Search

The tropical weathering of sedimentary kaolin deposits from the plateaux surrounding Manaus (Alter do Chao formation, Amazon basin, Brazil) leads to the in situformation of thick kaolinitic soils. The structural changes of kaolinite have been investigated quantitatively by infrared spectroscopy and electron paramagnetic resonance. Both techniques consistently show that each sample contains two types of kaolinite in various proportions. The progressive decrease in kaolinite order from the bottom to the top of the profile results from the gradual replacement of an old population of well-ordered kaolinite, typical of the underlying sedimentary kaolin, by a more recent generation of poorly ordered soil kaolinite. The vertical pattern of kaolinite replacement differs from that of the transformation of Fe oxides and oxyhydroxides previously observed in the same profile. The inherited fraction of well-ordered kaolinite ranges from 60% at a depth of 9 m to 30% in the upper levels of the soil. The persistence of sedimentary kaolinite in the upper horizons suggests that the rate of kaolinite transformation is relatively slow at the time scale of lateritic soil formation. Kaolinite inheritance unlocks the lateritic record of past weathering conditions.

A method is proposed to measure the absolute concentration of paramagnetic Fe3+ions in kaolinite from various geochemical environments using powder X-band electron paramagnetic resonance (EPR) data. An Fe3+-doped corundum sample is used as a concentration standard. The Fe3+signal is calibrated by calculating the powder EPR spectra of Fe3+ions in corundum and low-defect kaolinite. The paramagnetic Fe3+concentration in other samples is obtained by an extrapolation procedure. This study provides a direct assessment of the iron distribution between isolated structural Fe3+ions and other iron species, such as Fe3+concentrated phases and Fe2+ions. The concentration of isolated structural Fe3+ranges between 200–3000 ppm and represents less than half of the total iron within kaolinite crystals.

Structural Fe3+in kaolinites and dickites covering a broad range of disorder was investigated using electron paramagnetic resonance (EPR) spectroscopy at both the X and Q-band frequencies. A procedure based on a numerical diagonalization of the spin Hamiltonian was used to accurately determine the second and fourth-order fine-structure parameters. A least-squaresfitting method was also developed to model the EPR spectra of Fe3+ions in disordered local environments, including multimodal site-to-site distributions. Satisfactory fits between calculated and observed X and Q-band spectra were obtained regardless of the stacking order of the samples. In well-ordered kaolinite, Fe3+ions are equally substituted in sites of axial symmetry (Fe(II)sites, namely Fe(II)aand Fe(II)b) which were determined to be the two non-equivalent Al1 and Al2 sites of the kaolinite structure. In dickite, Fe3+ions were also found to be equally substituted for Al3+in the two non-equivalent Al sites of the dickite structure. In poorly ordered kaolinites, the distribution of the fine-structure parameters indicates that Fe3+ions are distributed between Fe(II)sites and other sites with the symmetry of the dickite sites. Hence, when stacking disorder prevails over local perturbations of the structure, the near isotropic resonance owing to Fe3+ions in rhombically distorted sites (Fe(I)sites) is a diagnostic feature for the occurrence of C-layers in the kaolinite structure, where C refers to a specific distribution of vacant octahedral sites in successive layers.

A new appraisal of radiation-induced defects (RID) in natural kaolinite, i.e., positive trapped holes on oxygen atoms, has been undertaken using Q-band EPR spectra, recorded at 93 K, of irradiated annealed and oriented kaolinite samples originating from various environments. Three different centers were identified. Two of the centers, A- and A’-centers, are trapped holes on oxygen from Si-O bonds. They have a distinct signature and orthogonal orientation, i.e., perpendicular and parallel to the (ab) plane, respectively. The third center, the B-center, is a hole trapped on the oxygen bonding Al in adjacent octahedral positions (AlVI-O−-AlVIbridge). This confirmed some previous assignments from the literature, some others are no longer considered as valid. A least squares fitting procedure is proposed to assess the RID concentration in any kaolinite. It allows a quantitative approach of the thermal stability of RID. Isochronal annealing shows that the thermal stability of the centers decreases in the order A, A′, B over the temperature range 0–450°C: (1) B-center is completely annealed above 300°C; (2) A′-center can be annealed by heating at 400°C for more than two hours; (3) A-center is stable up to 450°C. The activation energy and the magnitude of the mean half-life for A-center is evaluated through isothermal annealing at 350, 375 and 400°C, with Ea= 2.0 eV ± 0.2, and t½> 1012years at 300 K. The stability of A-center seems to decrease with increasing crystalline disorder. Nevertheless, it is high enough for radiation dosimetry using kaolinites from any environment on the Earth’s surface.

The substitution of Fe3+in the kaolinite structure is studied by EPR spectrometry and by FTIR spectrometry on a large set of kaolins from different origins (sedimentary and primary ores, soil kaolins). The IR bands at 3598 and 875 cm−1, observed in the literature only in the case of disordered kaolins or in Fe-rich environments (synthetic, lateritic), are revealed by high-resolution IR analysis, whatever the origin and the total Fe content of the samples. The EPR bands corresponding to Fe3+substituted in sites II of the octahedral sheet increase when the IR absorbance near 3600 cm−1increases. Two IR absorption bands near 4465 cm−1and 7025 cm−1are observed for the first time, both in transmission and diffuse reflectance on all samples. These bands are assigned to the combination of the 3598 and 875 cm−1bands and to the first harmonic of the band at 3598 cm−1, respectively. The area of the band at 4465 cm−1in diffuse reflectance is quantitatively correlated to the abundance of Fe3+located in centers II as measured by ESR. This directly confirms the assignment of the two IR bands at 3598 and 875 cm−1to OH stretching and deformation vibration bands in octahedral FE3+environment in the kaolinite structure, respectively. Effects due to the size of particles and to the main kaolins impurities on the near infrared spectra, are also discussed.