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Mineralogically, What is Asbestos and How Does it Form?
A.E. Williams-Jones
Department of Earth and Planetary SciencesMcGill 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 AsbestosSix type of asbestos have been recognized, but mineralogically they fall into two classes, serpentine (90%) and amphibole (10%)
Serpentine Class Amphibole ClassChrysotile (1)
Grunerite-cumminmgtonite (4) (amosite)
Tremolite-actinolite (5-6)
Riebeckite (crocidolite) (2)
Anthophyllite (3)
Serpentine ClassSerpentine forms at spreading centres as a result of hydration of the Earth’s mantle via reactions of the type:
Spreading Centre at Mid-ocean Ridge
Asthenospheric MantleLithospheric Mantle
Oceanic crust
Olivine Serpentine
Seawater is drawn down into lithospheric mantle
3Mg2SiO4 + 4H2O + SiO2aq Mg6Si4O10(OH)8)
Serpentine ClassThe 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).
Ophiolite
Erosion
Mantle
Continent
Continent
Mantle
Oceanic crust
Asbestos vein formation
Rock mass pulled apart undergoes brittle failure, fractures form and fibres grow from the two faces of the fracture.
Serpentinite Serpentinite
Chrysotile
Serpentine is normally a platy mineral and only becomes asbestiform when it grows in extensional veins
The Serpentine Class – Sheet Silicates
Si4O10-4
Mg6Si4O10(OH)8
• Silicon tetrahedra bond to each other to form a negatively charged sheet
• Sheets are bonded ionically by cations (Mg2+) in octahedral co-ordination
OH-
Serpentine
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 proto-chrysotile
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
Evans et al. (2013)
Lizardite sector fibre
Banded Iron Formation and AsbestosOxygenation 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 (Fe)7Si8O22(OH)2 + 4CO2 + ½O2
Magnetite Quartz Siderite Grunerite (Amosite)
3Fe3O4 + 8SiO2 + 2H2O + 2Na+ Na2Fe2+3Fe3+
2Si8O22(OH)2 + 2F2O3 +2H+
Magnetite Quartz Riebeckite (Crocidolite) Hematite)
Crocidolite)
Hot magma releases heat and fluids
Si4O11-6
Si4O11-6
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)
The Structure of Amphibole
OH-
Na+, Ca2+, Mg2+, Fe2+
The Structure of Amphibole
Tremolite in Chrysotile Deposits
5Mg6Si4O10(OH)8 + 12Ca2+ + 28SiO2 6Ca2Mg5Si8O22(OH)2 + 24H+ + 2H2O
Diorite dykes interact with serpentinite to produce rind of tremolite
Diorite
Tremolite
Diorite
Serpentinite
Nayebzadeh et al. (2001)
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?
Wood et al. (2006)
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
Chrysotile
Tremolite
Thank you