Fluorine surface speciation in South Andean volcanic ashes

ZAMPIERI, G.; BIA, G.; GARCIA, M.G.; BORGNINO, L.

CHEMICAL GEOLOGY

ELSEVIER SCIENCE BV

Lugar: Amsterdam; Año: 2020 vol. 532 p. 1 - 15

0009-2541

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 surfacedifferential charging of samples, XPS data have been collected using an electron flood gun during spectralacquisition. Ash leaching at different pH conditions was also performed with the aim of understanding thefactors that control the fluoride release to aqueous reservoirs. The results indicates that in the studied Patagonianashes, F is concentrated onto the ash surface, likely in the form of FeNa and FeCa salts but also within the glasslattice forming FeSi and FeAl bonds. Although the main F-source in the studied volcanic ashes is the aluminosilicateglass, this phase is stable at the neutral pH that dominates the aqueous reservoirs, thus the contributionto the pool of dissolved fluoride is minor. Higher contributions are associated with soluble fluoridesalts. In addition, the presence of coatings of FeCa compounds inhibits the release of fluoride within the volcanicglass lattice, at least during the initial stages of the ash-water interaction.