Reverse weathering

Reverse weathering generally refers to the formation of a clay neoformation that utilizes cations and alkalinity in a process unrelated to the weathering of silicates. More specifically reverse weathering refers to the formation of authigenic clay minerals from the reaction of 1) biogenic silica with aqueous cations or cation bearing oxides or 2) cation poor precursor clays with dissolved cations or cation bearing oxides.^[1]

The reverse weathering process can involve many different anions and cations, but can be summarised in the following simplified reaction:

Biogenic silica (SiO₂) + metal hydroxides (Al(OH)₄⁻) + dissolved cations (K⁺, Mg²⁺, Li⁺, etc.) + bicarbonate (HCO₃) → clay minerals + H₂O + CO₂^[2]

The formation of authigenic clay minerals by reverse weathering is not fully understood. Much of the research done has been conducted in localized areas, such as the Amazon Delta,^[3] Mississippi Delta, a palaeo-delta in Aínsa-Sobrarbe (Pyrenees) and in the Ethiopian Rift lakes,^[4] making a global understanding of the process difficult. Much of the driving force behind research into reverse weathering stems from constraining the chemical mass balance between rivers and oceans.^[5] Prior to the discovery of reverse weathering, the model of the chemical mass balance of the ocean predicted higher alkali metal and bicarbonate (HCO₃⁻) concentrations than was observed.^[5] The formation of authigenic clay minerals was initially thought to account for the entirety of this excess, but the discovery of hydrothermal vents challenged this, as removal of alkali-alkaline earth metals and HCO₃⁻ from the ocean occurs in these locations as well.^[5]