UK Dust Network – 1st WorkshopClaire Horwell24th 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.

XRD spectrum for Soufriere Hills dome-collaspe ash

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10 30 50 70 90Degrees 2 theta

Co

un

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Quartz STD

Cristobalite STD

Monterrat ash separate

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: Fe2+ + H2O2 Fe3+ + OH- + HO

Horwell et al. Environmental Research, 2003

Fe2+ release vs. hydroxyl radical production at 30 mins

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Fe2+ release (after 7 days, mol/m2)

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roxy

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(30

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MON SHI

MER ELR

SAK MSH

M03 FUE

LAN TUN

PIN PAC

ETN CER2

V1872 V1904

V1906 V1871

Minusil 1906A

Production of hydroxyl radicals

Basaltic

Andesitic/Dacitic

Tephritic/Phonolitic

Minusil 5 Quartz standardHorwell, 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.

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MVO287 prismatic cristobalite 50x obj

MVO287 cristob

Reference cristobalite

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

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SiO2 (Wt. %)

Al 2

O3

(W

t. %

)

MVO945vp

MVO945qz

MVO819pl

MVO332vp

MVO1236pl

MVO1236vp

MVO1236qz

MVO1406vp

MontR1vp

MontR1qz

MVO617

MVO287

quartz

euhedral cris.

platey cris.

Case Study 1 – Volcanic ashEarly 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 ashEarly 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:

y = 0.5256x - 0.505R2 = 0.9754

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< 10 m (cum. vol. %)

< 4

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%)

y = 0.1877x - 1.9179R2 = 0.8608

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< 63 m (cum. vol. %)

< 4

m (

cum

. vo

l. %

)Data collected by Malvern Mastersizer

2000 laser diffractometer.

n = 65 samples from volcanoes worldwide.