Document 48x7jyna4kRk5LDnajoqoVwa

10/08/2001 15:58 312-996-7822 SCIENCE LIBRARY SNViRONwenTAi. mssasch 16, JM-397 (197$) On the Utility of Dark-Field Electron Microscopy in the Determination of the Degree of Deformation In Chrysotile Asbestos: An Environmental Research Application K. Seshan Department of Materials Science and Engineering, Materials and Molecular Research Division, Lawrence Berkeley Laboratory. University of California, Berkeley. California 94770 Received June 21, 1977 Tbe degree of microsrytUUwe deformation in fiber* of chrysolite **be>to may be distin guished wine hick resolution dark-field electron microscopy. This i* demonstrated by com paring undefonned chrysotile with Union Internationale Contre !c Cancer (UICC) standard reference samples. The UICC sample* are shown to be partially deformed as a result of milting in the mining process- Samples of used and unused brake shoe lining dust were examined using this technique; it is shown that chrysotile asbestos in various stages of deformation--relatively utxjcformcd to heavily deformed--survives in automobile brake drum dust. Such dark-held images can serve to identify the source of asbestos found is environmental pollution sample*. INTRODUCTION There is a controversy io the literature whether or not fibers of chrygotile asbestos survive in automobile brake drum dust; two studies (Rohl et al,, 1976; Aisle et oL, 1976) report that the fibers do survive and earlier work (Lynch, 1968; Hickisb and Knight, 1970) reports that they do not. The latter claim that the chrysotile is converted to forsterite under the high temperatures attained in the braking process. As high resolution dark-field electron microscope images are sensitive to the degree of deformation, they can be used to distinguish between deformed and undeformed fibers and thus to resolve these two differing sets of results. It is shown using this technique that chrysotile asbestos fibers in various states of deformation--relatively undamaged to heavily deformed and recrystallized--exist in automobile brake drum dust. SAMPLE SELECTION AND PREPARATION Four samples were selected; (A) undeformed chrysotile ore samples from ser pentine outcrops of Calaveras County (California); (B) slightly deformed UICC reference standards of Canadian chrysotile milled during preparation to reduce fiber size (Timbrel! et al., 1969); (C) unused brake lining dust, collected during burnishing prior to installation of new brake shoes; and (D) brake drum dust collected from tbe front and rear brake drums of a State vehicle obtained during brake shoe service (Seshan and Smith, 1977). Samples were transferred directly to Formvar-coated electron microscope grids and coated with carbon on both sides. 0QI3-OTJ1/78/I6J J-WWSWWVO Cepyricft* W* by Actfank JtoM. Inc. Al} of rtprtduotior io guy tarn rf*rvod, SCF-FA-7252 PAGE