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