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.