Weathering and allophane neoformation in soils developed on volcanic ash in the Azores

International audienceOn Faial and Pico islands (Azores), we studied two profiles on basaltic pyroclasts that contain buried horizons, and we focussed on petrography, micropedology and mineralogy. Emphasis was given to weathering of the lapilli and ashes, and the neoformation of allophane. A combination of optical studies, in situ chemical analyses, X‐ray diffraction and infrared spectroscopy of clay fractions revealed that allophane is present both in the micromass of the groundmass, in alteromorphs after lapilli or pumice, and in clay coatings. Whereas most studies describe allophane as a colloidal fraction formed by the congruent and total dissolution of the ashes, this study shows evidence for the formation of allophane alteromorphs, due to leaching of Si and cations, with preservation of the original shapes of the tephra. The allophane alteromorphs often display optical characteristics that resemble those of palagonite. Increasing alteration is observed through three steps: (i) hydration of the glass associated with strong cation and Si leaching, (ii) allophane hypocoatings, and (iii) allophane alteromorphs with development of intragrain bridges. The chemical signature of the alteromorphs varies from a pure alumino‐silicate at one extreme to an Fe(Ti) enriched alumino‐silicate at the other. Between those two extremes, the colour grades from yellow to dark orange, with microzonations. An Al‐rich allophane composition is associated with gibbsite in the EUR6‐Pico profile, whereas at the base of the EUR5‐Faial profile, Si‐rich allophane is associated with halloysite. Some variations of the palaeo‐environment are also suggested by strong iron segregation observed with various secondary phases (ferrihydrite, haematite, iddingsite)