Asbestos
Asbestos is a commercial term that includes six regulated asbestiform silicate (silicon + oxygen) minerals. Because this group of silicate minerals can be readily separated into thin, strong fibers that are flexible, heat resistant, and chemically inert, asbestos minerals were once used in a wide variety of products. However, due to adverse health effects, the use of asbestos in the U.S. has been significantly decreased. In 2013, for example, the total amount used was only 950 tons, all of which was chrysotile, and was mined in Brazil. Many other countries still mine and use asbestos in insulation products due to less stringent health and safety regulations.
Type
Mineral
Mineral Classification
Silicate
Chemical Formula
Mg3(Si2O5)(OH)4 - Chrysotile; Fe7Si8O22(OH)2 – Amosite; Na2Fe2+3Fe3+2Si8O22(OH)2 – Crocidolite; Ca2Mg5Si8O22(OH)2 – Tremolite asbestos; Ca2(Mg, Fe)5(Si8O22)(OH)2 – Actinolite asbestos; (Mg, Fe)7Si8O22(OH)2 – anthophyllite asbestos
Streak
White
Mohs Hardness
2.5-3
Crystal System
Monoclinic, Orthorhombic
Color
gray, white, blue, green
Luster
Silky
Fracture
Fibrous
Description
Asbestos is a commercial term that includes six regulated asbestiform silicate (silicon + oxygen) minerals. Because this group of silicate minerals can be readily separated into thin, strong fibers that are flexible, heat resistant, and chemically inert, asbestos minerals were once used in a wide variety of products. However, due to adverse health effects, the use of asbestos in the U.S. has been significantly decreased. In 2013, for example, the total amount used was only 950 tons, all of which was chrysotile, and was mined in Brazil. Many other countries still mine and use asbestos in insulation products due to less stringent health and safety regulations.
Relation to Mining
(IMAR 7th Edition: Gross and Braun 1984, Alleman and Mossman 1997, Sinclair1959; Selikoff and Lee 1978)
Asbestos has been mined for more than 4,000 years with evidence of its use as early as 2500 BC. A small asbestos industry appeared in the early 1800s when the Italians established a textile manufacturing industry. Use of asbestos did not increase significantly until after 1880 when large chrysotile deposits in Canada and Russia, and amosite and crocidolite deposits in South Africa, were discovered. The development of these deposits provided a large and steady supply of asbestos, allowing the asbestos industry to expand. This expansion came to its peak in 1975 with more than 25 countries mining asbestos. Growing opposition to the use of asbestos began to affect worldwide demand beginning in the early 1970’s. Many mines, manufacturers, and installers or users of asbestos products began facing an increasing number of class-action lawsuits. As a result many of the short-fiber asbestos resources were dropped and replaced with substitutes. Today virtually the only form of asbestos still mined is long-fiber chrysotile asbestos.
Asbestos Mining: IMAR 7th Edition
Chrysotile mining developed through a succession of mining practices and equipment. Originally, the simplest hand methods were employed in shallow open-pit workings. Later, when pits reached considerable depth, overhead cableway derricks were used. Now, power shovels and heavy-duty trucks have replaced other loading and transporting equipment for open-pit quarry methods.
In its early history, most chrysotile was mined in Canada using underground methods, which included glory holes, shrinkage and sublevel stoping, and block caving. Now open-pit mining prevails. Surface mining offers advantages in recovery, grade control, economy, and safety. The shift to open-pit mining resulted mainly from the introduction of large-capacity power shovels, trucks with higher payload capacities, improved rock-drilling equipment, and new blasting agents and techniques.
In Africa, chrysotile is still mined underground. Most ore bodies are tabular in shape with a pronounced dip so that the economic limit for quarry mining is reached at a comparatively early stage. Ore widths in the larger mines commonly range from 20 to 60 m and as much as 120 m. Some ore bodies, notably in the Shabani District of Zimbabwe, are long. In one case, development extended for 5 km along the strike and was being developed or diamond drilled to more than 300 m in depth.
Several underground methods have been used. Sublevel stoping and caving may be initiated by blasting holes drilled upward from sublevel crosscuts, starting first on the hanging-wall side and retreating over a considerable width toward the footwall. Development and retreat also may be along the strike of the ore. In some cases, high pressures tend to develop from an arching effect. These pressures can be released by cutting a vertical slot that may extend to the surface. In the sublevel stoping method, a slot also may be opened across the center of the ore body. The holes that are fanned out from the sublevel drifts are blasted toward the slot, and mining proceeds as a systematic retreat in two directions away from the opening.
Uses
Asbestos is used to make heat resistant products. Long asbestos fibers are preferred, and short fibers are worth only a fraction of the price. The former uses of asbestos in building construction (fireproof ceiling panels in schools, for example) have largely disappeared, although asbestos is still used in making asbestos-cement products, automobile and truck brakes, roof castings, and applications where the fibers are encased in other materials and are unlikely to become free-floating.
Roofing products containing asbestos (asphalt coatings) account for more than half of U.S. consumption. Friction products such as brake pads, and gaskets, account for about another 20%. Asbestos is also used for some specialized concrete products.
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