UK Dust Network – 2nd WorkshopClaire Horwell14th March 2008

Mineralogy & Volcanology

Objectives of field:

• To use mineralogy and geochemistry to make rapid assessments of the potential health hazard of natural mineral particles.

• To understand WHY a mineral or dust may trigger a pathogenic respiratory response.

Sources: • Volcanoes– Volcanic ash– Aerosols (liquid & solid)

• Dust storms– Sourced from:

• Glacial forelands

• Debris fans

• Deserts

• Exposed lake beds

• Biogenic dust – bacteria, pollen, diatoms (silica)

• Quarrying and mining of rock15 µm

Phytolith (bacterium)

Beijing, China, 2003

Questions to be answered:

• Is the dust small enough to enter the lungs?

• What is the composition of the dust?

• Is shape a relevant factor (e.g. fibrous)?

• Is the surface of the dust reactive?

• Are individual particles ‘pure’?

• Is the crystalline structure of the mineral relevant?

Grain size

• Is the dust small enough to enter the lungs?

• Grain size analysis techniques:

– Laser diffraction

– SEM with image analysis

– Sieving• > 63 m only

• New predictive technique

Malvern Mastersizer 2000

Horwell, 2007, JEM

Grain size - volcanoes

Horwell, 2007, JEM

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0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

Equivalent spherical diameter ( m)

Vesuvius AD79

Merapi

Mt St Helens

Montserrat 5.6.99

Montserrat 12.7.03

Pinatubo

Taapaca

Tungurahua

El Reventador

Langila

Fuego

Etna

Cerro Negro

Pacaya

Shiveluch

Sakurajima

Vesuvius 1930

Uluwun

Composition of heterogeneous dusts

SEM image of volcanic ash

• Volcanic ash and eroded dusts are 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

ts

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)

Hyd

roxy

l ra

dic

al g

ener

atio

n

(30

min

s,

mo

l/m

2)

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 a dust 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 silica 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.

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.

0.266

0.267

0.268

0.269

0.27

0.271

0.272

0.273

0.274

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MVO287 prismatic cristobalite 50x obj

MVO287 cristob

Reference cristobalite

Raman spectra from cristobalite in dome rock

Cristobalite composition – 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.)

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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?

– Leverhulme proposal: Horwell, Williamson, Donaldson, Cressey (Fubini, Carpenter, Parman)

• Integration of work with toxicology.– Leverhulme proposal– Michnowicz PhD (April 2008)

• Application of techniques to other natural dusts e.g. coal and desert.

– Horwell NERC Fellowship.

A useful tool for predicting the respirable fraction:

y = 0.5256x - 0.505R2 = 0.9754

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< 10 m (cum. vol. %)

< 4

m

(cu

m. v

ol.

%)

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.