Hydration of the <1 μm size fraction of SWy-1 source clay (low-charge montmorillonite) was studied by modeling of X-ray diffraction (XRD) patterns recorded under controlled relative humidity (RH) conditions on Li-, Na-, K-, Mg-, Ca-, and Sr-saturated specimens. The quantitative description of smectite hydration, based on the relative proportions of different layer types derived from the fitting of experimental XRD patterns, was consistent with previous reports of smectite hydration. However, the coexistence of smectite layer types exhibiting contrasting hydration states was systematically observed, and heterogeneity rather than homogeneity seems to be the rule for smectite hydration. This heterogeneity can be characterized qualitatively using the standard deviation of the departure from rationality of the 00l reflection series (ξ), which is systematically larger than 0.4 Å when the prevailing layer type accounts for ~70% or less of the total layers (~25% of XRD patterns examined). In addition, hydration heterogeneities are not distributed randomly within smectite crystallites, and models describing these complex structures involve two distinct contributions, each containing different layer types that are interstratifed randomly. As a result, the different layer types are partially segregated in the sample. However, these two contributions do not imply the actual presence of two populations of particles in the sample. XRD profile modeling also has allowed the refinement of structural parameters, such as the location of interlayer species and the layer thickness corresponding to the different layer types, for all interlayer cations and RH values. From the observed dependence of the latter parameter on the cation ionic potential (v/r; v = cation valency and r = ionic radius) and on RH, the following equations were derived: which allow the quantification of the increase of layer thickness with increasing RH for both 1W (one water) and 2W (two water) layers. In addition, for 2W layers, interlayer H₂O molecules are probably distributed as a unique plane on each side of the central interlayer cation. This plane of H₂O molecules is located at ~1.20 Å from the central interlayer cation along the c* axis.

X-ray diffraction (XRD) and Mn K-edge extended X-ray absorption fine structure (EXAFS) spectroscopy were combined to elaborate a structural model for phyllomanganates (layer-type Mn oxides) lacking 3D ordering (turbostratic stacking). These techniques were applied to a sample produced by a common soil and freshwater bacterium (Pseudomonas putida), and to two synthetic analogs, δ-MnO₂ and acid birnessite, obtained by the reduction of potassium permanganate with MnCl₂ and HCl, respectively. To interpret the diffraction and spectroscopic data, we applied an XRD simulation technique utilized previously for well-crystallized birnessite varieties, complementing this approach with single-scattering-path simulations of the Mn K-edge EXAFS spectra. Our structural analyses revealed that all three Mn oxides have an hexagonal layer symmetry with layers comprising edge-sharing Mn⁴⁺O₆ octahedra and cation vacancies, but no layer Mn³⁺O₆ octahedra. The proportion of cation vacancies in the layers ranged from 6 to 17%, these vacancies being charge-compensated in the interlayer by protons, alkali metals, and Mn atoms, in amounts that vary with the phyllomanganate species and synthesis medium. Both vacancies and interlayer Mn were most abundant in the biogenic oxide. The diffracting crystallites contained three to six randomly stacked layers and have coherent scattering domains of 19–42 Å in the c* direction, and of 60–85 Å in the a-b plane. Thus, the Mn oxides investigated here are nanoparticles that bear significant permanent structural charge resulting from cation layer vacancies and variable surface charge from unsaturated O atoms at layer edges.

The diagenetic evolution of kaolin and illitic minerals in sandstones is described here. The structural characterization of these minerals, the possible reaction pathways leading to their crystallization, and the origin of the fluids involved are discussed specifically.While early precipitation of kaolinite is in general related to flushing by meteoric waters, subsequent diagenetic kaolinite-to-dickite transformation probably results from invasion by acidic fluids of organic origin. Dickite is the stable polytype in most sandstone formations and the kaoliniteto- dickite conversion is kinetically controlled.The conventional model of kaolin illitization, assuming a thermodynamic control in a closed system, is discussed and compared to an alternative model in which illitization of kaolin is not coupled to dissolution of K-feldspar (Bergeret al., 1997). In the latter model, illite crystallization at the expense of kaolin implies that an energy barrier is overcome either by an increased K⁺/H⁺activity ratio in solution or by a considerable temperature increase.

