Method Mineralogy FR

Laterites in the broad sense are weathering profiles with a typically simple mineralogy including mainly kaolinite, iron oxides and oxyhydroxides, possibly aluminum hydroxides and residual minerals from the parent rock such as quartz. The main processes of silicates weathering are known and vary as a function of water drainage and loss of ions in solution.

For instance, weathering of a primary mineral (feldspar) can be written for the monosiallitization process: 2 KAlSi3O8 + 9 H2O + 2 H+ Si2Al2O5(OH)4 + 4 H4SiO4 + 2K+

Feldspar Solution Kaolinite Silicic acid Free ions

However, the assemblages at microscopic scale are complex because the secondary minerals are finely divided and the profile is multiphased, hindering a full understanding of its evolution. One way to unravel the conditions of formation of secondary minerals is to reveal and date various generations of the same mineral (kaolinite, iron oxide) in the weathering profile.
In the RECA program, classical tools are first used to determine the nature of mineral phases in the profile, such as standard powder X-ray diffraction, optical microscopy and Scanning electron Microscopy (SEM). In the studied profiles from the Amazon Basin, these methods confirm the presence of kaolinite as a major phase in addition to iron oxides that are minor in loose horizons and dominant in Fe duricrusts.
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SEM micrograph of samples from loose horizons of a weathering profile near Manaus: a) topsoil (badly ordered kaolinites), b) saprolite (well-ordered kaolinites) (Credit photo: C. Ansart)
In addition, rotating anode X-ray diffraction is used in combination with Rietveld refinement to quantify iron oxides such as hematite / goethite and characterize them in terms of mean sizes of coherent domains and substituted aluminum content on micrometric single grains. The same undisturbed grains are subsequently used for (U-Th)/He dating, the mineral characterization being helpful for discussing the ages. This method allowed us to determine the Hem/(Goe+Hem) ratio and relate the age to the Al content of iron oxides of a Fe duricrusts in duricrusts from French Guiana.
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Scheme of rotating anode X-ray diffraction
Spectroscopies are also used to reveal different conditions of formation of kaolinite populations on small amounts of matter (mg to tens of mg), according to proxies of the structural order-disorder. Fourier transform infrared spectroscopy allows the measurement of OH stretching modes that are sensitive to stacking faults at the molecular scale. A profile near Manaus, developped on the sedimentary Alter do Chao formation, exhibits a clear trend with well-ordered kaolinite at the bottom (saprolite) to badly ordered kaolinite in the topsoil. These results are fully in line with EPR of the structural iron that is used as a probe of local crystalline disorder. They are also consistent with the stacking disorder assessed from X-ray diffraction (long-range disorder).
The mineralogy of secondary minerals thus evidences several conditions of formation (within or on several horizons), which will be meaningful for discussing their ages determined by (U-Th)/He (iron oxides) and EPR (kaolinites) dating, in order to reconstruct the evolution of the weathering profiles in combination with isotopic data.
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