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Asbestos minerals
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There are three major types of asbestos minerals, known as "white", "brown" and "blue" asbestos. The following description includes six asbestos minerals:
actinolite
amosite (brown asbestos)
anthophyllite
crocidolite (blue asbestos)
chrysotile (white asbestos)
tremolite
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All asbestos minerals occur naturally, and most belong to the amphiboles. The exception is chrysotile, which is a serpentine mineral.
The amphiboles are a very common group of rock-forming silicate minerals. Theoretically, asbestos may be present in all metamorphic rocks containing Mg-Fe amphiboles or serpentine, where the rock has been subject to shear stress.
Asbestos is mined in open-pit mines in i.a. Canada, Russia and the U.S.A.
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Chrysotile ("white asbestos")
Mg3[Si2O5](OH)4
Chrysotile is the most widely used industrial asbestos mineral, with the following characteristics:
Has bent fibers, often as bundles that split into fine fibrils
Can be spun for use in textiles
Can be used for adsorption of gases and solvents
High tensile strength
High thermal and electrical insulation capacity
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Chrysotile is the only asbestos mineral that has a low resistance to acids, but a high resistance to bases. Chrysotile has been used in asbestos cement, fireproofing, gaskets, adhesives, putty, joints, textiles, mats, paint etc.
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SEM picture of chrysotile. Magnification 2000x . Dashed line equals 10µm. The picture shows that the fibers form bundles of very fine fibrils. Chrysotile is destroyed at temperatures above 800 ºC. |
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EDX diagram of chrysotile.
Chrysotile is rich in magnesium (Mg) but has a low content of iron (Fe). |
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Anthophyllite
Mg7[Si8O22](OH)2; characteristics:
Straight fibers that split in the ends, forms bundles.
High chemical resistance
High thermal and electrical insulation capacity
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Has been used in, e.g. insulation material, adhesives, plastics, filling materials and reinforcement material etc.
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Anthophyllite asbestos at 2500x magnification (SEM). Dashed line equals 10µm. Anthophyllite asbestos forms either straight fibers that split in the ends, or fiber bundles. Anthophyllite disintegrates at temperatures above 950ºC.
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EDX diagram of anthophyllite. Anthophyllite has a lower silica (Si) content than chrysotile. The Mg conctration is higher than that of Fe.
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Tremolite and actinolite
Ca2(Mg, Fe2+)5[Si8O22](OH)2
Tremolite contains calcium (Ca) and magnesium (Mg), whereas in actinolite, Mg is replaced by iron (Fe). Both minerals occur as asbestiform and non-asbestiform.
Straight fibers, that split in the ends (asbestos), but also as fibers with the ratio length:thickness <3
Occurs only in natural materials: talc, dolomite etc.
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The SEM picture shows tremolite, partly asbestiform (very fine fibers) and partly non-asbestiform fibers. Tremolite usually forms smaller fiber bundles than the other asbestos minerals.
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Amosite ("brown asbestos")
Fe7[Si8O22](OH)2
Amosite is a trade name for the amphiboles belonging to the cummingtonite - grunerite solid solution series. It is named as an acronym from Asbestos Mines of South Africa. Amosite is stable up to temperatures of 600 – 900 ºC. Amosite can be found in e.g. pipe insulation material and fireproofing.
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- Straight fibers that split in the ends
- Forms bundles
- Can be spun
- High tensile strength (but not as high as chrysotile or crocidolite)
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Amosite at 2500x magnification (SEM). Dashed line equals 10µm.
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Amosite is an iron-rich amphibole with a low magnesium content.
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Crocidolite ("blue asbestos")
Na2Fe5[Si8O22](OH)2
Crocidolite forms very fine fibers. The fibers split in the ends and can form bundles.
High tensile strength
Can be spun
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Crocidolite can be found in asbest-cement products, filters, gaskets and insulation etc.
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Crocidolite at 2500x magnification.
Dashed line equals 10µm.
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Crocidolite is an iron-rich amphibole. Unlike amosite, the crocidolite structure also contains sodium (Na).
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