UK Dust Network – 2nd Workshop Claire Horwell 14 th March 2008
description
Transcript of UK Dust Network – 2nd Workshop Claire Horwell 14 th March 2008
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
0
5
10
15
20
25
30
35
40
45
50
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
0
100
200
300
400
500
600
700
800
900
1000
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
0
2
4
6
8
10
12
0 1 10 100 1000
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
100 200 300 400 500 600 700 800 900 1000
0
0.2
0.4
0.6
0.8
1
1.2
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.)
0
0.5
1
1.5
2
2.5
3
94 95 96 97 98 99 100 101
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
0
2
4
6
8
10
12
14
16
18
20
0 5 10 15 20 25 30 35
< 10 m (cum. vol. %)
< 4
m
(cu
m. v
ol.
%)
y = 0.1877x - 1.9179R2 = 0.8608
-5
0
5
10
15
20
0 20 40 60 80 100
< 63 m (cum. vol. %)
< 4
m (
cum
. vo
l. %
)Data collected by Malvern Mastersizer
2000 laser diffractometer.
n = 65 samples from volcanoes worldwide.