International Geologiical Congress - Oslo 2008


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MGH-01 Earth and health - medical geology - Part 2


Formation of volcanic cristobalite: Implications for health hazards


Claire Horwell, Durham University, Institute of Hazard & Risk Research (United Kingdom)
Ben Williamson, Camborne School of Mines, University of Exeter (United Kingdom)
Jennifer Le Blond, University of Cambridge (United Kingdom)


Volcanic cristobalite is found in abundance within the Soufrière Hills, Montserrat volcanic ash. It is formed during, and post-emplacement of the lava dome within the crater. The concentration and grain size of the cristobalite in the volcanic ash has led to concern over its potential health hazard. This detailed study of the form, composition, quantity and size of the cristobalite crystals within the dome rock highlights the possibility that the cristobalite may not be as toxic as expected because its composition is impure. XRD results show that up to 29 % of the bulk rock composition is cristobalite. The cristobalite in the Soufrière Hills dome is found in two forms - as euhedral crystals and as platey crystals, both of which are likely to have formed by vapour-phase deposition of silica. This is the first time that platey cristobalite has been documented. The crystals grow within amygdales, often completely filling the vesicles and cracks in the dome rock, thereby significantly decreasing its porosity. We found little evidence of cristobalite formed by devitrification of volcanic glass, although we did observe quartz as a devitrification product in old samples. Electron microprobe analysis shows that the cristobalite is impure, with Si having being substituted with cations, predominantly Al2O3 (1-1.5 wt. %). Raman spectrometry within an SEM allowed confirmation of the polymorphic composition of individual crystals. The results have bearing for the ongoing discussion on the potential toxicity of volcanic ash. The impure nature of the cristobalite may act to inhibit its toxicity in the lung as Al is known to inhibit silica toxicity. The cristobalite crystals have undergone phase transition from beta to alpha types. The resulting volume change cracks may act as fracture points during fragmentation of the dome rock within pyroclastic flows, thereby providing a mechanism for the formation of respirable particles.


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