Sabine Petit

Halloysite clay minerals - A review

Abstract Halloysite clay minerals are ubiquitous in soils and weathered rocks where they occur in a variety of particle shapes and hydration states. Diversity also characterizes their chemical composition, cation exchange capacity and potassium selectivity. This review summarizes the extensive but scattered literature on halloysite, from its natural occurrence, through its crystal structure, chemical and morphological diversity, to its reactivity toward organic compounds, ions and salts, involving the various methods of differentiating halloysite from kaolinite. No unique test seems to be ideal to distinguish these 1:1 clay minerals, especially in soils. The occurrence of 2:1 phyllosilicate contaminants appears, so far, to provide the best explanation for the high charge and potassium selectivity of halloysite. Yet, hydration properties of the mineral probably play a major role in ion sorption. Clear trends seem to relate particle morphology and structural Fe. However, future work is required to understand the possible mechanisms linking chemical, morphological, hydration and charge properties of halloysite.

Talc as nucleating agent of polypropylene: morphology induced by lamellar particles addition and interface mineral-matrix modelization

The efficiency of talc used as nucleating agent (0.5% by weight) in polypropylene (PP) was determined taking into account the particle size d50, particle morphology and BET specific surface areas. These findings were compared to a mineral with similar properties, pyrophyllite. Talc samples with the finest particle sizes induce a significant increase in the starting crystallization temperature of PP and irrespective of the particle size d50, pyrophyllite was found to be less efficient than talc. X-Ray results show that PP oriented crystallization due to talc or pyrophyllite addition, corresponds to an epitaxial growth whereby the mineral c*-axis is merged with the PP b*-axis. Microscopic observations revealed that in the presence of talc, nuclei density of PP increased strongly. In addition, a large number of nuclei was observed to appear everywhere on the talc surface. A PP-talc interface model is proposed by matching the (001) talc plane and the (010) PP plane. In this model, 3% of PP cell accommodation on talc is necessary with a 15° angle between PP chains elongation and the crystallographic directions of talc. Hexagonal rings on talc surface are believed to represent hydrogen bonds with PP methyl groupings. This fine structure relation between talc and PP is discussed, and is used to characterize the differences observed between the efficiency of talc and that of pyrophyllite.

Characterization of octahedral substitutions in kaolinites using near infrared spectroscopy

Fourier transform infrared (FTIR) spectroscopy investigations in the near infrared (N1R) region of synthetic and natural kaolinites with various octahedral substitutions have been carried out in order to elucidate the relationships between the substituted cations and specific features of the NIR spectra. The combination modes of the OH stretching and bending vibrations characterizing Fe(III), Ga(III) and Cr(III) octahedral substitutions are identified in the NIR region at 4466, 4498 and 4474 cm-1, respectively, and the first overtones of the OH stretching vibrations at 7018, 7018 and 6986 cm-1, respectively. As far as we know, the bands of kaolinites containing Ga(III) or Cr(III) have not been reported yet. For both Ga(III) and Cr(III), the NIR observations explain why the bending vibration bands of AlGaOH and AlCrOH groups are not observed in the middle infrared (MIR) region.

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

Characterization of hydroxy-interlayered vermiculite and illite/smectite interstatified minerals from the weathering of chlorite in a cryorthod

{\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}}

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

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.

Pedogenic formation of kaolinite-smectite mixed layers in a soil toposequence developed from basaltic parent material in Sardinia (Italy)

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.