Document GKxvjkDMzoza9n8MjjrqwJd0Y

CAM-13 National Bureau of Standards Specia1 Publication 506. Proceedings of the Workshop on Asbestos: Definitions and Measurement Methods held at NBS, Gaithersburg, MD, July 18-20, 1977. (Issued November 1978) NOTE: This manuscript was not presented at the Workshop but was submitted for inclusion in the Proceedings. PRACTICAL ASPECTS OF TALC AND ASBESTOS C. J. Parmentier and G. J. Gill Cyprus Industrial Minerals Company Talc Division Los Angeles, California 90071 Abstract The present day controversy and misunderstanding regarding talc and asbestos has existed for many years. This paper reviews some of the reasons for the widespread public misconception that all tales contain asbestos. The experiences of a major talc producer are discussed in relation to occurrences of talc and asbestos, and the analytical techniques required to substantiate a talc-asbestos relationship are reviewed. Key Words: Asbestos; scanning electron microscopy; talc; transmission electron microscopy. Introduction Numerous environmental, occupational safety, and health agencies have recognized the medical hazards associated with asbestos minerals and have issued regulations for their use. However, during the past five years an increasing amount of controversy has been encountered regarding terms, definitions, and measurement methods. Due to the ambiguities in these criteria and various misconceptions regarding the mineralogy of talc, disagreements also exist between industry, medical researchers, state and federal regulatory agencies concerning talc and asbestos. This controversy affects a variety of industries, ranging from the ta1c producers themselves to the industrial consumers, their insurance carriers, and even the household consumer of the numerous products containing talc . This presentation will highlight some observations and experiences of a major talc producer, in relation to occurrences of talc and asbestos . Analytical techniques used for identification of asbestos minerals and fibers in a talc matrix are also discussed. Asbestos: As used in this presentation, the term "asbestos" pertains to fibrous forms of actino- lite, tremolite, anthophyllite, and chrysotile. All of the six recognized asbestos minerals are not included in this review as the above mineral types are the only asbestos minerals this laboratory has found to occur in tales to date. 403 SC-RTV-5042 CoogleDigitized by The terms acicular, lath, bladed, cleavage fragments, rods, needle, and columnar are often used as synonyms for the term fiber or asbestiform. It is the term 11 fiber11 which appears in need of a uniform definition. As used in this presentation, a fiber is described as a particle with a length to width ratio of 10:1 or greater. The present OSHA definition [1] 1 allows a length to width ratio of 5 or more to 1, with a maximum diameter of 3 ~m. Figure 1 illustrates and compares the simulated appearance of particles with the aforemenntioned ratios. The 5:1 length to width ratio allows blocky particles to be classified as fibers, illustrating the need for more realistic definitions and size criteria of a fiber. ASPECT RATIOS 3:1 5:1 10:1 Figure 1. Illustrated by the above drawing are the geoetric shapes which have been defined as fibers. The Asbestos Regulation CFR 1910. lOOL stipulates that a particle with an aspect ratio of 2:1 or greater is a fiber. However, these criteria were further modified by OSHA field information meorandum 174-92, November 21, 1974. The OSHA modification changed the criteria for a fiber from a 3:1 aspect ratio to a 5:1 length-to-width ratio with a maximum diameter of 3 ~m. The 10:1 aspect ratio is used at the Cyprus Industrial Minerals laboratory. Talc The term 11 talc11 has also been misused in published studies and requires definition. Talc refers to a specific mineral with a hydrated magnesium silicate composition of laminar or sheet structure. The natural platelet diameter of a talc varies, depending on deposit location and geological condition. Both the size and orientation of the laminar plates, with respect to each other, determine the characteristic form of the talc, e.g., massive or steatite talc versus foliated or micaceous talc. It is this characteristic talc structure which most often deten~ines end use applications. The large micaceous or foliated tales are ideal for cosmetic applications; while the finer particle size, 111assive or steatite type tales are most often used in filler and extender applications. The purity and locations of the talc deposit, in addition to the degree of benefication, also have a significant effect upon the end use application. 1 Figures in brackets indicate the literature references at the end of this paper. 