Fibres

European Conference on
Asbestos Risk and Management
Rome, 4-6 December 2006
STRUCTURAL AND MICROSTRUCTURAL
CONTROL ON CHRYSOTILE DISTRIBUTION IN
SERPENTINITES FROM EASTERN LIGURIAN
OPHIOLITES
Laura GAGGERO, Laura CRISPINI, Pietro MARESCOTTI,
Cristina MALATESTA, Monica SOLIMANO
Dipartimento per lo Studio del Territorio e delle sue Risorse,
Università degli Studi, Genova, Italy
WHERE AND WHY
Where: Several outcrops of serpentinites, from the very-low grade
metamorphic ophiolites of the Northern Apennine in eastern Liguria
(Italy).
WHERE AND WHY
Why:
9Need for pilot studies based on microstructural and
mineralogical investigations, to assess the distribution and
approximate volumes of asbestos minerals and their potential
airborne fibre contribution.
9The asbestos hazard due to quarry activity was brought into
evidence by judicial proceedings
9The quarry areas (I.e. total rock exposure), represent the worstcase scenario to evaluate the global asbestos diffusion over
ophiolitic districts.
WHERE AND WHY
Why:
The Regione Liguria acknowledges the law N° 257/92 by issuing the
“Piano di protezione e decontaminazione dai pericoli derivanti
dall’amianto” ( D.C.R. N° 105, 20/12/1996)
9
9
9
9
9
9
9
9
Censimenti dei siti estrattivi
Censimento di strutture civili ed
industriali
Censimento delle imprese
REGIONE LIGURIA
Attività di rimozione e bonifica
Smaltimento di rifiuti contenenti
LEGENDA DELLE AREE A POTENZIALE RISCHIO AMIANTO
amianto
COMPLESSI LITOLOGICI
GRUPPO
RETINATURA
Controlli sulla salubrità ambientale
Serpentiniti, Serpentinoscisti
A
SCURA
ed Eclogiti
Corsi di formazione
Anfiboliti, Gabbri,
Metagabbri e Metabasiti
B
MEDIA
Brecce ofiolitiche, Basalti e
Metabasalti
C
CHIARA
WHERE AND WHY
Why:
9Need for pilot studies based on microstructural and
mineralogical investigations, to assess the distribution and
approximate volumes of asbestos minerals and their potential
airborne fibre contribution.
9The asbestos hazard due to quarry activity was brought into
evidence by judicial proceedings
9The quarry areas (I.e. total rock exposure), represent the worstcase scenario to evaluate the global asbestos diffusion over
ophiolitic districts.
WHERE AND WHY
Why:
9Need for pilot studies based on microstructural and
mineralogical investigations, to assess the distribution and
approximate volumes of asbestos minerals and their potential
airborne fibre contribution.
9The asbestos hazard due to quarry activity was brought into
evidence by judicial proceedings
9The quarry areas (I.e. total rock exposure), represent the worstcase scenario to evaluate the global asbestos diffusion over
ophiolitic districts.
GEOLOGIC SETTING OF EASTERN LIGURIA
Bargonasco - Val Graveglia Massif
(Val di Vara
Graveglia Unit)
Supergroup,
Bracco-
Northern Apennines ophiolites represent
slices of the Jurassic Ligure-Piemontese
oceanic
basin
separating
paleoEuropean and Adria continental blocks.
Menna et al., in press
GEOLOGIC SETTING OF EASTERN LIGURIA
•Basement: two cycles of ocean floor metamorphic events
(from high-T amphibolite facies to low-T hydrothermal conditions;
conditions
Cortesogno & Lucchetti, 1984).
The sea-floor hydration led to widespread serpentinization of
peridotites (Cimmino et al., 1979; Piccardo et al., 1988) and
hydrous assemblages, including several generations of serpentine
minerals (mostly chrysotile and lizardite), chlorite, brucite and
mixed-layer
phyllosilicates,
developed
on
the
mantle
assemblages (Piccardo et al., 2004).