More than a century of non-ferrous metallurgical activities have had a severe impact on the natural environment leading, in most heavily contaminated sites, to a complete loss of the vegetation cover (that is, desert-like areas) or to the selection of metal-hyperaccumulator plant species. Identifying the chemical forms of toxic metals is of vital importance for a realistic assessment of the chemical risk posed by their presence in soils and selecting effective remediation technologies. In this study, X-ray diffraction (XRD), X-ray texture goniometry, and powder and polarized extended X-ray absorption fine structure (EXAFS, P-EXAFS) have been used to investigate quantitatively the speciation of Zn in soils contaminated by three smelters from northern France and Belgium, and coupled synchrotron-based micro-X-ray radiation fluorescence (mu SXRF) and micro-EXAFS (mu EXAFS) were also used for one of these soils. Of these techniques, the application of P-EXAFS and mu EXAFS to molecular environmental science was unprecedented, and we show that their complementarity greatly improves the sensitivity of powder EXAFS to identify the nature of metal-containing minerals in soils. Franklinite (ZnFe ₂ O ₄ ), willemite (Zn ₂ SiO ₄ ), hemimorphite (Zn ₄ Si ₂ O ₇ (OH) ₂ .H ₂ ), and Zn-containing magnetite ([Fe,Zn]Fe ₂ O ₄ ) were identified in dense soil fractions by XRD and powder EXAFS. These primary minerals originate from atmospheric fallout of Zn dusts emitted during the pyrometallurgical smelting process, and they act as the main source of Zn in contaminated soils. In all soil samples, Zn released in solution during the weathering of these high-temperature minerals is taken up partly by phyllosilicates and, to a lesser extent, by Mn and Fe (oxyhydr) oxides. Zn-containing phyllosilicates were identified by comparing powder EXAFS spectra to a library of model compounds and from the noteworthy angular dependence of EXAFS spectra collected on self-supporting films of clay soil fractions. Analysis of higher correlations in EXAFS spectra suggests that the local structure around Zn in phyllosilicates is trioctahedral. The phyllomanganate Zn-sorbed birnessite and Zn-containing Fe grains having a delta FeOOH-like local structure were unambiguously identified by mu SXRF-mu EXAFS. In birnessite Zn is sorbed in the interlayer space above/below vacant sites and can be either 4-fold or 6-fold coordinated depending, presumably, on the anionic stacking of birnessite layers. Based on this micro-mineralogical investigation, a satisfactory fit of the three identified Zn species (that is, phyllosilicate, Mn, and Fe (oxyhydr)oxides) to experimental powder EXAFS spectra of all clay soil fractions was obtained. The significance, origin, and stability of Zn-phyllosilicates are discussed. Specifically, we show that the formation of Zn-containing phyllosilicates is consistent with calculated thermodynamic solubilities. For the range of measured Zn (super 2+) ( approximately 10 ppm), Si(OH) ₄ (10-20 ppm), and H (super +) (5.6<pH6) or near saturation (pH<6) with respect to the trioctahedral Zn phyllosilicate, Zn-kerolite. Finally, the plausibility of the formation of (Zn, Al) hydrotalcite-like species contemplated by Julliot (1999) is critically assessed.

