Thirty six bentonite samples from 16 different locations were examined in order to demonstrate the applicability of a new Rietveld description approach for quantitative phase analysis. X-ray diffraction patterns of the bulk material were obtained and analyzed by the Rietveld method. The samples contain up to ten different minerals, with dioctahedral smectite as the major component. A model for turbostratic disorder of smectites was formulated inside a structure-description file of the Rietveld program BGMN. The quality of the refinements was checked using an internal standard mineral (10.0 or 20.0 wt.% corundum) and by cross-checking results with X-ray fluorescence (XRF) data. The corundum content was reproduced with only small deviations from the nominal values. A comparison of the chemical composition obtained by XRF and the composition as re-calculated from quantitative Rietveld results shows a satisfactory agreement, although X-ray amorphous components such as volcanic glasses were not considered. As a result of this study, the Rietveld method combined with the new structure model for turbostratic disorder has proven to be a suitable method for routine quantitative analysis of bentonites with smectites as the dominant clay minerals.

The isotopic signature of soil water is influenced by mineral-water interaction. During the hydration of clay, different minerals deplete free water in heavy isotopes. The extracted soil water (dehydration water) gathered from clay-rich soils is generally more depleted in the heavy isotopes than the spike water, making results obtained for different soil types difficult to compare. Isotope effects observed at the mineral-water interface highlight potential explanations for eco-hydrological separation of water pools. Copyright © 2016 John Wiley & Sons, Ltd.

The boundary conditions of saponite formation are generally considered to be well known, but significant gaps in our knowledge persist in respect to the influence of solution chemistry, temperature, and reaction time on the mineralogy, structure, stability, and chemical composition of laboratory-grown ferrous saponite. In the present study, ferrous saponite and Mgsaponite were synthesized in Teflon-lined, stainless steel autoclaves at 60, 120 and 180°C, alkaline pH, reducing conditions, and initial solutions with molar Si:Fe:Mg ratios of 4:0:2, 4:1:1, 4:1.5:0.5, 4:1.75:0.25, and 4:1.82:0.18. The experimental solutions were prepared by dissolution of sodium orthosilicate (Na4SiO4), iron(II)sulfate (FeSO4·6H2O) and magnesium chloride salts (MgCl2·6H2O with ≤ 0.005 mass% of K and Ca) in 50 mL ultrapure water that contained 0.05% sodium dithionite as the reducing agent. The precipitates obtained at two, five and seven days of reaction time were investigated by X-ray diffraction techniques, transmission electron microscopy analysis, infra-red spectroscopy, and thermo-analytical methods.The precipitates were composed mainly of trioctahedral ferrous saponite, with small admixtures of co-precipitated brucite, opal-CT, and 2-line ferrihydrite, and nontronite as the probable alteration product of ferrous saponite. The compositions of the obtained ferrous saponites were highly variable, (Na0.44−0.59 K0.00−0.05 Ca0.00−0.02) (Fe2+0.37−2.41 Mg0.24−2.44 Fe3+0.00−0.28)Σ2.65−2.85 [(Fe3+0.00−0.37 Si3.63−4.00)O10](OH)2, but show similarities with naturally occurring trioctahedral Fe and Mg end members, except for the Al content. This suggests that a complete solid solution may exist in the Fe-Mg-saponite series.A conceptual reaction sequence for the formation of ferrous saponite is developed based on the experimental solution and solid compositions. Initially, at pH ≥ 10.4, brucite-type octahedral template sheets are formed, where dissolved Si-O tetrahedra are condensed. Subsequent reorganization of the octahedra and tetrahedra via multiple dissolution-precipitation processes finally results in the formation of saponite structures, together with brucite and partly amorphous silica. The extent of Fe2+ incorporation in the octahedral template sheets via isomorphic substitution is suggested to stabilize the saponite structure, explaining (i) the abundance of saponite enriched in VIFe2+ at elevated Fe supply and (ii) the effect of structural Fe on controlling the net formation rates of ferrous saponite.

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