•During Late Cretaceous-Early
Cenozoic orogenic events, the
ophiolites were deformed and
metamorphosed
(prehnitepumpellyite facies conditions; T
= 275±25 °C; P = 2.5±0.5 kbars;
kbars
Lucchetti et al., 1990).
Menna et al., in press, redrawn after
Cortesogno et al. 1987; Principi et al. 1992
As a result, the serpentinites of
the Bargonasco-Val Graveglia
Massif
are
extensively
tectonized and characterized by
several generations of fault and
fractures developed during
polyphase ductile and brittle
deformations associated both
with ocean floor and orogenic
tectono-metamorphic events.
THE SERPENTINITE
QUARRIES
1)
Three serpentinite quarries located in the Petronio Valley
(Bargonasco
Bargonasco village north of Sestri Levante) that were dismissed from
less than one, five and about ten years, respectively.
2)
The Ponte Nuovo serpentinite – chert quarry in the lower Vara
Valley.
Valley At present, only cherts are exploited.
At all sites, the serpentinite was exploited to obtain concrete aggregates.
The quarried serpentinites were affected by several deformational events
associated with recrystallization; as a consequence, a complex fabric (i.e.
geometric) pattern results in the rock, and different serpentine group
minerals were developed in different textural position.
The exposed outcrops of the quarries have been characterized following
the scheme of the UNI EN ISO 14689-1 (“Indagini e prove geotecniche –
Identificazione e classificazione delle rocce”) modified and adapted for the
specific case of the asbestos-bearing serpentinities.
Cava Bargonasco
Ch
GBr
SBr
Conoidi detritiche
Cava Ponte Nuovo
S
THE SERPENTINITE QUARRIES
Ponte Nuovo
Bargonasco
The exposed outcrops of the quarries
have been characterized ⇒ UNI EN
ISO 14689-1 (“Indagini e prove
geotecniche
–
Identificazione
e
classificazione delle rocce”) modified
and adapted for the specific case of the
asbestos-bearing serpentinities.
THE MASSIVE SERPENTINITE
PONTE NUOVO:
The pristine granular to
NUOVO
porphyroclastic tectonitic lherzolite is serpentinized.
o A fine-grained aggregate of prevailing lizardite, minor
chrysotile + magnetite developing mesh and ribbon
textures overgrows the groundmass olivine.
Ponte Nuovo
o Porphyroclastic texture: serpentine pseudomorphs on
pristine olivine or pyroxene (bastite porphyroblasts).
porphyroblasts
Bargonasco
0.5 mm
BARGONASCO:
BARGONASCO Massive serpentinites represent 2025% of the volume of the quarry fronts and occurs as
subspheroidal or irregular shaped metric to decametric
lenses that show a relatively low degree of fracturing.
THE FRACTURED SERPENTINITE
Fractured serpentinites:
serpentinites 10 to 70% of the
quarry fronts.
Fractures: 10 to 100 mm thick; partially or
completely filled by light green to whitish
fibrous and/or columnar minerals.
The main fracture systems are interconnected
and form a network that subdvide the
massive bodies in polyhedric blocks varying
in size from 2000 to 200 mm.
Also several irregularly distributed and less
pervasive fracture systems occur mainly
related to local stresses.
< 1mm
1 mm
c
Brittle structures without significant mineral
recrystallisation
A) structures such as slickenside,
slickenside elongated
mineral fibres, intersection lineations between S-C
surfaces that give indications of the direction of
tectonic movement, and
B) structures such as stepwise fibre growth,
growth
sigmoids and rotation of a pre-existing foliation
(kinematic indicators).
GEOMETRY OF VEINS
•The thickness of the veins ranges between few mm to some cm.
•Some veins developed along reactivated joint surfaces.
•In syntaxial veins the filling fibrous chrysotile nucleated from the host rock. Fibres are
perpendicular to the rockwall of the vein and bend to the centre; this indicates that the oldest
fibres are at the selvedge, without important compositional discontinuity between the host rock
and the early vein-filling mineral.
