Probing electrolytes’ impact on interfacial layers that influence olivine dissolution

Olivine (magnesium iron silicate) is a prime contender for CO2 mineralization in aqueous environments via natural weathering or industrial processes. However, upon its dissolution, a silica-rich alteration layer (SAL) gets formed, which can drastically slow down the chemical reactions at the mineral-electrolyte interface. SALs properties are linked to their evolving porosity, thickness and chemical composition. However, the way these give raise to passivation remains to be clarified.
To get more insights on olivine SALs, we used Confocal Raman (CRM), Scanning Electron (SEM), and Fluorescence Lifetime Imaging (FLIM) Microscopies. We monitored the dissolution kinetics and SAL formation at the same initial pH across different acids. With CRM we observed the formation of a µm-sized SAL when olivine is dissolved in H2SO4, while no SAL formation was detected when using HCl. SAL Raman spectra evidenced a serpentine-like silicate structure with contributions from Iron (II, III) oxides, indicating that the oxidizing characteristics of sulphate anions are crucial in the formation of this SAL. This evidence was then compared with SEM and FLIM data to evaluate the interplay between dissolution kinetics and SALs formation and properties. Our findings underscore the significant influence of electrolyte chemistry on the alteration layer's composition and growth.