A.M. Jaunet

Mineral micro- and nano-variability revealed by combined micromorphology and in situ submicroscopy

Abstract A combination of micromorphology and different in situ sub microscopical techniques, performed on thin sections of soil, saprolite, and rock samples, was used to assess possible relationships between micro- and nano-variability and macro- and micro-environmental conditions of mineral transformation during weathering. Four examples, presented in this study illustrate variability in morphology, chemistry and mineralogy of secondary minerals present in pseudomorphs after primary silicate minerals in volcanic soils present in the humid tropics. The study was based on micromorphological characterization of secondary (clay) minerals, with complementary submicroscopical analysis such as scanning electron microscopy with energy dispersive X-ray analysis (SEM–EDXRA), performed on uncovered thin sections, step scan X-ray diffraction (SSXRD) and transmission electron diffraction (TEM–EDS) carried out either on disturbed powder samples or undisturbed microsamples, obtained by micro-drilling in thin sections. These techniques highlight a very complex mineralogy in pseudomorphs after primary minerals, including secondary minerals that generally are assessed to be formed under contrasting physicochemical conditions (gibbsite and 2:1 phyllosilicates in a pseudomorph after plagioclase; halloysite, kaolinite, smectite and aluminum and iron oxi-hydroxides in pseudomorphs after pyroxenes). The study by TEM of undisturbed microfabrics of secondary minerals in a clay pseudomorph after pyroxene revealed typical distribution patterns, with mineralogical compositions varying over some microns. The combination of techniques gave an improved insight on the complexity and variability of the nature composition and distribution of secondary minerals formed during weathering. It indicates the influence of sequential weathering processes on mineral micro- and nano-variability.

Micromorphology and submicroscopy of isotropic and anisotropic Al/Si coatings in a Quaternary Allier terrace, (France)

Abstract Jongmans, A.G, Oort, van F., Buurman, P. and Jaunet, A.M. 1994. Micromorphology and submicroscopy of isotropic and anisotropic Al/Si coatings in a Quaternary Allier terrace, (France). In: A.J. Ringrose-Voase and G.S. Humphreys (Editors), Soil Micromorphology: Studies in Management and Genesis. Proc. IX Int. Working Meeting on Soil Micromorphology, Townsville, Australia, July 1992. Developments in Soil Science 22, Elsevier, Amsterdam, pp. 285–291.

Mineralogy and structural organization of a red to yellow soil sequence in Cuba ― relationships with soil properties

Abstract A ferrallitic soil sequence from red to yellow developed on carbonate rocks in Cuba is studied. Clay content is very high (80 to 90 %) and composition differs mainly in the relative proportion of hematite and goethite. The main modification concerns the organization of constituents. Kaolinite crystallites are very small and independent in yellow soil resulting in a massive fabric. These crystallites are associated in red soil giving one μm aggregates and micropeds (500 μm). Higher Al content (TEM + microprobe) suggests that the cement is more an Al than an Fe coating. Soil compaction and consequent waterlogging are likely to be responsible for such a soil structure evolution. CEC, specific surface and porosity are different in the two kinds of soils and then all the physical properties.