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 ClassAmphibole Class Chrysotile (1) Grunerite-cumminmgtonite (4) (amosite) Tremolite-actinolite (5-6) Riebeckite (crocidolite) (2) Anthophyllite (3)

Serpentine Class Serpentine 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 Mantle Lithospheric Mantle Oceanic crust OlivineSerpentine Seawater is drawn down into lithospheric mantle 3Mg 2 SiO 4 + 4H 2 O + SiO 2 aq Mg 6 Si 4 O 10 (OH) 8 )

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). Ophiolite Erosion Mantle 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 Chrysotile Serpentine is normally a platy mineral and only becomes asbestiform when it grows in extensional veins

The Serpentine Class – Sheet Silicates Si 4 O Mg 6 Si 4 O 10 (OH) 8 Silicon tetrahedra bond to each other to form a negatively charged sheet Sheets are bonded ionically by cations (Mg 2+ ) in octahedral co-ordination OH - Serpentine

Lizardite and Chrysotile Chrysotile is the asbestiform variety of the serpentine group mineral lizardite (Mg 6 Si 4 O 10 (OH) 8 ), which is a sheet silicate Lizardite sheets in a serpentinite

Serpentinisation Mantle olivine (Mg 2 SiO 4 ) is hydrated to form lizardite (Mg 6 Si 4 O 10 (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 Asbestos Oxygenation of the oceans and atmosphere by cyanobacteria lead to oxidation of soluble Fe 2+ to insoluble Fe 3+ producing banded iron formation (BIF) Fe 3 O 4, Fe2O 3, SiO 2 FeCO 3

Metamorphism of BIF Produces Crocidolite and Amosite Fe 3 O 4 + 8SiO 2 + 4FeCO 3 + H 2 O (Fe) 7 Si 8 O 22 (OH) 2 + 4CO 2 + ½O 2 Magnetite Quartz Siderite Grunerite (Amosite) 3Fe 3 O 4 + 8SiO 2 + 2H 2 O + 2Na+ Na 2 Fe 2+ 3 Fe 3+ 2 Si 8 O 22 (OH) 2 + 2F 2 O 3 +2H + Magnetite Quartz Riebeckite (Crocidolite) Hematite) Crocidolite) Hot magma releases heat and fluids

Si 4 O Double chain silicates with the general formula AB 2 C 5 Si 8 O 22 (OH) 2 Riebeckite (Na 2 (Fe 2+,Fe 3+ )Si 8 O 22 (OH) 2, Tremolite (Ca 2 Mg 5 Si 8 O 22 (OH) 2, Anthophyllite (Mg,Fe)2(Mg,Fe) 5 Si 8 O 22 (OH) 2 Cummingtonite-Grunerite (Ditto) The Structure of Amphibole OH - Na +, Ca 2+, Mg 2+, Fe 2+

The Structure of Amphibole

Tremolite in Chrysotile Deposits 5Mg 6 Si 4 O 10 (OH) Ca SiO 2 6Ca 2 Mg 5 Si 8 O 22 (OH) H + + 2H 2 O 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

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