mots clés: Berthierine; Chlorite; Fe Metal; Nuclear Waste; Saponite; Smectite; Vermiculite; Berthierine

Domaines: Géochimie; Milieux et Changements globaux; Géologie appliquée; Minéralogie

Régine Mosser-Ruck, Michel Cathelineau, D. Guillaume, Delphine Charpentier, D. Rousset, et al.. Effects of temperature, pH, and iron/clay and liquid/clay ratios on experimental conversion of dioctahedral smectite to berthierine, chlorite, vermiculite, or saponite. Clays and Clay Minerals, 2010, 58 (2), pp.280-291. ⟨10.1346/CCMN.2010.0580212⟩. ⟨hal-00566986v2⟩