404 CoogleDigitized by Talc and Asbestos Many misconceptions and misunderstandings of talc have existed due to improper use of terms such as tremolite talc, asbestiform talc, and fibrous talc. The majority of these improper terms were used to describe talc from the New York State talc district. In many earlier medical studies, which indicated a high incidence of talcosis, pneumoconiosis and mesothelioma among talc miners and millers from St. Lawrence County, New York, the supposition is made that talc dust was the cause . Kleinfeld and workers in 1967 [2] compared the mortality among talc miners and millers in New York State to findings among asbestos workers and noted similar pathological findings, to quote, "Rather characteristic was the presence of elongated, terminally clubbed bodies indistinguishable from asbestos bodies as seen in asbestosis." The talc dust exposure consisted predominantly of talc admixed with other silicates such as serpentine and tremolite, carbonates and a small amount of free silica, according to Or. Kleinfeld. Additionally, Or. Kleinfeld cites numerous other publications in which medical studies were performed on workers engaged in the milling or mining of the "fibrous variety of talc" [3-6] . However, none of these studies have differentiated between the mineral talc and the asbestiform variety of tremolite. In a later publication [7], studies of New York State talc deposits and their asbestos contents were carried out. Mineralogical analyses of these materials indicated that all samples were predominantly asbestos. The amount of asbestos impurities encountered in commercial talc ranges from <0. 1 percent to over 50 percent. The majority of commercial talc samples analyzed have asbestos contents ranging from <0.2 percent to a maximum of 2 percent. Commercial production talc samples from New York State show a range of 20 percent to over 50 percent asbestos. A few additional commercial tales have an asbestos content of 5 to 15 percent. Figures 2 and 3 illustrate the asbestos contents of typical New York commercial tales . Figure 2. Talc-Fiber No. 1 from International Talc Co. Analyzed as asbestiform tremolite with minor talc content. 405 CoogleDigitized by Figure 3. New York talc product used in the paint industry. Actual talc content is less than 40 percent. Majority of fibers are asbestos-tremol ite, identified by TEM/SAEO. Cyprus Industrial Minerals Co. is one of the largest talc producers in the world. We have a continuing exploration program and over the past ten years have characterized talc deposits worldwide, in addition to analyzing numerous commercial talc samples. With this background and experience we feel justified in stating that not all tales contain, or are associated with, asbestos. We also feel confident in stating that our ultra-fine grind Montana talc is the standard of talc purity throughout the world. Although high-purity talc deposits such as the Montana tales are rare, asbestos-free tales are not uncommon. Based on our experience and bank of analytical data, we are aware that some tales on the market do contain varying amounts of asbestos minerals. The most commonly encountered asbestos impurities in talc are tremolite and anthophyllite. There have been occurrences in which chrysotile asbestos has been found in selected talc samples. However, these occurrences are considered unusual but not unlikely. Analytical Techniques The methodology used for the analysis of asbestos fibers in tales is a subject of considerable controversy. Following is a summarized description of the techniques used by the Cyprus laboratory and the reasons for their use. Illustrated in Table 1 is the typical test procedure used to analyze talc for possible asbestos contamination. Although x-ray diffraction is shown as the initial method of analysis, it must be remembered that the primary function of the Cyprus Industrial Minerals laboratory is the characterization of tales. If we were concerned with analyzing tales for the presence of asbestos only, we would consider scanning electron microscopy as the most applicable technique to initiate analyses by screening samples for the presence of fibers. 406 CoogleDigitized by Table 1. Analytical procedure flow sheet XRD l ASBESTOS FREE no fibers SEM fibers found 1 TEM/SAED Scanning Electron Microscopy (SEM) has two very unique advantages compared to optical microscopy techniques . First is the capability to accurately identify a fiber by tilting the specimen, thus viewing a particle from various angles. Additionally, the added depth of focus characteristic of SEM allows the complete particle to be studied at the same time. Figure 4 is included to illustrate this advantage, and shows the typical curled edge of a talc platelet. If a similar talc platelet was observed by optical microscopy, the limiting depth of focus would dictate that only the curled edge was in focus and would appear as a fiber, or the plate would be in focus and appear as a separate particle. Figure 4. This micrograph illustrates the morphology of a typical curved talc platelet. The problems associated with attempting to define a fibrous structure by optical microscopy and it 1imited depth of focus can be seen. The optical microscope would make the above particle appear to be two separate particles, one of which would appear as a fiber and the other as a platelet. 