GEOSCIENCE

Asbestos can be divided into two broad groups, serpentine and amphibole asbestos.
Chrysotile, which is hydrous magnesium silicate with the chemicalformula
Mg3 Si2O5(OH)4, is the fibrous form of serpentine asbestos ;it is the most valuable type and constitutes the bulk of all commercial asbestos. Chirysotile’s strong, silky fibers are easily spun and can withstand temperatures up to 2750°c.
The vast majority of chrysotile asbestos occurs in serpentine, a type of rock formed by the alteration of ultramafic igneous rocks such as peridotite under low-and medium-grade metamorphic condition. Serpentine is thought to form from the alteration of olivine by hot, chemically active, residual fluids emanating from cooling magma. The chrysotile asbestos forms veinlets of fiber within the serpentine and may comprise up to 20% of the rock.
At least five varieties of amfibole asbestos are known, but crocidolite, a sodium-iron amphibole with the chemical formula Na2(Fe+3)2(Fe+2)3Si8O22(OH)2, is the most common. Crocidolite which is also known as blue asbestos, a long, coarse, spinning fiber that is stronger but more brittle than chrysotile and also less resistant to heat. The other varieties of amphibole asbestos have fewer uses and are used primarily for insulation.
Crocidolite is found in such metamorphic rocks as slates and schists . It is thought that crocidolite forms by the solid-state alteration of other minerals within the high-temperature and high-pressure environment that results from deep burial.
Despite its widespread use, the federal environmental protection Agency (EPA) has instituted a gradual ban on new asbestos products. The ban was imposed because some form of asbestos can cause lung cancer and scarring of the lungs if its fibers are inhaled. Because the EPA apparently paid little attention to the issue of risks versus benefits when it enacted this rule, the U.S. fifth circuit court of Appeals overturned the EPA ban on asbestos in 1991. The current policy (1993) of the EPA mandates that all forms of asbestos be treated as identical hazards. Yet studies indicate that only the amphibole forms constitute a known health hazard. Chrysotile, whose fibers tend to be curly, does not become lodged in the lungs. Furthermore, its fibers are generally sotuble and disappear in tissue. In contrast, crocidolite has long straight, thin fibers that penetrate the lungs and stay there.This fibers irritate the lung tissue and, over a long period of time, can lead to lung cancer. Thus, crocidolite, and not Chrysotile, is overwhelmingly responsible for asbestos related lung cancer. Because about 95% of the asbestos in place in the United States is Chrysotile, many people are questioning whether the dangers from asbestos have been some what exaggerated. Removing asbestos from buildings where it has been installed might cost as much as $100 billion, and some recent studies have indicated that the air in buildings containing asbestos has essentially the same amount of airborne asbestos fibers as the air outdoors. In fact , unless the material containing of airborne the asbestos is disturbed, asbestos does not shed fibers. Furthermore, improper removal of asbestos can lead to contamination. In most cases of improper removal, the concentration of airborne asbestos fibers is far higher than if the asbestos had been left in place. The problem of asbestos contamination is a good example of how geology affects our lives and why a basic knowledge of science is important .