Diffusion of Water through the Dual-Porosity Swelling Clay Mineral Vermiculite.

Prediction of water and solute migration in natural clay-based materials requires a detailed understanding of the roles played by different porosity types (around or inside clay particles) on the overall transfer process. For smectite, a reference material for the design of migration models, this discrimination is complex because of osmotic swelling of the structure under water-saturated conditions. Diffusion experiments with a water tracer (HDO) were conducted on 0.1-0.2, 1-2, and 10-20 µm size fractions of Na-vermiculite, a swelling clay mineral with no osmotic swelling. Results obtained for the two finest fractions suggest that osmotic swelling and the associated impact on pore structure are responsible for the low De values reported in the literature for smectite compared to those of vermiculite. When considering only interparticle porosity, De values for vermiculite are similar to those reported for nonporous grains (Na-kaolinite and Na-illite). This indicates that interparticle porosity has a primary effect on the overall water diffusion process, whereas interlayer porosity is shown to imply a small proportion of HDO adsorption. This study provides evidence that vermiculite is a promising reference mineral for the understanding of the roles played by pore structure and mineral-water interaction in the transport properties of water in claystones and for associated refinement of dual-porosity diffusion models.

Untangling the environmental from the dietary: dust does not matter.

Both dust and silica phytoliths have been shown to contribute to reducing tooth volume during chewing. However, the way and the extent to which they individually contribute to tooth wear in natural conditions is unknown. There is still debate as to whether dental microwear represents a dietary or an environmental signal, with far-reaching implications on evolutionary mechanisms that promote dental phenotypes, such as molar hypsodonty in ruminants, molar lengthening in suids or enamel thickening in human ancestors. By combining controlled-food trials simulating natural conditions and dental microwear textural analysis on sheep, we show that the presence of dust on food items does not overwhelm the dietary signal. Our dataset explores variations in dental microwear textures between ewes fed on dust-free and dust-laden grass or browse fodders. Browsing diets with a dust supplement simulating Harmattan windswept environments contain more silica than dust-free grazing diets. Yet browsers given a dust supplement differ from dust-free grazers. Regardless of the presence or the absence of dust, sheep with different diets yield significantly different dental microwear textures. Dust appears a less significant determinant of dental microwear signatures than the intrinsic properties of ingested foods, implying that diet plays a critical role in driving the natural selection of dental innovations.

Anodic activation of iron corrosion in clay media under water-saturated conditions at 90 degrees C: characterization of the corrosion interface.

To understand the process governing iron corrosion in clay over centuries, the chemical and mineralogical properties of solids formed by free or anodically activated corrosion of iron in water-saturated clay at 90 degrees C over 4 months were probed using microscopic and spectroscopic techniques. Free corrosion led to the formation of an internal discontinuous thin (<3 microm thick) magnetite layer, an external layer of Fe-rich phyllosilicate, and a clay transformation layer containing Ca-doped siderite (Ca(0.2)Fe(0.8)CO(3)). The thickness of corroded iron equaled approximately 5-7 microm, consistent with previous studies. Anodic polarization resulted in unequally distributed corrosion, with some areas corrosion-free and others heavily corroded. Activated corrosion led to the formation of an inner magnetite layer, an intermediate Fe(2)CO(3)(OH)(2) (chukanovite) layer, an outer layer of Fe-rich 7 A-phyllosilicate, and a transformed matrix layer containing siderite (FeCO(3)). The corroded thickness was estimated to 85 microm, less than 30% of the value expected from the supplied anodic charge. The difference was accounted for by reoxidation at the anodically polarized surface of cathodically produced H(2)(g). Thus, free or anodically activated corroding conditions led to structurally similar interfaces, indicating that anodic polarization can be used to probe the long-term corrosion of iron in clay. Finally, corrosion products retained only half of Fe oxidized by anodic activation. Missing Fe probably migrated in the clay, where it could interact with radionuclides released by alteration of nuclear glass.