Asbestiform

These fibrous minerals share several properties which qualify them as asbestiform fibers (/) they are found in large clusters which can be easily separated from the host matrix or cleaved into thinner fibers (1) (2) the fibers exhibit high tensile strengths (1) (J) they show high length diameter ratios, from a minimum of 20 up to >1000 (1) (4) they are sufficiendy dexible to be spun and (5) macroscopicaHy, they resemble organic fibers such as cellulose (2). 

The usual definition of asbestos fiber excludes numerous other fibrous minerals which could be qualified as asbestiform following the criteria Hsted above. However, it appears the term asbestos has traditionally been attributed only to those varieties which are commercially exploited (1,2). 

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. 

Hansen, K. and Mossman, B.T. (1987). Generation of superoxide (Oi ) from alveolar macrophages exposed to asbestiform and nonfibrous particles. Cancer Res. 47, 1681-1686. 

The most common, and certainly the most familiar, inorganic fibers are the mineral fibers known as asbestos. Several minerals have been mined as asbestos (Fig. 1.1 A, C see chapter 2 for a detailed discussion of the asbestos minerals). These minerals are widespread in nature, but mining is economical only when they occur as continuous fibrous aggregates (veins) crosscutting rock masses. The veins are made up of hairlike fibers in parallel array that, on close inspection, appear bent (Fig. I.IB). Not surprisingly, a mineralogical term, asbestiform, has been used to describe this subset of inorganic materials. 

The crystal structures of all amphibole minerals, including the asbestiform varieties, are most easily understood as variations on a basic structural unit called an I-beam. The term I-beam alludes to the cross-sectional shape of the three-part structure consisting of corner-linked (Si04) and/or (A104) tetrahedra (T) linked together into a double-tetrahedral chain that sandwiches a layer of edge-shared and R Og octahedra (O). R elements, which... 

Silicate minerals that usually occur as spherulitic aggregates of fibers have formed as a result of the alteration of the many minerals subsumed within the category of biopyriboles. Alteration of the micas under hydrothermal conditions produces compositional variants on recrystallization such as hydrous muscovite. Some of these samples have been labeled asbestiform, probably because they are found in veins that criss-cross rock masses. Fibrous micaceous minerals also occur as discrete disseminated particles, although few detailed analyses of crystallites from the disperse occurrences have been made. Fibrous mica found in veins usually grades (composition-ally) into members of the serpentine mineral group, the clays or the chlorites. 

In his recent book, Dietrich (1985) described the common habits of tourmaline as being acicular, filiform, asbestiform, and chalcedonylike, and as whiskers. This last term indicates that tourmaline composition fibers have been synthesized. The wide range of forms for minerals in this group probably accounts for some of the early confusion when samples of tourmaline were equated with asbestos. Schorl, now known to be NaFe Al6(B03)3Si60ig(0H)4, usually occurs as dark green or black acicular crystals. Based on their appearance alone, schorl could easily be mistaken for an amphibole. 

Veblen, D. R., P. R. Buseck, and C. W. Burnham (1977). Asbestiform chain silicates, new minerals and structural groups. Science 198 359-365. 

Using radioactive labeled UICC samples, the deposition and distribution of asbestiform fibers in the pulmonary cavity have been studied. For example, after thirty minutes of inhalation, the deposition of fibers in the respiratory track was shown to be proportional to the median aerodynamic particle diameter for the two UICC chrysotiles, amosite, anthophyllite, and crocidolite. The percentage of total deposited fiber in the lower respiratory tract varied inversely as the square root of the particle diameter (Morgan et al., 1975). 

Differences in clearance rates between several asbestiform samples (chry-sotile, amosite, and crocidolite) were detected chrysotile was cleared most rapidly, corcidolite was deposited and retained in larger amounts than amo-... 

Breslow, L. (1984). Nonoccupational health risks of asbestiform fibers. Committee on Nonoccupational Health Risks of Asbestiform Fibers, Board of Toxicology and Environmental Health Hazards, Commission on Life Sciences, National Research Council, Washington, DC. 

Fig. 2.8B. From Zoltai, T. (1978). Asbestiform and acicular mineral fragments. Ann. N.Y. Acad. Sci. 330. Fig. 17, p. 635. With permission of N.Y. Acad. Science.

Although there are a number of contradictory reports regarding the effects of talc, the contradiction has been ascribed to the differences in mineral composition of the various talcs, which include pure talc, talc associated with silica and other nonasbestiform minerals, and talc containing asbestiform fibers such as tremolite and anthophyllite. ... 

Selevan SG et al Mortality patterns among miners and millers of non-asbestiform talc preliminary report. J Environ Pathol Toxicol 2 273-284, 1979... 

AMOSITE. Amosite is a long-fiber gray or greenish asbestiform mineral related to the cummingtonite-grunente series, and is of economic... 

Ross, M. R.A, Kuntze, and R.A, Clifton in B. Levadie, ed., Definition for Asbestos and Other Health Related Silicates. ASTM STP 834, American Society for Testing and Materials, Philadelphia, PA. 1984, pp. 139-147 W. J. Campbell and co-workers, Selected Silicates Minerals and ThcF Asbestiform Varieties, 1C 8751, US. Bureau of Mines, Washington, DC, 1977, pp. 5-17, 33. 

Caillere, S., 1936a. Etude de quelques silicates magnesiens a facies asbestiform ou papyrace n ap-partenant pas au groupe de l entigorite. Bull.Soc.Franc. Mineral., 59 353-374. 

Two molecular types of silicates are referred to as asbestos. Chrysotile is a magnesium silicate built upon a layered structure of silicate rings and Mg(OH)2. The layered structure causes the sheets to roll into cylinders approximately 200A in diameter. Amphibole asbestos may contain a variety of cations but is built upon a double chain silicate structure. The chrysotile asbestos is always found as an asbestiform crystal while the amphiboles may be either acicular or asbestiform. 

---