The geochemistry of Co at the Earth's surface is closely associated with that of manganese oxides. This geochemical association results from the oxidation of highly soluble Co (super 2+) to weakly soluble Co (super 3+) species, coupled with the reduction of Mn (super 4+) or Mn (super 3+) ions, initially present in the manganese oxide sorbent, to soluble Mn (super 2+) . The structural mechanism of this Co immobilization-manganese oxide dissolution reaction was investigated at the buserite surface. Co-sorbed samples were prepared at different surface coverages by equilibrating a Na-exchanged buserite suspension in the presence of aqueous Co (super 2+) at pH 4. The structure of Co-sorbed birnessite obtained by drying buserite samples was determined by X-ray diffraction (XRD) and powder and polarized EXAFS spectroscopy. For each sample we determined the proportion of interlayer cations and layer vacancy sites, the Co (super 2+) /(Co (super 2+) +Co (super 3+) ) ratio, the nature of Co sorption crystallographic sites, and the proportion of interlayer vs. layer Co. From this in-depth structural characterization two distinct oxidation mechanisms were identified that occur concurrently with the transformation of low pH monoclinic buserite to hexagonal H-rich birnessite (Drits et al. 1997; Silvester et al. 1997). The first mechanism is associated with the fast disproportionation of layer Mn (super 3+) according to 2Mn (super 3+) _layer -->Mn (super 4 + ) _layer +[] _layer + Mn (super 2+) _solution , where [] denotes a vacant site. Divalent Co sorbs above or below a vacant site ([] ₁ ) and is then oxidized by the nearest Mn (super 3+) _layer . The resulting Co (super 3+) species fills the [] ₁ position while the reduced Mn migrates to the interlayer or into solution creating a new vacant site ([] ₂ ). This reaction can be written: Co (super 2+) _solution +[] ₁ + Mn (super 3+) _layer --> Co (super 2+) _interlayer + [] ₁ Mn (super 3+) _layer --> Co (super 3+) _interlayer + [] ₁ + Mn (super 2+) _layer --> Co (super 3+) _layer + [] ₂ + Mn (super 2+) (sub sol/inter) . This mechanism may replicate along a Mn (super 3+) -rich row, and, because the density of vacancies remains constant, it can result in relatively high Co concentrations, as well as domains rich in Co (super 3+) _layer -Mn (super 4+) _layer . During the low-pH buserite transformation, about one-half of the layer Mn (super 3+) that does not disproportionate migrates from the layer to the interlayer space creating new vacancies, with the displaced Mn (super 3+) residing above or below these vacancies. The second oxidation mechanism involves the replacement of Mn (super 3+) _interlayer by Co (super 3+) _interlayer ; the latter may eventually migrate into layer vacancies depending on the chemical composition of octahedra surrounding the vacancy. The criterion for the migration of Co (super 3+) into layer vacancies is the need to avoid Mn (super 3+) _layer -Co (super 3+) _layer -Mn (super 3+) _layer sequences. The suite of chemical reactions for this second mechanism can be schematically written: Co (super 2+) _solution + Mn (super 3+) _interlayer + [] --> Mn (super 2+) _solution + Co (super 3+) _interlayer + [] --> Mn (super 2+) _solution + Co (super 3+) _layer , the last step being conditional. In contrast to the first mechanism, this second mechanism decreases the density of vacant sites. At high surface coverage, Co-sorbed birnessite contains a substantial amount of unoxidized Co (super 2+) _interlayer species despite some non-reduced Mn (super 3+) in the sorbent. This result can be explained by the sorption of Co (super 2+) _solution ? onto vacant sites located in Co (super 3+) _layer - and Mn (super 4+) _layer -rich domains devoid of Mn (super 3+) . The number and size of these domains increase with the extent of oxidation and the total Co concentration in the solution, and this accounts for the decreasing capacity of buserite to oxidize Co. The weight of structural evidence indicates that Co is oxidized by Mn (super 3+) rather than Mn (super 4+) . Thermodynamic considerations indicate that under the solution pH conditions employed in this study Mn (super 3+) is the more likely electron sink for the oxidation of Co (super 2+) . This study also shows that the high affinity of Co for manganese oxides is not only due to its oxidation to weakly soluble Co (super 3+) species, but also because of the reducted layer strains from the substitution of Co (super 3+) for Mn (super 3+) . Results obtained for these model compounds were compared with those for natural Co-containing asbolane and lithiophorite (Manceau et al. 1987). This comparison indicates that the different structural mechanisms explored in the laboratory can satisfactorily account for the observations made on natural samples. Specifically, the present study proves that Co substitutes for Mn in natural phyllomanganates and allows us to eliminate the possibility of precipitation of discrete CoOOH particles.