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Formation of cristobalite nanofibers during explosive volcanic eruptions

¹Departamento de Geología, Universidad de Chile, Plaza Ercilla 803, Santiago, Chile
²Departamento de Ingeniería Mecánica, Universidad de Chile, Avenida Beauchef 850, Torre Central, Santiago, Chile
³Department of Earth Sciences, University of Bristol, Wills Memorial Building, Queen's Road, Bristol BS8 1RJ, UK
⁴Departamento de Geofísica, Universidad de Chile, Blanco Encalada 2002, Santiago, Chile

Correspondence: *E-mail: mreich{at}ing.uchile.cl.

High-resolution transmission electron microscopy (HRTEM) observations of unaltered volcanic air-fall deposits from the ongoing lava dome explosive eruption at Chaitén Volcano, Chilean Patagonia, revealed the presence of highly crystalline silica nanofibers in the respirable fraction of the volcanic ash (<4 µm). The nanofibers are identified as the high-temperature (>240 °C), beta form of cristobalite, with average lengths of hundreds of nanometers and widths on the order of tens of nanometers. We propose that the beta-cristobalite nanofibers are formed during explosive eruptions by the reduction of amorphous silica by carbon monoxide to its reactive suboxide SiO, which is later oxidized to form one-dimensional crystalline silica nanostructures. Nucleation and growth of the nanofibers are enhanced by the high surface area of the micrometer- to nanometer-sized fragments of silica glass in the volcanic column. The formation of nanocrystalline cristobalite fibers during explosive lava dome eruptions poses new challenges for the assessment of the short- and long-term health hazards associated with the respirable nanofibrous components of volcanic ash.