Unravelling the formation paths of amorphous hydroxyaluminosilicates and hydrous ferric silicates: Evidence from silicon isotopes

Silicate mineral weathering and the resulting formation of clay minerals are key processes at the Earth’s surface. Reverse weathering reactions regulate the evolution of ocean pH and chemistry, atmospheric carbon dioxide budget, soil formation and associated nutrient and element transfer within local and global biogeochemical cycles. A key element released during weathering processes is silicon (Si), that enters the biogeochemical cycle as dissolved silicic acid (Si(OH)₄ or DSi). During the transport of DSi through the hydro-, bio-, pedo- and lithosphere towards the ocean, DSi is involved in the formation of new silicate minerals (i.e., clay minerals) in particular in the critical zone (CZ). Here, DSi is frequently precipitating as gel-like, amorphous phases such as short range ordered hydroxyaluminosilicate phases (HAS: e.g., allophane) or as hydrous ferric silicates (HFS: e.g., hisingerite). These highly reactive minerals are known precursors to the formation of important soil clay minerals e.g., within the smectite group.

The individual reaction pathways and the environmental controls underlying HAS and HFS formation within the CZ are not yet well constrained. Si isotope fractionation throughout HAS and HFS formation is a key tool to decode and assess such enigmatic reaction mechanisms. Therefore, a series of allophane-hisingerite precipitation experiments has been performed to investigate Si isotope fractionation to potentially resolve mechanisms and conditions underlying the formation of HAS and HFS phases. Kinetic and equilibrium Si isotope fractionation between reactive fluid and solid phases are studied at high temporal resolution. Isotope exchange mechanisms are investigated using the three-isotope method, which is a novel proxy to trace the direction and the progress of low-temperature water-mineral/rock interactions.