Chrysotile

In humans chrysotile (cleared in months) might have less effect in inducing cancer than the amphi-bole fibres (cleared in years). A detailed elaboration of this argument has been published by Berry (1999). 

SV-40-transformed human bronchial epithelial cells co-cultured (transwell system) with human blood monocytes and exposed to 100 fig chrysotile B per 10 cells showed deoxyribonucleic acid strand lesions induced by the reactive oxygen intermediates released from the mononuclear phagocytes engulfing the fibres (Kienast et al. 2000). The addition of 200 U catalase (EC 1.11.1.6) per ml or 100 /iM desferoxamine to the culture medium blocked almost completely the induction of DNA strand lesions in this system (maximum 85 %). 

Inhalation of quartz dust (F. Gross 1927, Klos-terkOtter 1957, Dhom and Sauer 1967) or mixed dusts containing quartz (Schiller 1958) just as a single intratracheal instillation of quartz (Schiller 1961, 1963) induced epithelial lesions resulting in a disability to remove alveolar macrophages containing dust and free particles impinged upon the airway surface. 

Experiments to study the influence of electrostatic charges of dust particles on their retention in rat lungs showed that electric charges do not influence the retention of quartz dust particles in deep 

Influence of silica inhalation on the pulmonary clearance of Listeria monocytogenes. Pre-exposure of rats to sihca (15 mg/m ) for 59 days (6 h/day, 5 days/week) caused substantial increases in the number of lavagable neutrophils and lactate dehydrogenase activity compared with the air control, whereas sihca inhalation for both 21 and 59 days significantly enhanced the pulmonary clearance of Listeria monocytogenes compared with air controls (Antonini et d. 2000). 

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Dispersion staining is useful for rapid deterrnination of refractive index and dispersion. It is appHed most often, however, for needle-in-a-haystack detection of any particular substance in a mixture such as chrysotile in insulation, cocaine in dust samples, quartz in mine samples, or any particular mineral, eg, tourmaline, in a forensic soil sample. 

Chrysotile is in the serpentine mineral group aU others are amphiboles. 

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

Chrysotile fibers can be extremely thin, the unit fiber having a diameter of approximately 25 nm (0.025 p.m). Industrial chrysotile fibers are aggregates of these unit fibers which usually exhibit diameters from 0.1 to 100 p.m their lengths range between a fraction of a mm to several cm, though most of the chrysotile fibers used are shorter than 1 cm. 

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). 

Fig. 4. Microscopic structure of chrysotile fibers (10). Reprinted with permission.

The extent of substitution of magnesium and siUcon by other cations in the chrysotile stmcture is limited by the stmctural strain that would result from replacement with ions having inappropriate radii. In the octahedral layer (bmcite), magnesium can be substituted by several divalent ions, Fe ", Mn, or Ni ". In the tetrahedral layer, siUcon may be replaced by Fe " or Al ", leaving an anionic vacancy. Most of the other elements which are found in vein fiber samples, or in industrial asbestos fibers, are associated with interstitial mineral phases. Typical compositions of bulk chrysotile fibers from different locations are given in Table 3. 

In contrast to chrysotile fibers, the atomic crystal stmcture of amphiboles does not inherentiy lead to fiber formation. The formation of asbestiform amphiboles must result from multiple nucleation and specific growth conditions. Also, whereas the difference between asbestiform and massive amphibole minerals is obvious on the macroscopic scale, the crystalline stmctures of the two varieties do not exhibit substantial differences. Nonfibrous amphiboles also exhibit preferential cleavage directions, yielding fiber-shaped fragments. 

Asbestos fibers used in most industrial appHcations consist of aggregates of smaller units (fibrils). This is most evident with chrysotile which exhibits an inherent, weU-defined unit fiber. Typical diameters of fibers in bulk industrial samples may reach several tens of micrometers fiber lengths are on the order... 

The morphological variance appears more important with chrysotile than with amphiboles. The intrinsic stmcture of chrysotile, its higher flexibiUty, and interfibnl adhesion (10) allow a variety of intermediate shapes when fiber aggregates are subjected to mechanical shear. Amphibole fibers are generally more britde and accommodate less morphological deformation during mechanical treatment. 

Fig. 6. Fiber length distribution for (a) a long sample (group 4) and (b) a short sample (group 7) of chrysotile successive length classes separated by 50 p.m.

Chrysotile fibers tend to become negative after weathering and/or leaching. 

On the other hand, both chrysotile and the amphiboles exhibit a high degree of chemical inertia towards strong alkaHes over extended periods. At high temperatures, reactions with alkaHes (NaOH, KOH, Ca(OH)2) become significant over relatively short periods for example, crocidoHte was reported to be attacked by potassium hydroxide above 100°C (22). 

Other Bulk Physical Properties. The hardness of asbestos fibers is comparable to that of other crystalline or glassy siHcates. Compared to glass fibers, amphiboles have similar hardness values, while chrysotile shows lower hardness values. 

The friction coefficients of asbestos fibers are also different for chrysotile and amphiboles (when measured against the same material). Compared to glass fibers, the friction coefficients decrease in the order chrysotile, amphiboles, glass fibers. 

The high electrical resistivity of asbestos fibers is weU-known, and has been widely exploited in electrical insulation appHcations. In general, the resistivity of chrysotile is lower than that of the amphiboles, particularly in high humidity environments (because of the availabiHty of soluble ions). For example, the electrical resistivity of chrysotile decreases from 1 to 2100 MQ/cm in a dry environment to values of 0.01 to 0.4 MQ/cm at 91% relative humidity. Amphiboles, on the other hand, exhibit resistivity between 8,000 and 900,000 MQ/cm. 

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). 

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