Mineralogically, What is Asbestos
and How Does it Form?
A.E. Williams-Jones
Department of Earth and Planetary Sciences
McGill University, Montreal, Canada
What is Asbestos?
Asbestos is a mineral that crystallises in some rocks to form fibres. These
fibres are commonly less that 1 micron in diameter, and may be > 5 cm long
Chrysotile
Tremolite
Asbestos Relative to the Human Hair
Amosite asbestos fibres as
viewed with a scanning
electron microscope
Human hair
Types of Asbestos
Six type of asbestos have been recognized, but mineralogically they fall
into two classes, serpentine (90%) and amphibole (10%)
Serpentine Class
Chrysotile (1)
Amphibole Class
Riebeckite (crocidolite) (2)
Anthophyllite (3)
Grunerite-cumminmgtonite (4) (amosite)
Tremolite-actinolite (5-6)
Serpentine Class
Serpentine forms at spreading centres as a result of hydration of the Earth’s
mantle via reactions of the type:
3Mg2SiO4 + 4H2O + SiO2aq
Olivine
Mg6Si4O10(OH)8)
Serpentine
Spreading Centre at Mid-ocean Ridge
Seawater is drawn
down into lithospheric
mantle
Serpentine Class
The serpentine is exposed in ophiolites, which represent oceanic crust and
mantle, that has been obducted onto the continent. Quebec produced most of
this asbestos (chrysotile).
Continent
Oceanic crust
Mantle
Ophiolite
Erosion
Continent
Mantle
Asbestos vein formation
Serpentine is normally a platy mineral and only becomes asbestiform
when it grows in extensional veins
Rock mass pulled apart undergoes brittle failure, fractures form and
fibres grow from the two faces of the fracture
.
Chrysotile
Serpentinite
Serpentinite
The Serpentine Class – Sheet Silicates
• Silicon tetrahedra bond to each other to form a negatively charged sheet
• Sheets are bonded ionically by cations (Mg2+) in octahedral co-ordination
Si4O10-4
OH-
Serpentine
Mg6Si4O10(OH)8
Lizardite and Chrysotile
Chrysotile is the asbestiform variety of the serpentine group mineral
lizardite (Mg6Si4O10(OH)8), which is a sheet silicate
Lizardite sheets in a
serpentinite
Serpentinisation
Mantle olivine (Mg2SiO4) is hydrated
to form lizardite (Mg6Si4O10(OH)8)
Experimental
alteration of olivine to
lizardite
Lizardite sheets
rolling to form protochrysotile
Normand et al. (2002)
The Transformation of Lizardite to
Chrysotile
High resolution transmitted electron microscope image showing
lizardite (Lz) partly transformed to chrysotile (C)
The Structure of Chrysotile
The octahedral and tetrahedral layers, which are planar in lizardite, are
concentrically distributed in chrysotile to form cylinders within cylinders that
are weakly held together by van der Waals forces
High Resolution TEM Images of
Chrysotile
Note the concentric cylinders of octahedral and tetrahedral layers loosely
held together by hydrogen bonding
Lizardite sector fibre
Evans et al. (2013)
Banded Iron Formation and Asbestos
Oxygenation of the oceans
and atmosphere by
cyanobacteria lead to
oxidation of soluble Fe2+ to
insoluble Fe3+ producing
banded iron formation (BIF)
Fe3O4, Fe2O3, SiO2
FeCO3
Metamorphism of BIF Produces
Crocidolite and Amosite
Fe3O4 + 8SiO2 + 4FeCO3 + H2O
Magnetite Quartz Siderite
3Fe3O4 + 8SiO2 + 2H2O + 2Na+
Magnetite Quartz
(Fe)7Si8O22(OH)2 + 4CO2 + ½O2
Grunerite (Amosite)
Na2Fe2+3Fe3+2Si8O22(OH)2 + 2F2O3 +2H+
Riebeckite (Crocidolite) Hematite)
Crocidolite)
Hot magma releases
heat and fluids
The Structure of Amphibole
Double chain silicates with the general formula AB2C5Si8O22(OH)2
Riebeckite (Na2(Fe2+,Fe3+)Si8O22(OH)2, Tremolite (Ca2Mg5Si8O22(OH)2,
Anthophyllite (Mg,Fe)2(Mg,Fe)5Si8O22(OH)2 Cummingtonite-Grunerite (Ditto)
Si4O11-6
OHNa+, Ca2+,
Mg2+, Fe2+
Si4O11-6
The Structure of Amphibole
Tremolite in Chrysotile Deposits
Diorite dykes interact with serpentinite to produce rind of tremolite
5Mg6Si4O10(OH)8 + 12Ca2+ + 28SiO2
6Ca2Mg5Si8O22(OH)2 + 24H+ + 2H2O
Serpentinite
Tremolite
Diorite
Diorite
The contradiction of Québec chrysotile
mines
Québec chrysotile mines contain less than 1% tremolite but the lungs of
diseased workers from these mines contain more tremolite than chrysotile.
Why?
Nayebzadeh et al. (2001)
A Possible Explanation for the
Tremolite Anomaly
The structure of chrysotile, a rolled sheet silicate, is intrinsically unstable
enabling it to dissolve more easily in lung fluids than tremolite
The calculated rates of
dissolution of chrysotile and
tremolite in synthetic lung fluid
Tremolite
Chrysotile
Wood et al. (2006)
Thank you