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Structure and distribution of allophanes, imogolite and proto-imogolite in volcanic soils

Levard C., Doelsch E., Basile-Doelsch I., Abidin Z., Miche H., Masion A., Rose J., Borschneck D., Bottero J.Y.. 2012. Geoderma, 183-184 : p. 183-184.

DOI: 10.1016/j.geoderma.2012.03.015

Despite half a century of intensive research, the structure and presence of short-range ordered aluminosilicates in volcanic soils is still the focus of debate. Imogolite is easily distinguishable because of its tubular structure, whereas allophane compounds-usually described as spheres-are harder to identify, especially because of their variable structure and occurrence patterns. In addition, the local structure of allophanes can be very similar to that of proto-imogolite (imogolite precursor). Strangely, this similarity is seldom considered in most characterization studies. In this context, our study focuses on the structure of two Al-rich short range-ordered aluminosilicates of two different origins, from: (i) an Andosol B horizon (Andosol sample); and (ii) a weathered pumice grain (pumice sample). These natural samples were compared to a synthetic proto-imogolite. The three samples were analyzed using experimental tools that are commonly used for the identification of these nanophases (chemical composition, X-ray diffraction, nuclear magnetic resonance, Fourier transform infrared spectroscopy and transmission electron microscopy). The three samples exhibited the same local structure, but significant differences were observed at a larger scale. The pumice sample clearly showed ring-shaped particles, while the Andosol sample and the synthetic proto-imogolite were amorphous. Our results suggest that poorly ordered proto-imogolite, rather than Al-rich allophones and imogolites, is present in Andosol horizons. We believe that the mineralogy of these phases needs to be reassessed along with their growth mechanisms.

Mots-clés : sol volcanique; propriété physicochimique du sol; structure chimique; silicate; andosol; rayon x; spectroscopie infrarouge; spectroscopie rmn; réunion; france; nanoparticule

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