Fluorine surface speciation in South Andean volcanic ashes

Abstract

Fresh volcanic ash samples collected during the eruptions of the Hudson (1991), Chaitén (2008), Puyehue (2011), Calbuco (2015) and Copahue (2016) volcanoes were analyzed in order to identify the fluorine (F) bearing phases. XPS and HR-TEM was used to determine the solid speciation of F. In order to decrease the surface differential charging of samples, XPS data have been collected using an electron flood gun during spectral acquisition. Ash leaching at different pH conditions was also performed with the aim of understanding the factors that control the fluoride release to aqueous reservoirs. The results indicates that in the studied Patagonian ashes, F is concentrated onto the ash surface, likely in the form of FNa and FCa salts but also within the glass lattice forming FSi and FAl bonds. Although the main F-source in the studied volcanic ashes is the aluminosilicate glass, this phase is stable at the neutral pH that dominates the aqueous reservoirs, thus the contribution to the pool of dissolved fluoride is minor. Higher contributions are associated with soluble fluoride salts. In addition, the presence of coatings of FCa compounds inhibits the release of fluoride within the volcanic glass lattice, at least during the initial stages of the ash-water interaction.