Surface acid alteration of juvenile Andean pyroclastic rocks: effect of glass composition on the development of surface fluoride-bearing minerals

During eruption, sulphur dioxide gas interacts with glass shards, favouring the formation of sulphuric acid (H₂SO₄) aerosols and coatings enriched in hazardous elements such as fluorine (F). This study discusses the formation of F-bearing compounds during acidic weathering of Andean juvenile pyroclastic rock emitted from Puyehue and Lanín volcanoes and their potential to release F into aqueous reservoirs. Surface chemical composition and solid speciation of Si, Al, and O were performed by XPS. Batch experiments were carried out to understand the release of F from S-bearing compounds.

When pyroclastic rocks are partially altered in H₂SO₄, a mixture of S-bearing compounds precipitates on the surface of the rock grains. These compounds precipitate as a result of the weathering process of pyroclastic rocks after hydrolysis of the aluminate and silicate groups. The composition of the precipitated compounds is influenced by the magmatic source. Consequently, mafic particles exhibit precipitated compounds enriched in Al, Mg, Ca, and Fe and are more altered compared to felsic particles. The hydrolysis process also promotes the release of F, which was originally associated with the aluminosilicate network. Once released, a proportion of the dissolved F either precipitates or re-adsorbed onto S-bearing compounds, eventually being transported into the environment.