X-ray photoelectron spectroscopy of Mt. Etna volcanic ashes

X-ray photoelectron spectroscopy has been employed for the chemical characterization of the surface of volcanic ashes erupted by Mt Etna during the 2013 activity. The data, coupled with bulk mineralogical and chemical analysis, showed the strong F and S enrichment and the depletion on Si, Ca, Na and K of the nanometric external shell. The surface chemical modification may be ascribed to the interaction in the eruptive plume of volcanic particles, mainly formed by glass, and volcanic gasses. The processes of dissolution caused by acid compounds are responsible of the depletion of alkaline and alkaline earth elements followed by the deposition on the surface of strongly soluble sulphate and fluoride as pointed out by the further depletion of these elements after leaching experiments. The presence in the plume of HF may cause the formation of volatile SiF4 and determine the consequent low Si abundance in the particle surface.