Amosites

Fig. 1. Asbestos fibers (chrysotile, crocidoHte, and amosite) as separated from host rock and their massive varieties (antigorite, riebeckite,...

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 dispersion of amphiboles in concentrated HQ. solutions also leads to partial leaching, the rate of which depends on the metal cations present. With crocidoHte, only small amounts of magnesium and sodium are extracted in these conditions, whereas amosite Hberates substantial quantities of iron and magnesium. Overall, tremoHte appears to exhibit the highest resistance to acid leaching. 

With respect to magnetic properties, the intrinsic magnetic susceptibiHty of pure chrysotile is very weak. However, the presence of associated minerals such as magnetite, as weU as substitution ions (Fe, Mn), increases the magnetic susceptibiHty to values around 6 x 10 m /kg. With amphiboles, the magnetic susceptibiHty is much higher, mainly because of the high iron content typically, amosite and crocidoHte exhibit susceptibiHty values of 100 and 75 X 10 m /kg, respectively (23). 

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. 

Other fiber classification schemes have been devised for chrysotile fibers, but historically the QS grade system has been used as a reference other classification schemes usually have correspondence scales for conversion to the QS values. Amosite can be classified according to the QS grade system, but crocidohte requkes a different scheme (mainly due to the harshness of these fibers). 

Loose asbestos fibers, or formulations containing asbestos fibers for spray coatings, have been widely used in the building industry for fire protection and heat or sound insulation. Such applications used mainly chrysotile or amosite but, because of health concerns, this practice has been discontinued. 

Respiratory fibrogens Fibrogenic dusts e.g. Free crystalline silica, (quartz, tridymite, cristobalite), asbestos (chrysotile, amosite, crocidolite etc.), talc. 

Identify the type of asbestos (or assume that it is crocidolite or amosite, to which stricter controls are applicable than to chrysotile). 

In the 1987 Regulations, asbestos is defined as any of the following minerals crocidolite, amosite, chrysotile, fibrous anthophyllite, fibrous actinolite, fibrous tremolite and any mixture containing any of these. Before carrying out work on any substance suspected of being asbestos, a competent person must be called to advise on its possible... 

Asbestos It is not the name of a distinct mineral species but is a commercial term applied to fibrous varieties of several silicate minerals such as amosite and crocido-lite. These extremely fine fibers are useful as fillers and/or reinforcements in plastics. Property performances include withstanding wear and high temperatures, chemical resistance, and strengths with high modulus of elasticity. When not properly handled or used, like other fibrous materials, they can be hazardous. 

Ammonium chloride fume Ammonium perfluorooctanoate Ammonium sulphamate Amosite, see Asbestos n-Amyl acetate sec-Amyi acetate Aniline and homologues " Anisidine (o>. p-isomers)... 

Amosite Chrysotile Crocidolite Other forms Asphalt (petroleum) fumes 0.5 fibre/cc" 2 flbres/cc" 0.2 fibre/cc" 2 fibres/cc 5 ... 

Asbestos S P Chrysolite Amosite Crocidolite Building products Insulation and removal operations Fillers in various industries Motor vehicle assembly Polymers, plastics filler... 

Evans, P.H., Brown, R.C. and Poole, A. (1983). Modification of the in vitro activities of amosite asbestos by surface derivati-zation. J. Toxicol. Environ. Health 11, 535-543. 

---