The term asbestos is a generic designation referring usually to six types of naturally occurring mineral fibers that are or have been commercially exploited. These fibers belong to two mineral groups: serpentines and amphiboles. The serpentine group contains a single asbestiform variety: chrysotile; five asbestiform varieties of amphiboles are known: anthophyllite asbestos, grunerite asbestos (amosite), riebeckite asbestos (crocidolite), tremolite asbestos, and actinolite asbestos. These fibrous minerals share several properties which qualify them as asbestiform fibers: they are found in bundles of fibers which can be easily separated from the host matrix or cleaved into thinner fibers; the fibers exhibit high tensile strengths, they show high length: diameter (aspect) ratios, from a minimum of 20 up to greater than 1000; they are sufficiently flexible to be spun; and macroscopically, they resemble organic fibers such as cellulose. Since asbestos fibers are all silicates, they exhibit several other common properties, such as incombustibility, thermal stability, resistance to biodegradation, chemical inertia toward most chemicals, and low electrical conductivity.
The term asbestos has traditionally been attributed only to those varieties that are commercially exploited. The industrial applications of asbestos fibers have now shifted almost exclusively to chrysotile. Two types of amphiboles, commonly designated as amosite and crocidolite are no longer mined. The other three amphibole varieties, anthophyllite asbestos, actinolite asbestos, and tremolite asbestos, have no significant industrial applications presently.
The microscopic and macroscopic properties of asbestos fibers stem from their intrinsic, and sometimes unique, crystalline features. As with all silicate minerals, the basic building blocks of asbestos fibers are the silicate tetrahedra which may occur as double chains (Si4O11)-6, as in the amphiboles, or in sheets (Si4O10)-4, as in chrysotile. In the case of chrysotile, an octahedral brucite layer having the formula (Mg6O4 (OH)8)-4 is intercalated between each silicate tetrahedra sheet. Asbestos fibers used in most industrial applications consist of aggregates of smaller units (fibrils). This is most evident with chrysotile that exhibits an inherent, well-defined unit fiber.
The identification of asbestos fibers can be performed through morphological examination, together with specific analytical methods to obtain the mineral composition and/or structure. Morphological characterization in itself usually does not constitute a reliable identification criterion. Hence, microscopic examination methods and other analytical approaches are usually combined.
Most of the asbestos mining operations are of the open pit type, using bench drilling techniques. The fiber extraction (milling) process must be chosen so as to optimize recovery of the fibers in the ore, while minimizing reduction of fiber length. Dry milling operations are the most widely used. In the production, or industrial applications, of asbestos fibers, several parameters are considered critically important. The measurement of fiber length is important since the length determines the product category in which the fibers will be used and, to a large extent, their commercial value. The most widely accepted method for chrysotile fiber length characterization in the industry is the Quebec Standard test. A second industrially important fiber-length evaluation technique is the Bauer-McNett classification.