•Antitaxial veins have straight chrysotile fibres in the middle of the vein, grading to bent fibres
close to the wallrock; this growth pattern suggests that the filling derives from exotic input and
that the fibres at the centres are older than at the selvedge. The “crack and seal” mechanism
can account for this geometry.
•The most common veins are composite fibre veins and host both inward and outward fibre
growth. The geometry of the fibres indicates that the fracture was filled during extension
(syntaxial growth) and shearing (crack and seal).
seal
•Also the intersection relationships between older and younger vein and the optical continuity
among fibres support that extension and shearing was simultaneous, i.e. occurred under the
same kinematic regime.
•Finally, an extensional phase occurred, characterised by the development of cm thick veins.
This phase is scarcely represented at Ponte Nuovo, but is widespread in the Bargonasco
quarries.
GEOMETRY OF VEINS
The thickness of the veins ranges between few mm to some cm.
Some veins developed along reactivated joint surfaces.
In syntaxial veins the filling fibrous chrysotile nucleated from the host rock. Fibres are
perpendicular to the rockwall of the vein and bend to the centre; this indicates that the oldest
fibres are at the selvedge, without important compositional discontinuity between the host rock
and the early vein-filling mineral.
Antitaxial veins have straight chrysotile fibres in the middle of the vein, grading to bent fibres
close to the wallrock; this growth pattern suggests that the filling derives from exotic input and
that the fibres at the centres are older than at the selvedge. The “crack and seal” mechanism
can account for this geometry.
The most common veins are composite fibre veins and host both inward and outward fibre
growth. The geometry of the fibres indicates that the fracture was filled during extension
(syntaxial growth) and shearing (crack and seal).
seal
Also the intersection relationships between older and younger vein and the optical continuity
among fibres support that extension and shearing was simultaneous, i.e. occurred under the
same kinematic regime.
Finally, an extensional phase occurred, characterised by the development of cm thick veins.
This phase is scarcely represented at Ponte Nuovo, but is widespread in the Bargonasco
quarries.
GEOMETRY OF VEINS
The thickness of the veins ranges between few mm to some cm.
Some veins developed along reactivated joint surfaces.
In syntaxial veins the filling fibrous chrysotile nucleated from the host rock. Fibres are
perpendicular to the rockwall of the vein and bend to the centre; this indicates that the oldest
fibres are at the selvedge, without important compositional discontinuity between the host rock
and the early vein-filling mineral.
Antitaxial veins have straight chrysotile fibres in the middle of the vein, grading to bent fibres
close to the wallrock; this growth pattern suggests that the filling derives from exotic input and
that the fibres at the centres are older than at the selvedge. The “crack and seal” mechanism
can account for this geometry.
The most common veins are composite fibre veins and host both inward and outward fibre
growth. The geometry of the fibres indicates that the fracture was filled during extension
(syntaxial growth) and shearing (crack and seal).
seal
Also the intersection relationships between older and younger vein and the optical continuity
among fibres support that extension and shearing was simultaneous, i.e. occurred under the
same kinematic regime.
Finally, an extensional phase occurred, characterised by the development of cm thick veins.
This phase is scarcely represented at Ponte Nuovo, but is widespread in the Bargonasco
quarries.
GEOMETRY OF VEINS
Vein set 2
Vein set 1
Massive serpentinite
GEOMETRY OF VEINS
Vein set 2
Massive σ
Vein set 1
GEOMETRY OF VEINS
VEIN FILLING MINERALS
Analyzed by:
•transmitted light polarising
microscope,
•extraction from veins or
separation by magnetic
methods, XR powder
diffraction on minerals
•quantitative in situ electron
microprobe analyses by SEMEDS on selected massive
samples.
Trm
Chr
Fracture networks Î
chrysotile.
Local calcite Î stepwise
fracture opening.
The late veining Î cm-long
linear fibres of chrysotile and
intergrown chrysotile +
tremolite.
Tremolite – actinolite Î
gabbro or basalt dikelets.
