Alain Decarreau

Partitioning of divalent transition elements between octahedral sheets of trioctahedral smectites and water

Using trioctahedral smectites synthesized at low temperature (25 and 75°C). partition coefficients have been determined for M2+ transition metals (Mn, Fe, Co, Ni, Cu, Zn) between octahedral sheets of smectites and water. These coefficients D(M2+−Mg) = (M2+)/(Mg) solid/(M2+)/(Mg) liquid have high values near 104 for Cu, 1000 for Ni, Co, Zn, 300 for Fe and 30 for Mn. All transition metals are strongly stabilized in the magnesian solid phase, even Mn which leads to noncrystallized products. Within the range of experimental uncertainties, it is found that tetrahedral substitution of Si by Al and differences in temperature (from 25 to 75°C) have no influence on partition coefficients. Experimental data are closely related to thermodynamic properties of the cations and on this basis other partition coefficients can be calculated, for the (M2+ − Fe2+) pair for instance. The behaviour of transition metals is explained using crystal field theory.

Crystal-chemistry of talc: A near infrared (NIR) spectroscopy study

Abstract The crystal-chemistry of fifteen samples of talc from various localities and origins having a relatively wide range of Fe, Al, and F contents, have been studied mainly by Fourier-transform infrared (FTIR) spectroscopy, in both the near infrared (NIR) and middle infrared (MIR) regions, and by Mössbauer spectroscopy. For the first time, assignments have been made for the 2νOH bands in talc, and Mg/Al substitutions were revealed using NIR. Less than 0.01 atom of Fe2+ or Al per half unit cell can be detected from the 2νOH region, whereas the sensitivity is about half in the νOH region. The amounts of F also can be quantified by NIR measurement, using the F inductive effect on the width of the 2νMg3OH band. The NIR data show that Fe2+ and Mg are distributed randomly in their octahedral sheet. Actual structural formulae of natural talcs can be obtained from NIR and Mössbauer spectroscopies only, even for impure talc samples. NIR spectroscopy is simple to use and is a very powerful tool to study the crystal chemistry of talc.

Infrared evidence of dioctahedral-trioctahedral site occupancy in palygorskite

A Mg-rich palygorskite sample from phosphorite deposits of Ganntour (Morocco) with the structural formula

INFLUENCE OF SYNTHESIS pH ON KAOLINITE “CRYSTALLINITY” AND SURFACE PROPERTIES

{\rm{S}}{{\rm{i}}_8}\left( {{\rm{M}}{{\rm{g}}_{2.6}}{\rm{A}}{{\rm{l}}_{1.19}}{\rm{Fe}}_{{\rm{0}}{\rm{.33}}}^{{\rm{III}}}{\square_{0.88}}} \right){\rm{C}}{{\rm{a}}_{0.056}}{\rm{N}}{{\rm{a}}_{0.024}}{{\rm{K}}_{0.104}}{{\rm{O}}_{20}}{\left( {{\rm{OH}}} \right)_2}{\left( {{\rm{O}}{{\rm{H}}_2}} \right)_{4 \cdot }}4{{\rm{H}}_{\rm{2}}}{\rm{O}}

HYDROTHERMAL SYNTHESIS, BETWEEN 75 AND 150°C, OF HIGH-CHARGE, FERRIC NONTRONITES

Si8(Mg2.6Al1.19Fe0.33III□0.88)Ca0.056Na0.024K0.104O20(OH)2(OH2)4⋅4H2O, was studied by FTIR spectroscopy. In both OH-stretching and OH-bending regions, there is evidence of dioctahedral Al2□OH, AlFe□OH and trioctahedral Mg3OH features, leading to a di-trioctahedral crystallochemical model of octahedral site occupancies in ribbons of Ganntour palygorskite.This model, established through the IR spectroscopy study of a Mg-rich palygorskite, seems to be appropriate for many other palygorskites with lower Mg content in the octahedral sheet.

SYNTHESIS OF KAOLINITE WITH A HIGH LEVEL OF Fe3+ FOR Al SUBSTITUTION

Numerous 2:1 clay minerals were synthesized between 100 and 250°C. For each clay mineral, octahedral sheets were occupied by two different cations A and B, the atomic ratio R=A(A+B) varying from zero to one. Synthetic clays were studied by XRD and IR Spectroscopy with a view to determine the actual distribution of the two cations. Random distribution of octahedral cations is rarely observed, even for homovalent substitutions. In this case similarities in chemical properties (electronegativities for example) appear more important than similarities in ionic radii. For one (Al-Fe)-beidellite, with a R value of 0.5, I.R. data suggest an ordered distribution of the two cations. Segregation of octahedral cations is very common and leads either to clusters within octahedral sheets [kerolites and (Ni-Mg)-talcs, (Fe3+Mg)-beidellites] or to an immiscibility of two different phases (Al-beidellite and Mg-saponite). So smectites with intermediate chemical compositions between di and trioctahedral end-members cannot be considered as homogeneous and random solid-solutions.

Accurate crystal chemistry of ferric smectites from the lateritic nickel ore of Murrin Murrin (Western Australia). I. XRD and multi-scale chemical approaches

Hydrothermal syntheses were performed at various pH values and temperatures to induce variability in kaolinite defect density. Temperature of synthesis ranged from 200 to 240°C, for 21 d. Initial pH at room temperature ranged from 0.5 to 14. The starting material was a hydrothermally treated gel, with an atomic Si/Al ratio of 0.93, partly transformed into kaolinite.Kaolinite was obtained for a wide range of pH. Although no influence of temperature on “crystallinity” (i.e., defect density) was observed, the effect of pH was important. A continuous series was obtained from a low-defect kaolinite, with high thermal stability and a hexagonal morphology for the most acidic final pH, to a high-defect kaolinite, with low thermal stability and lath shape for the most basic final pH. These variations of kaolinite properties appear related to the pH dependence of kaolinite surface speciation. Increasing pH value results in increased cation adsorption on the kaolinite external surfaces and increases in the elongation of particles.

Hematite and goethite from duricrusts developed by lateritic chemical weathering of Precambrian banded iron formations, Minas Gerais, Brazil

Abstract The IR spectra of NH4+-saturated smectites were examined in terms of their charge characteristics. The ν4 NH4+ band near 1440 cm-1, observed in the DRIFTS spectra (obtained without use of a KBr matrix), was assigned to the vibrations of NH4+ ions compensating the negative charge of the clays. When KBr was used as a diluting matrix, the ν4 NH4+ band was located at 1400 and/or 1440 cm-1. The band at 1400 cm-1, related to NH4Br, originated from the replacement of NH4+ in the clay by K+ from the KBr. For swelling clay minerals this band indicates that layers have permanent low charge density and/or variable charge. For non-swelling clay minerals, the 1400 cm-1 band characterizes the presence of variable charges only. The ν4 NH4+ band at 1440 cm-1 suggests that NH4+ in the clay was not replaced by K+ from KBr and remains in the interlayer space of the clay minerals. This absorption is due to NH4+ compensating only permanent charge in the interlayers, or part of the interlayers with a high charge density. The presence of both bands at 1400 cm-1 and 1440 cm-1 in the IR spectrum suggests that the clays studied have a heterogeneous interlayer charge.