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 (<10−40) the main mineralogical sequences are as follows:



1. up to 150°C, under neutral pH, and L/C > 5: dioctahedral smectite (di-sm) → 7 A Fe-rich phase (berthierine, odinite–cronstedtite) 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 150°C, under alkaline pH (10–12), and L/C > 5: di-sm → Fe di-sm (±palygorskite) for a small Fe/C ratio (0.1);

3. at 300°C, 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 10–12).



Low temperatures (<150°C) 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.

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EFFECTS OF TEMPERATURE, pH, AND IRON/CLAY AND LIQUID/CLAY RATIOS ON EXPERIMENTAL CONVERSION OF DIOCTAHEDRAL SMECTITE TO BERTHIERINE, CHLORITE, VERMICULITE, OR SAPONITE

The study of the transformation of smectite to illite, chlorite or vermiculite via interstratified clay minerals needs precise qualitative and quantitative determinations of the different layers in the mixed- layer clays and is generally based on X-ray diffraction (XRD) patterns after specific treatments of the clay samples. Saturation with K or Mg followed by ethylene glycol (EG) solvation are classical methods used to identify high-charge smectite and vermiculite. These procedures have been applied to two experimental clays, one composed of smectite layers and the second, a mixture of vermiculite and smectite layers. Different methods of glycolation (EG vapor or liquid EG) produce significant differences in the XRD patterns. Comparison with literature data indicates that K-saturated, high-charge smectite (~0.8 < total charge <1/unit-cell) and Mg-vermiculite (whatever its charge) do not expand in ethylene glycol vapor (d values ~14y15 A E ). Expansion to 17 A E in liquid ethylene glycol occurs for Mg-vermiculite with a total charge of <~1.2/unit-cell and for K-saturated, high-charge smectite, when the tetrahedral charge is <~0.7/unit-cell. This study shows that: (1) procedures of glycolation need to be standardized; (2) the use of saturation protocols using both liquid ethylene glycol and ethylene glycol vapor yields useful additional information about the distribution of charges in clay minerals.

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Effects of Ethylene Glycol Saturation Protocols on XRD Patterns: A Critical Review and Discussion

Interactions between metallic iron and clay minerals have been extensively studied under low-temperature anoxic conditions owing to their potential impact on the long-term safety of high-level nuclear waste disposal in deep geological repositories. To complement the studies investigating the destabilization mechanisms and rates of various initial clay minerals, the prediction of the storage long-term performance requires a comprehensive characterization of the reaction products. The Fe-rich 1:1 phyllosilicates resulting from interactions at 80 °C and in the absence of O 2 between metallic iron and smectites with contrasting compositions are thus characterized chemically and structurally using various experimental techniques (X-ray and electron diffractions, infrared, energy-dispersive, and electron energy loss spectroscopies, and high-resolution electron microscopy). Cronstedtite and odinite are the two Fe-rich 1:1 phyllosilicates formed under the experimental conditions investigated, both species differing from their relative contents and the average valence state of structural Fe. No parental link has been evidenced between the two minerals despite their contrasting crystal morphologies and thermodynamical predictions. The formation of the 1:1 phyllosilicates apparently results from the destabilization of the initial smectite through the formation of an intermediate gel.

Experimental investigation of smectite interaction with metal iron at 80 °C: Structural characterization of newly formed Fe-rich phyllosilicates

Mineralogical reactions in the Tournemire argillite after in-situ interaction with steels

The efficiency of deep geological storage systems relies on the confinement properties of the natural or engineered clay-rich material: low permeability and diffusivity, high retention and swelling capacity. The reactivity of clay barriers (CB) is intimately linked to the nature and properties of their constitutive minerals. The CB react due to changes in the initial physicochemical conditions (pH and redox potential, aqueous species concentrations) and/or the introduction of “foreign” materials (iron, steel, concrete, glass, bitumen, etc.). This chapter describes the impact on CB, for different storage systems, as a result of the dissolution of primary minerals and precipitation of secondary minerals, as well as modifications of mineral properties (especially related to the exchange capacity, the interlayer cation content, and the swelling ability of clay minerals) and transport properties through modifications in porosity, permeability and tortuosity.

D. Charpentiera

Papers

Bentonite–iron interactions under alkaline condition: An experimental approach