U.S. DEPARTMENT OF THE INTERIOR U.S. GEOLOGICAL SURVEY Asbestos: Geology, Mineralogy, Mining, and Uses by Robert L. Virta1 Open-File Report 02-149 Prepared in cooperation with Kirk-Othmer Encyclopedia of Chemical Technology, Online Edition, Wylie- Interscience, a division of John Wiley & Sons, Inc., New York, NY This report is preliminary and has not been reviewed for conformity with U.S. Geological Survey editorial standards (or with the North American Stratigraphic Code). Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the U.S. Government. 1Reston, VA Abstract ...................................................................................................... 4 1. Introduction ................................................................................................. 5 2. History ..................................................................................................... 5 3. Geology and Fiber Morphology ................................................................................. 5 3.1 Chrysotile ............................................................................................... 6 3.2 Amphiboles ............................................................................................. 6 4. Crystal Structure of Asbestos Fibers .............................................................................. 7 4.1 Chrysotile ............................................................................................... 7 4.2 Amphiboles ............................................................................................. 7 5. Properties of Asbestos Fibers ................................................................................... 8 5.1 Fiber Length Distribution .................................................................................. 8 5.2 Physico-chemical Properties ................................................................................ 8 Thermal Behavior ......................................................................................... 8 Tensile Strength .......................................................................................... 9 Asbestos Fibers in Aqueous Media ........................................................................... 9 Other Bulk Physical Properties .............................................................................. 9 5.3 Surface Properties ....................................................................................... 10 Surface Area ............................................................................................ 10 Surface Charge in Aqueous Media .......................................................................... 10 Adsorption and Surface Chemical Grafting ................................................................... 10 6. Analytical Methods and Identification of Asbestos Fibers ........................................................... 10 6.1 Microscopic Methods .................................................................................... 10 6.2 Instrumental Methods for Bulk Samples ..................................................................... 10 7. Production ................................................................................................. 11 8. Mining and Milling Technologies .............................................................................. 11 9. Fiber Classification and Standard Testing Methods ................................................................ 12 9.1 Dry Classification Method ................................................................................. 12 9.2 Wet Classification Method ................................................................................ 12 9.3 Other Fiber Evaluation Methods ............................................................................ 12 10. Industrial Applications ...................................................................................... 13 11. Alternative Industrial Fibers and Materials ...................................................................... 13 12. Health and Safety .......................................................................................... 14 13. Regulation ................................................................................................ 14 Bibliography ................................................................................................. 15 Table 1. Asbestos Fiber Production ............................................................................... 17 Table 2. Geological Occurrence of Asbestos Fibers .................................................................. 17 Table 3. Elemental Analysis of Asbestos Fibers ..................................................................... 18 Table 4. Physical and Chemical Properties of Asbestos Fibers ......................................................... 19 Table 5. World Production of Asbestos by Principal Producing Countries, 1920-2000a,b, tons ................................ 20 2 Table 6. Utilization of Asbestos Fibers by Product Category and Process in 2000 .......................................... 21 Table 7. Uses of Asbestos ...................................................................................... 22 Table 8. Asbestos Substitutes and Relative Costs .................................................................... 23 Figures Figure 1. Asbestos fibers (chrysotile, crocidolite, and amosite) as separated from host rock and their massive varieties (antigorite, riebeckite, cummingtonite-grunerite) ........................................................................... 24 Figure 2. Electron micrographs of asbestos fibers: (a) chrysotile; (b) crocidolite ........................................... 24 Figure 3. Silicate framework of chrysotile and amphiboles: (a) the sheet silicate structure of chrysotile, analogous to that of micas; (b) the double-chain silicate structure found in amphiboles ............................................................ 25 Figure 4. Microscopic structure of chrysotile fibers .................................................................. 25 Figure 5. Microscopic structure of amphibole fibers ................................................................. 26 Figure 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 :m .................................................................................... 26 Figure 7. Thermal analysis curves for asbestos: (a) chrysotile; (b) crocidolite (in inert atmosphere) ............................ 27 Figure 8. World production of asbestos, 1931–2001 ................................................................. 27 Figure 9. Schematic of a typical asbestos milling flowline (30 mesh-590 :m) .............................................. 28 3 ABSTRACT 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 -6 -4 (Si4O11) , as in the amphiboles, or in sheets (Si4O10) , as in chrysotile. In the case of chrysotile, an octahedral brucite layer having the -4 formula (Mg6O4 (OH)8) 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