CATACLASTIC SERPENTINITES
Serpentinite Î fine-grained matrix with small relic volumes of the massive rock.
The veins filled with chrysotile easily disperse their content.
In case of cataclastic incoherent matrix, the fibres are to be considered free.
free
MECHANISMS OF FIBRE CONCENTRATION
MILLING OF MASSIVE LIZARDITE SERPENTINITE
Milling of fibrous chrysotile increases the percentage of fibres,
due to the perfect lengthwise cleavage of the fibre.
lizardite
serpentinite pieces, releases scarce fibrous minerals if
Milling, i.e. size reduction between few mm to some cm of
the material is devoid of filled veins.
SIZE REDUCTION OF FIBROUS VEIN FILLING
In syntaxial veins, the chrysotile fibres grow consistently with the host
serpentinite and can thus be assumed as a mechanically continuous
material at the microscale.
antitaxial veins and in the more widespread
composite veins, characterised by disjunction between wall-rock and
Conversely, in the
vein, and by contrasting fibre orientation along a lateral cross section, fibrous
chrysotile under mechanical stress results more liable to be released.
ANALYTICAL SEQUENCE
D.M. 14/5/1996 reference normative synthetic materials and quarried
blocks and aggregates
INCONSISTENCIES
# 1 rock volumes show a farther wide range of textures and the frequency and
distribution of potential asbestos sources is variable and
dependent on the tectonic style.
# 2 geologic systems are time-variable Î natural weathering on the rock
surface or runoff over the detrital stone heaps can enhance or inhibit the
fibre dispersion.
As a whole, our investigation integrated the following procedures:
I)
Field survey [UNI EN ISO 14689-1 (“Indagini e prove geotecniche –
Identificazione e classificazione delle rocce”) adapted to asbestos-bearing
serpentinites]. …. could be further improved e.g.. high-resolution image analysis
of the quarry front
II)
Sampling is strategic to result representative of the vein population
III)
Mineralogical analyses on vein filling by STEREOSCOPIC, MOLP +
MODAL ANALYSIS, SEPARATION of fibrous minerals, XRPD. Selected
samples Î SEM + EDS
However, an ANALYTICAL BIAS resides I) in the approximation to extend the
modal abundances in 2D to the rock volume at 3D and mostly II) in the
analysis of the quarry front, that is operator-dependent.
TO SUM UP..
Accuracy in evaluating asbestos in the natural background Î Liguria or
Piedmont, significant volumes of ophiolitic lithologies outcrop.
GOAL Î assessment of the fracture volume
the volume of infilling minerals and
the definition of cessible and free fibres
Porphyroclastic, mesh- or ribbon-textured serpentinite as MASSIVE
FREE FIBRES Î fibrous minerals already detached from the rock or the
host vein, and liable to enter the sedimentary cycle, hydrosphere,
atmosphere or biosphere.
CESSIBLE FIBRES Î asbestos minerals in the host rock under different
textures, liable to be RELEASED for natural or anthropic friction and/or
comminution, are subject to local tectonic and mineralogical factors.
QUANTIFICATION Î Integration of mesoscale to microscopic data.
Structural analysis, frequency of veins in the rock volume.
Arrangement of mineral fibres within veins, mineral species
CONCLUSIONS
•“Ophiolite” Î asbestos-bearing rocks with different fibre “dilution”.
•Asbestiform phases Î brittle deformation regime & development of
vein filling.
•Rock fracturing is therefore critical for the origin and concentration of
asbestos. Open or filled fractures act as discontinuities and disrupt the
rock body;
•Reactivation of early fracture nets under the cataclastic events
•Fibres Î NOT from the corresponding massive mineral or rock body.
ÎMILLING (i.e. quarry operations) Î scarce fibres , dilution of
asbestos fibres dispersed in the rock.
ÄCONCENTRATION mostly occurs by natural processes
(cataclasis, detrital aprons, pedogenesis) starting from a veined rock
body.
Ä Airborne fibre dispersion is both natural and anthropic.