Régine Mosser-Ruck

Experimental synthesis of chlorite from smectite at 300°C in the presence of metallic Fe

Abstract The alteration and transformation behaviour of montmorillonite (bentonite from Wyoming, MX-80) in low-salinity solutions (NaCl, CaCl2) in the presence of metallic Fe (powder and 8 × 4 × 1 mm plate) and magnetite powder was studied in batch experiments at 300°C to simulate the mineralogical and chemical reactions of clays in contact with steel in a nuclear waste repository. The evolutions of pH and solution concentrations were measured over a period of 9 months. The mineralogical and chemical evolution of the clays was studied by XRD, SEM, Transmission Mossbauer Spectroscopy and TEM (EDS, HR imaging and EELS). Dissolution of the di-octahedral smectite of the starting bentonite was observed, in favour of newly formed clays (chlorite and saponite), quartz, feldspars and zeolite. The formation of Fe-chlorite was triggered by contact with the metallic Fe plate and Fe-Mg-chlorite at distance from the Fe plate (>2 mm).

Experimental study of the transformation of smectite at 80 and 300°C in the presence of Fe oxides

Abstract The alteration and transformation behaviour of montmorillonite (Wyoming bentonite) was studied experimentally to simulate the mineralogical and chemical reaction of clays in contact with steel in a nuclear waste repository. Batch experiments were conducted at 80 and 300°C, in low-salinity solutions (NaCl, CaCl2) and in the presence or otherwise of magnetite and hematite, over a period of 9 months. The mineralogical and chemical evolution of the clays was studied by XRD, SEM, transmission Mössbauer spectroscopy and EDS-TEM. Experimental solutions were characterized by ICP-AES and ICP-MS. The main results are that no significant change in the crystal chemistry of the montmorillonite occurred at 80°C, while at 300°C, the presence of Fe oxides leads to a partial replacement of montmorillonite by high-charge trioctahedral Fe2+-rich smectite (saponite-like) together with the formation of feldspars, quartz and zeolites.

A reinvestigation of smectite illitization in experimental hydrothermal conditions: Results from X-ray diffraction and transmission electron microscopy

Abstract The hydrothermal reactivity of the <1 μm fraction, K-saturated SWy-2 Wyoming low-charge montmorillonite was studied in the 250-400 °C temperature range with reactions lasting between 5 and 120 days, with a solid/solution mass ratio of 1:10, and in 1 mol/L KCl solution. From X-ray diffraction (XRD) profile modeling results on K-saturated and ethylene-glycol solvated samples the illitization process appears to occur as a progressive replacement of expandable layers by layers with illitic behavior, in a single illite-smectite phase. However this treatment overestimates the amount of illite layers because of the presence of smectitic non-expandable layers. This was revealed by calcium exchange of the products, which causes re-expansion of the apparent illite layers. The illitization model then obtained consists of four phases with increasing illite content: discrete smectite, a randomly interstratified mixed-layered mineral (MLM) and two ordered MLMs that progressively replace the phases with less illite content. This polyphase model is consistent with results from transmission electron microscopy and chemical microanalysis, which show several types of particle morphology with different interlayer K content and Al-for-Si substitution that seem to correspond to the several phases detected by XRD. Thus, the actual illitization mechanism of smectite in our experiments is not a progressive reaction but a dissolution-precipitation process following the Ostwald step rule in which metastable smectite transforms into illite through a series of metastable illite-smectite phases.

A MULTI-TECHNIQUE CHARACTERIZATION OF CRONSTEDTITE SYNTHESIZED BY IRON–CLAY INTERACTION IN A STEP-BY-STEP COOLING PROCEDURE

The cooling of steel containers in radioactive-waste storage was simulated in a step-by-step experiment from 90 to 40ºC. Among newly formed clay minerals observed in run products, cronstedtite was identified by a number of analytical techniques (powder X-ray diffraction, transmission electron microscopy, and scanning electron microscopy). Cronstedtite has not previously been recognized to be so abundant and so well crystallized in an iron—clay interaction experiment. The supersaturation of experimental solutions with respect to cronstedtite was due to the availability of Fe and Si in solution, as a result of the dissolution of iron metal powder, quartz, and minor amounts of other silicates. Cronstedtite crystals are characterized by various morphologies: pyramidal (truncated or not) with a triangular base and conical with a rounded or hexagonal cross-section. The pyramidal crystals occur more frequently and their polytypes (2M1, 1M, 3T) were identified by selected area electron diffraction patterns and by automated diffraction tomography. Cronstedtite is stable within the 90-60ºC temperature range. At temperatures of ⩽ 50ºC, the cronstedite crystals showed evidence of alteration.