407 CoogleDigitized by The second advantage of SEMis the easily varied magnification capabilities with sufficient resolution to distinguish the presence of very fine fibers. In contrast, optical microscopy techniques can allow a large number of very fine fibers such as chrysotile to go undetected. Langer, Selikoff, et al., s~ate "Many particles found in lung tissue are submicroscopic, measuring as little as 200 X(0.02 microns) in diameter, there- fore requiring e 1ectron microscopy" [ 1]). Suzuki and Churg a1so stated the preponderance of submicroscopic asbestos fibers observed in the successive steps in the development of the asbestos body were less than 1 1-1m in length, necessitating the use of the electron microscope [14]. Figures 5 and 6 are included to illustrate the SEM capabilities by revealing numerous chrysotile fibers present in a talc sample spiked with 1.5 percent chrysotile . Many of these fibers can be observed in figure 6 with sizes approaching single fiber diameters of ~20 nm (200 ~).well below the theoretical resolution limit of optical microscopy . Figure 5. CTFA spiked talc by SEM. 1.5% chrysotile, 0.5% tremolite x 2800 at 40 . .. Figure 6. Spiked talc by SEM. 1. 5% chrysotile x 13,5000 at 35. 408 CoogleDigitized by Scanning electron microscopy for initial sample screening also has advantages compared to transmission electron microscopy methods. The sample preparation is considered easier and less time-consuming than TEM, which requires preparation of filmed specimen grids for sample mounting. Additionally, and most significant is the fact that by SEM screening of talc samples for fiber presence, the amount of sample examined is approximately an order of magnitude greater than TEM methods (0. 1 mg vs. <0. 01 mg). This difference becomes a major factor when investigating talc samples for trace fiber content (<1000 ppm.). Transmission Electron Microscopy and Selected-Area Electron Diffraction This powerful analytical technique is capable of furnishing a combination of morphological and crystal structure data of small single fibers which can result in very conclusive identification. However, this method requires a good deal of operator expertise and accuracy of data. We feel that fiber analysis via electron diffraction, supplemented with morphological data, provides a positive identification. We have observed, however, many cases where workers have identified minerals through using only partial diffraction patterns; that is to say, only the high angle reflections (low numerical spacings). These spacings are very similar between silicate minerals making absolute identification impossible without the more conclusive low angle reflections (high numerical spacings) . Additionally, due to the unique possibi 1ities of unusua1 mineral occurrences, which are more the expected than the unexpected in mineralogy, the morphology of a fiber does not, and should not indicate asbestos. Along these lines, some examples of unexpected trace mineral fibers which have been encountered and subsequently identified in specific talc samples or deposits are: zeolite fibers (mordenite), clay mineral fibers (attapulgite-polygorskite), and a rare fibrous variety of antigorite-serpentine known as picrolite [12,15]. These materials are not asbestos by definition. All of the indicated fib~rs have similar d-spacings in the high angle reflections (numerically lower than ~s.oo A), indicating the necessity for using only the numerically high, low angle spacings for positive identification. Although SAED allows positive and undisputed identification of mineral fibers, only under specific conditions are SAED patterns considered positive identification. The following criteria must be met before an SAED pattern is considered accurate: 1. The camera constant must be determined on the same grid and same sample area as the SAED pattern, using a known standard. Preferred accuracy is obtained by shadowing the specimen preparation with a gold or aluminum metal which allows the camera constant to be ca1cul ated directly on the SAED pattern and acts as an i nterna1 camera constant. 2. The indexed d-spacings should include the most intense low angle spacing~. Any pattern without d-spacings numerically higher than ~s.oo A (low angle) should be questionable, as many silicate minerals have similar d-spacings in the higher reflection angles. 