11 12 Because of their large surface area and reactivity, nanometric-size soil mineral phases 13 have a high potential for soil organic matter stabilization, contaminant sorption or soil 14 aggregation. In the literature, Fe and Al phases have been the main targets of batch-15 synthesized nanomineral studies while nano-aluminosilicates (Al and Si phases) have 16 been mainly studied in Andic soils. In the present work, we synthesized secondary 17 nanophases of Fe, Al and Si. To simulate a system as close as possible to soil conditions, 18 we conducted laboratory simulations of the processes of (1) biotite alteration in acidic 19 conditions producing a Al Si Fe Mg K leachate solution and (2) the following 20 neoformation of secondary nanophases by titrating the leachate solution to pH 4.2, 5 21 and 7. The morphology of the nanophases, their size, crystallinity and chemistry were 22 characterized by TEM-EDX on single particles and their local atomic structure by EXAFS 23 (Extended X-ray Absorption Fine Structure) at the Fe absorption K-edge. The main 24 nanophases formed were amorphous particles 10 to 60 nm in size whose composition 25 (dominated by Fe and Si) was strongly controlled by the pH conditions at the end of the 26 titration. At pH 4.2 and pH 7, the structure of the nanophases was dominated by the 27 polymerization of Fe, which was hindered by Al, Si, Mg and K. Conversely, at pH 5, the 28 polymerization of Fe was counteracted by precipitation of high amounts of Si. The 29 synthetized nanophases were estimated to be rather analogous to nanophases formed in 30 natural biotite-bearing soils. Because of their small size and potential high surface 31 *Manuscript 2 reactivity, the adsorption capacities of these nanophases with respect to the OM should 32 be revisited in the framework of soil C storage. 33