Presentation on theme: "UK Dust Network – 1 st Workshop Claire Horwell 24th May 2007 Cutting edge techniques for the analysis of volcanic ash and other natural particles."— Presentation transcript:
UK Dust Network – 1 st Workshop Claire Horwell 24th May 2007 Cutting edge techniques for the analysis of volcanic ash and other natural particles
Objectives: To use mineralogy and geochemistry to make rapid assessments of the potential health hazard of volcanic ash (and other natural dusts). To understand WHY a mineral or dust may trigger a pathogenic respiratory response.
Questions to be answered: Is the dust small enough to enter the lungs? What is the composition of the dust? Is the surface of the dust reactive? Are individual particles ‘pure’? Other ideas?
Grain size Is the dust small enough to enter the lungs? Grain size analysis techniques: –Laser diffraction 70 analyses from around the world –SEM with image analysis –Sieving > 63 m only New predictive technique Malvern Mastersizer 2000 Horwell, 2007, accepted for publication
Composition of heterogeneous dusts SEM image of volcanic ash Volcanic ash is often composed of tens of minerals. Some are considered toxic e.g. crystalline silica. Analytical techniques: –SEM-EDX gives individual particle compositions but not polymorphs. –XRD-PSD gives quantity of minerals in a bulk sample. High res. so no overlap between plagioclase and cristobalite. –Raman-SEM allows polymorphic determination of individual crystals/particles.
Reactivity of surfaces Electron Spin Resonance detects free and surface radicals. Radicals formed by breaking bonds during fragmentation Radicals are highly reactive, damaging DNA, proteins, lipids etc. Likely to be one of several triggering mechanisms for chronic lung disease.
Production of silica surface radicals Horwell et al. Environmental Research, 2003 Soufrière Hills dome-collapse ash shows no generation of silica radicals (peaks expected at point A). Distinctive curve and peak (at point B) shows interaction of iron. Crystalline silica alone (Talvitie residue) has less iron but no significant generation of silica radicals.
Production of hydroxyl radicals: Fenton Reaction: Fe 2+ + H 2 O 2 Fe 3+ + OH - + HO Horwell et al. Environmental Research, 2003
Production of hydroxyl radicals Basaltic Andesitic/ Dacitic Tephritic/ Phonolitic Minusil 5 Quartz standard Horwell, Fenoglio & Fubini, in review
Purity of crystalline silica One could say that if the ash is respirable and contains x-silica then it is a potential health hazard. BUT toxicological and epidemiological evidence appears to suggest that volcanic silica isn’t very toxic. We can use mineralogy to determine WHY x-silica is/ is not toxic. The problem: Difficult to analyse differences in composition at the nano- scale. Timeliness: New technology for high resolution micro-analysis e.g. TEM-EDX, FIB thinning etc.
How pure is volcanic x-silica? SEM-EDX spectrum of cristobalite 1 mm Crystalline silica in volcanic ash may be modified by more-inert components. E.g. it is known that Al ameliorates toxicity. Evidence: SEM-EDX work indicates that silica particles are impure. The silica particles may be modified by: –occlusion by glass –intergrowth with glass or plagioclase –substitution of Si from atomic structure by Al & Na.
Early results – dome rock Cristobalite in dome rock vugh 1 mm –In dome rock we see euhedral and platey crystals which have grown in cracks and vesicles by vapour-phase deposition. –Raman-SEM confirms these are cristobalite. Horwell, Williamson & Le Blond, in prep. Raman spectra from cristobalite in dome rock
Early results – dome rock 1 mm Electron microprobe shows that the cristobalite is compositionally distinct from volcanic quartz, containing impurities of Al and Na. Horwell, Williamson & Le Blond (in prep.) Cristobalite structure quartz euhedral cris. platey cris.
Case Study 1 – Volcanic ash Early results 1 mm SEM-EDX Elemental maps In thin section, cracked appearance. Devitrification of glass also produces crystalline silica. Blue = Si Pink = K Green = Al
Case Study 1 – Volcanic ash Early results 1 mm SEM-EDX Elemental maps In thin section, cracked appearance. Devitrification of glass also produces crystalline silica. Would not fragment as microlites. Blue = Si Pink = K Green = Al ?
Where do we go from here? 1 mm Could alpha and beta forms of cristobalite and quartz have different toxicities? Extremely unusual to preserve beta variety but impurities potentially make it possible. Integration of work with toxicology. Application of techniques to other natural dusts e.g. coal and desert.
A useful tool for predicting the respirable fraction: Data collected by Malvern Mastersizer 2000 laser diffractometer. n = 65 samples from volcanoes worldwide.