Influence of anions on the formation of a passivating Fe-rich interfacial layer during olivine dissolution

To better understand the rate and mechanism of olivine dissolution under acidic conditions, we exposed mm-sized ‘Norwegian olivine’ pebbles to hydrochloric, nitric and sulfuric acid at 65 °C and varying pH, for up to 50 days and monitored the evolution of Mg, Fe, and Si concentrations as well as pH. The evolution of the surface of the pebbles was examined ex-situ with Confocal Raman microscopy, Scanning Electron Microscopy (SEM), energy-dispersive X-ray Spectroscopy (EDX) and X-ray Photoelectron Spectroscopy (XPS). For initial pH values of 1 and 2, we observed distinct anion effects. Most notably, initial olivine dissolution was fastest in sulfuric acid. When the pH of the solution gradually increased to values beyond 3, a visible surface alteration layer was formed and dissolution stopped. Raman spectra, SEM-EDX, and XPS consistently demonstrate that the surface layer, which is most prominent in sulfuric and least prominent in hydrochloric acid, is enriched in iron oxides. XPS data show a co-existence of Fe2+ and Fe3+ species. Raman spectra suggest initial precipitation of goethite and magnetite followed by hematite at later stages of dissolution. We propose that the dissolution process is governed by an initial dissolution of Fe2+ followed by (partial) oxidation in solution to Fe3+ and subsequent re-precipitation of Fe-(oxy)hydroxides that slow down further dissolution.