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Chrysotile and Asbestos Amphiboles: Two Different Fibre Types
"Asbestos" is not a mineral in itself. It is a collective term given to a group of minerals whose crystals occur in fibrous forms. The term "asbestos" was adopted for commercial identification.
The six minerals commonly referred to as asbestos come from two distinct groups of minerals. One group is known as serpentines (chrysotile, white asbestos); while the other group is the amphiboles (amosite, brown asbestos; Crocidolite, blue asbestos; Anthophyllite; Tremolite; and Actinolite). While both are silicate minerals, the two groups are chemically and mineralogically distinct.
Chrysotile is a sheet silicate which is formed as a very thin rolled sheet. The sheet is about 8 angstoms thick (0.8 nanometers thick). It is composed of a sandwich of magnesium and silica. In the lung, the acid environment of the macrophage scavenger cell quickly breaks apart the sheet structure causing the fiber to decompose into small pieces. These pieces can then be readily cleared from the lung. If the fiber is swallowed and ingested it is attacked by the even stronger acid environment (hydrochloric acid, PH 2) in the stomach.
Chrysotile has been known for over 2000 years, being used initially for cremation cloths, oil lamp wicks and other textiles. But, it is only in the 19th Century that Chrysotile was first mined commercially in the Urals (Russia), Italy and Canada.
This is in contrast to the amphibole asbestos fibers which are formed as solid rods/fibers. The structure of an amphibole is a double chain of silicate tetrahedral which makes it very strong and durable. The external surface of the crystal structures of the amphiboles is quartz-like, and has the chemical resistance of quartz. The amphibole fibers have negligible solubility at any pH that might be encountered.
All these fibres are non-flammable. The major difference between chrysotile and amphiboles is related to its chemical composition, its acid-resistant properties and its effects on health. In contrast with amphiboles, chrysotile does not persist in the lungs after inhalation; it is quickly eliminated by the body. A prolonged exposure to high concentrations of chrysotile fibres is required for a clinical manifestation of pulmonary damage to appear. In the past, such high exposures were frequent; it is no longer the case today. On the other hand, because of their toxicity and their high biopersistence, amphiboles are mainly responsible for mesothelioma and pulmonary diseases even caused after a short or moderate exposure.
From Asbestos to Chrysotile
Today, chrysotile is the only asbestos fibre commercialized. The industry now only markets dense and non-friable materials in which the chrysotile fibre is encapsulated in a matrix of either cement or resin. These products include chrysotile cement building materials, friction materials, gaskets and certain plastics.
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