3. Where applicable, unit cell parameters should also be used to supplement fiber identification. X-Ray Diffraction X-ray diffraction (XRD) is a standard mineralogical technique. Within levels of detectability and lack of interfering reflections, it allows the identification and quantitation of asbestos minerals in talc. The major 1imitations of XRD are the lack of morphological data and the lower levels of sensitivity can allow a large number of asbestos fibers to go undetected. The information derived from XRD is invaluable for the characterization of a talc, and is of considerable help in the understanding of the mineralogical processes of forma- 409 CoogleDigitized by tion and the possible impurities present. The background information of mineral phases present aids and expedites the TEM-SAED interpretation of unknown fibers. Optical Microscopy Optical microscopic techniques are considered of minimal value in the analyses of fine-grained complex silicate mi nera 1 mixtures, and often result in ambiguous data of questionable value. As previously mentioned, optical microscopy techniques can allow a large number of very fine fibers such as chrysotile to go undetected [7, 13]. Due to these findings and the health and economic considerations involved, optical microscopy is not used at the Cyprus laboratory for asbestos in talc analyses or airborne asbestos fiber analyses. Conclusions This presentation has reviewed the misunderstandings and widespread misconceptions that all tales contain, or are associated with, asbestos. Some of the reasons for these misconceptions were shown to be the lack of mineral background in the use of terms and definitions. However, the use of inadequate analytical methods has been shown to be a major cause. The use of ambiguous analytical techniques such as optical microscopy for reasons of expense, lack of experience, or ease of analysis has far-reaching economic and health implications. Only TEM-SAED techniques supplemented by SEM appear to give the necessary degree of accuracy needed for positive results. The existence of large deposits of high-purity, asbestos-free talc are well documented, and it is hoped that future references to talc will be more clearly defined as to proper mineral content. In view of the evidence presented from both the medical and mineralogical science fields, it is evident that state and federal regulatory agencies need to redefine terms, definitions, and analytical methods for the assessment of asbestos and talc. This applies to both airborne exposure and bulk samples containing asbestos. References [1] Tremolite and Talc, U.S. Department of Labor, OSHA field Information Memorandum #74-92, November 21, 1974. [2] Kleinfeld, M., Messite, J., Kooyman, 0., et al., Mortality among Talc Miners and Millers in New York State, Arch. Environ. Health, J..1, 663-667 (1967). [3] Kleinfeld, M., Messite, J., and Tabershaw, I. R., Talc Pneumoconiosis, Arch. Indus- trial Health, ].g, 66-72 (July 1955). -- -- [4] Kleinfeld, M., et al., Lung Function in Talc Workers: A Comparative Physiologic Study of Workers Exposed to Fibrous and Granular Talc Dusts, Arch. Environ. Health, _2, 559-566 (Nov. 1964). -- --- [5] Messite, J., Redding, G., and Kleinfeld, M., Pulmonary Talcosis, A Clinical and Environmental Study, Ind. Health Perspect., 20, 408-413 (1959). [6] Siegal, W., Smith, A. R. , and Greenburg, L., The Dust Hazard in Tremol ite Talc Mining, Including Roentgenological Findings in Talc Workers, Amer. ~ Roentgen., 49, 11-29 (Jan. 1943). --- [7] Kleinfeld, M., Messite, J., and Langer, A., A Study of Workers Exposed to Asbestiform Minerals in Commercial Talc Manufacture, Environ. Res., ~. 132 (1973). [8] Rohl, A. N. and Langer, A. M., Identification and Quantitation of Asbestos in Talc, Environ. Health Perspec., _2, 95-109 (1974). [9] Rohl, A. N., Asbestos in Talc, Environ. Health Perspec., _2, 129-132 (1974). 410 CoogleDigitized by [10] Rohl, A. N., Langer, A. M., and Selikoff, I. J . , Environmental Asbestos Pollution Related to Use of Quarried Serpentine Rocks, Science, 196, 1319-1322 (June 1977). [ 11] Mumpton, F. A. , Characterization of Chrysoti1 e Asbestos and Other Members of the Serpentine Group of Minerals, Siemens Review XLI (1974). [12] Zussman, J., Brindley, G. W., and Comer, J. J., Electron Diffraction Studies of Serpentine Minerals, Amer. Minerals, 42, 133-153 (1957) . [13] Ehrenreich, T., Mackler, A. 0. , Langer, A. M. , Selikoff, I. J., Identification and Characterization of Pulmonary Dust Burden in Pneumoconiosis, Annals of Clinical laboratory Science,~. No. 2, 118-131 (1973) . [14] Suzuki, Y. and Churg, J., American~ Path., 55, 79 (1969). [15] Deer, W. A., Howie, R. A., and Zussman, J., Rock-Forming Minerals , Vol. 3, Sheet Silicate 173-174. 411 CoogleDigitized by