Eugine Whint 7/12/2005
Chemistry 5G
Asbestos.
Asbestos (Greek a-, "not"; sbestos, "extinguishable") is a group of fibrous metamorphic minerals. The name is derived for its historical use in lamp wicks; the resistance of asbestos to fire has long been exploited for a variety of purposes. It was used in fabrics such as Egyptian burial cloths and Charlemagne's tablecloth, which, according to legend, he threw in a fire to clean.
When asbestos is used for its resistance to fire or heat, the fibres are typically mixed with cement or woven into fabric or mats. It was used in brake shoes and gaskets for its heat resistance, on electric oven and hotplate wiring for its electrical insulation at elevated temperature, and in buildings for its flame-retardant and insulating properties, its tensile strength, flexibility, and resistance to chemicals. However, the inhalation of some kinds of asbestos fibres is now thought to cause various illnesses, including cancer, and thus most uses of asbestos are banned in many countries. Fibre glass has been found to be a suitable substitute for thermal insulation and woven ceramic fibre performs as well or better as an insulator of high-temperature electrical conductors.
Most respirable asbestos fibres are invisible to the unaided human eye because their size is about 3.0-20.0 µm in length and can be as thin as 0.01 µm. Fibres ultimately form because when these minerals originally cooled and crystallized, they formed by the polymeric molecules lining up parallel with each other and forming oriented crystal lattices. These crystals thus have three cleavage planes as other minerals and gemstones have. But in their case, there are two cleavage planes that are much weaker than the third direction. Thus when sufficient force is applied they tend to break along their weakest directions, resulting in a linear fragmentation pattern and hence a fibrous form. This fracture process can keep occurring over and over until they have been broken down to their smallest unit dimensions. For this reason, one larger asbestos fibre can ultimately become the source of hundreds of much thinner and smaller fibres in a normal environment over the course of time. As they get smaller and lighter, they become more mobile and more easily entrained (wafted) into the air, where human respiratory exposures typically result.
Confusingly, the Modern Greek word asbestos means quicklime.
Types of asbestos.
Chrysotile, or white asbestos, is obtained from Canadian serpentine rocks. It is less friable (and therefore less likely to be inhaled) than the other types and is the type most often used industrially. Chrysotile should not be confused with chrysolite, a synonym of olivine. There is some evidence that this form of asbestos is not actually harmful when inhaled. Amosite, or brown asbestos, is an amphibole from Africa.
Crocidolite, or blue asbestos, is an amphibole from Africa and Australia. It is the fibrous form of riebeckite. Blue asbestos is commonly thought of as the most dangerous type of asbestos. Notes: Serpentine rocks are those with curled fibres. Amphiboles have straight, needle-like fibres.
The amphiboles, in their fibrous form, are friable and therefore the most carcinogenic, although they also exist in safer non-fibrous forms.
Other asbestos minerals, such as tremolite, actinolite and anthophyllite are less-used industrially but can still be found in a variety of construction materials and insulations and occur in a few consumer products, such as talcum powders and vermiculite.
