Asbestos fibers, types amphibole

In accordance with demonstrated differences between the various asbestos fiber types, the workplace regulation in many countries specifies different exposure limits for chrysotile and the amphiboles (45). Moreover, to alleviate estabHshed, or apprehended, risk from substitute fibers, the regulation often specifies maximum exposure limits for synthetic fibers (46) values of exposure limits adopted in leading industrial countries are coUected in Table 8. 

Mineralogy Asbestos is a name for a set of naturally fibrous silicate minerals. Asbestos minerals have been used because of their high tensile strength, high heat resistance, resistance to chemical attack, and ability to be woven into cloth (Dupres et al. 1984). These properties vary by asbestos fiber type. There are six accepted types of asbestos fibers namely, chrysotile, amosite, crocidolite, antho-phyllite, tremolite, and actinolite. These fibers can be separated into two broad mineralogical groups chrysotile or amphiboles, the latter including amosite, crocidolite, tremolite, actinolite, and anthophyllite. Table 2.1.6 lists the diseases caused by asbestos. While Table 2.1.6 is correct in broad outline, there are considerable discrepancies in the abil-... 

The two main amphibole asbestos fibers are amosite and crocidoHte, and both are hydrated siHcates of iron, magnesium, and sodium. The appearance of these fibers and of the corresponding nonfibrous amphiboles is shown in Figure 1. Although the macroscopic visual aspect of clusters of various types of asbestos fibers is similar, significant differences between chrysotile and amphiboles appear at the microscopic level. Under the electron microscope, chrysotile fibers are seen as clusters of fibrils, often entangled, suggesting loosely bonded, flexible fibrils (Fig. 2a). Amphibole fibers, on the other hand, usually appear as individual needles with a crystalline aspect (Fig. 2b). 

The evolution in the world production of asbestos fibers since 1950 is illustrated in Table 5 (5) after a peak near 1980, production leveled off after 1985 at 4.2 4.3 X 10 t. Changes in the production of the two main producers, Canada and the former USSR, over the same period are also illustrated. These figures show a substantial decrease in the Canadian production with a concomitant increase in the former USSR production. During recent years, several other countries, namely Brazil, Zimbabwe, and China, have substantially increased their production of chrysotile. Most of China s production, as well as the limited production of many other countries, is used in local industrial appHcations. South Africa is the only country where the three main types of asbestos are produced (chrysotile, crocidoHte, and amosite), and the only significant producer of amphibole fibers. 

A further consensus developed within the scientific community regarding the relative carcinogenicity of the different types of asbestos fibers. There is strong evidence that the genotoxic and carcinogenic potentials of asbestos fibers are not identical in particular mesothelial cancer is mostiy, if not exclusively, associated with amphibole fibers (43). 

The fractional breakdown of the recent world production of the various fiber types shows that the industrial applications of asbestos fibers have now shifted almost exclusively to chrysotile. Two types of amphiboles, commonly designated as amosite and crocidolite are still being used, but their combined production is currently less than 2% of die total world production. The odier three amphibole varieties, anthophyllite, actinolite, and tremolite, have no significant industrial applications presently, This... 

It is important to recognize that asbestos is not a single substance, but is the generic name for a family of six related poly silicate fibrous minerals of which one (chrysotile) belongs to the serpentine family and five (actinolite, amosite, anthophyllite, crocidolite, and tremolite) belong to the amphibole family. These minerals differ from each other in physical and chemical properties, and each mineral can exist in a wide range of fiber sizes. These differences between fiber type and, more importantly, fiber size (length and diameter) are believed to be important determinants of the health risks posed by asbestos. 

Along these same lines, further work would be helpful in defining other fiber characteristics that are important determinants of carcinogenicity. It is suspected, for example, that amphiboles, such as crocidolite and tremolite asbestos, are more likely to cause mesothelioma than chrysotile, but it is not certain if this is attributable to differences in fiber length alone or to differences in chemical properties (e g., fiber morphometry, iron content, durability in biological fluids and tissues). Consequently, additional animal studies of the relative carcinogenic potency of airborne asbestos fibers of different types (e.g., chrysotile versus amphibole asbestos), carefully matched with regard to fiber size distribution, may be valuable. 

Microscope slides (72 mm x 25 mm x 0.8 mm approximately) and coverslips should be of the best optical quality and of the correct thickness for obtaining quality images (usually 0.17 mm). In the case of counting a particular fiber, an assumption has to be made about the cross-sectional profile of the fiber type chrysotile is usually assumed to be cylindrical in shape, while the amphiboles are considered to have a thickness-to-width ratio of 1.6 1. Discrimination of asbestos fibers using the morphology and refractive index in such cases is aided by prior identification of the fiber types present in the bulk material using PLM or other methods. 

Amianthus A term from ancient times applied to the exceptionally long, silky, relatively fine, and highly flexible mineral fibers capable of being woven. The actual usage establishes that the term referred to amphibole-asbestos. No longer in common use in English, amianthus was occasionally used to refer to all types of asbestos. 

ASBESTOS. The tenn asbestos is a generic designation referring usually to six types of naturally occurring mineral fibers which are or have been commercially exploited, These fibers are extracted from certain varieties of hydrated alkaline silicate minerals comprising two families seipe.ntin.es and amphiboles. The serpentine group contains a single fibrous variety chrysotile five fibrous forms of amphiboles are known anthophyllite, amosite, crocidolite. tremolite, and actinolite... 

Asbestos, the first inorganic fiber material used, is currently still exclusively produced from natural mineral deposits. It is formed by the hydrothermal conversion of basic and ultrabasic volcanic rock (olivine and pyroxene) to serpentine upon which the actual asbestos formation takes place leading to two asbestos sorts with different structures serpentine asbestos and amphibole asbestos. Asbestos can be produced synthetically by several hours heating of a polysilicic acid/metal oxide mixture (e.g. Mg, Fe, Co, Ni) in water at 300 to 350°C and 90 to 160 bar. The properties of four important asbestos types are summarized in Table 5.2-2. 

The different structure types lead to very different properties. Unlike amphibole, chrysotile fibers exhibit a positive zeta potential, which is important for its utilization as a filtration and clarifying agent in the food and drink industries as well as for the separation of viruses and pyrogens from pharmaceutical solutions. Due to the accessibility of its hollow fibers, chrysotile asbestos is unstable to acids unlike amphibole. 

PHYSICAL PROPERTIES a group of six different minerals occurring naturally in the environment most common type is white others are blue, gray, or brown long thin fibers similar to fiberglass not volatile not soluble serpentine asbestos chrysotile mineral, strong, resistant to chemicals and heat amphibole asbestos brittle fibers, resistant to chemicals and heat... 

Termolite is also an amphibole and its structure is more acicular, tubular, or lamellar. Its crystals are reported to be monoclinic. Termolite is probably the most used fibrous amphibole asbestos -type fiber, but crocidolite has been equally popular. Both fibers have been little more than specialty items in the United States with chrysotile fortunately being the primary fiber of choice. 

The primary useful property of phosphates that perhaps nothing else approaches, is the safety record of phosphates in manufacturing plants and households around the world. It was with this fact in mind that two candidates for new mineral fibers were chosen. There were at least eight other candidates that could have been selected. As noted, there are two general molecular forms of serpentine minerals that are referred to as asbestos. These are the chrysotile and the amphibole types. These two forms of asbestos are about as similar, chemically and physically, as sodium chloride and sucrose both are white crystalline solids. 

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