EFFECTS OF ETHYLENE GLYCOL SATURATION PROTOCOLS ON XRD PATTERNS: A CRITICAL REVIEW AND DISCUSSION

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 ≈14–15 Å). Expansion to 17 Å 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) glycolation procedures 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.

Oxidation of mudstone in a tunnel (Tournemire, France): consequences for the mineralogy and crystal chemistry of clay minerals

Abstract The excavation of a tunnel through a mudstone formation provides an opportunity to examine the effects of the modification of the physical and chemical environment on the rock. The mineralogical and chemical consequences of hydration-dehydration cycles and of oxidation have been evaluated in the case of the Toarcian mudstone formation at the Tournemire experimental site (France). Studies by X-ray diffraction and tansmission electron microscopy of both altered and preserved samples show that the introduction of air and condensed water causes the oxidation of pyrite and the subsequent generation of acid and sulphate-rich waters at the micron scale, in the local environment of pyrites. The fluid-clay particle interactions around the oxidized pyrites induce: (1) a statistical enrichment in Si of the I-S clay minerals; (2) an increase in the Fe(III)/Fe total ratio in some of the I-S particles; and (3) the dissolution of illite layers in mixed-layer I-S. These evolutions are consistent with the results of numerical modelling which reproduced the interaction between the clay particles and the acid water.

Mineralogical Evolution of A Claystone After Reaction with Iron Under Thermal Gradient

The design of the repository for high-level nuclear waste (HLW) in France consists of a multiple-barrier system including steel canisters in a clay host rock. The system will undergo temperature variations in time and space, the heat source being the HLW within the canisters. The effect of a thermal gradient in space on the Fe-claystone interaction was investigated here by applying a thermal gradient (150–300°C and 80–150°C) to a mix of claystone, Fe, and an aqueous chloride solution over periods of 3 and 6 months. Following the reaction, the starting clay minerals (mostly illite and mixed-layer illite-smectite) evolved toward chlorite, Fe-serpentine, Fe-saponite, mixed-layer chlorite-smectite, or mixed-layer serpentine-smectite as a function of temperature. Iron corrosion made the medium basic and reductive. Magnesium enrichment of clay minerals was observed in the hottest part of the experiment due to Mg migration under the thermal gradient. Reaction progress was enhanced at the lowest temperatures, compared to batch experiments.

Évolution minéralogique des argilites en zone sous-saturée oxydée : exemple des parois du tunnel de Tournemire (Aveyron, France)

Abstract Digging of underground work such as that of the Tournemire tunnel in a Toarcian argillite formation, has an impact on the surface mineralogy of the rock blocks. In spite of its very low water content, the argillite is reactive as a consequence of the modification of the redox conditions. Thus, atmospheric oxygen in presence of adsorbed water induces pyrite corrosion and formation of secondary mineral phases such as gypsum, celestite, jarosite and Fe-hydroxides, which play a significant role on the water–rock equilibria of the mechanically disturbed zone.

Experimental alteration of Mg-vermiculite under hydrothermal conditions: formation of mixed-layered saponite-chlorite minerals

Abstract The hydrothermal reactivity of a Mg-vermiculite from Santa Olalla (Spain) was studied experimentally at 150 and 300°C, 75 and 100 bars, respectively, in (Na,K,Ca,Mg) chloride and sulphate solutions (liquid/solid ratio = 10). The formation of mixed-layer clays is demonstrated by FT-IR spectroscopy, X-ray diffraction (XRD) and electron microprobe. The gradual decrease of the 3220 cm-1 infrared band intensity, characteristic of free water, is interpretated as the dehydration of the interlayer Mg cation of the vermiculite. The presence of ‘brucite type’ sheets is also observed by the increase of 3555 cm-1 and 3420 cm-1 band intensities. The XRD results show that run products are able to expand when glycolated. Microprobe analyses document the decrease of IVSi content and a significant enrichment in Mg in the run products. The formation of a mixed-layer clay comprising saponite and chlorite layers is proposed.