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Reprinted from Prinud in England Volume 18 First quarter January 1970 \ tmhe icrosc Incorporating "Crystal Front". Founded 1937 January-April 1970 Volume 18 No. 1 Annual Subscription: 5. S15.Seconddass postage paidat New York. Published by: Microscope Publications Ltd., 2 McCrone Mews, Belsize Lane, London, N.W.3., England. Identification of Asbestos Fibers by Microscopical Dispersion Staining* Y. JULIAN and W. C. McCRONE McCrone Associates, Inc., Chicago, Illinois, U.S.A. Abstract The Cherkasov focal screening dispersion staining procedure has been successfully applied to the identification of asbestos. The various types of asbestos can be differentiated by noting the refractive index of the Cargille liquid giving matching wavelengths in the region near 550 nm. It is not necessary to use polarized light although the much more definitive data obtained with polarized light may eventually permit identification of the mine from which each asbestos came. Although too few samples ofamosite and crocidolite have been studied, it appears that these two types can be differentiated by dispersion staining. In Cargille liquid ng3C 1 -680 the central stop without polars will show colors in the blue magenta region (X0 =-550-650 nm) for amosite and in the golden yellow region (>.,,= 400 - 500 nm) for croci dolite. Furthermore, with polarized light crocidolite will show lower birefringence than amosite; in terms of X<, difference in a given liquid amosite will show 160--190 nm and crocidolite 120-140 nm. Finally, the higher value of X* is observed for the vibration direction parallel to the length for crocidolite (three samples) but perpendicular to the length for amosite (two samples). * Presented at INTER/MICRO-69, London. England. I f Through publications1-3 and current research, the pneumoconiotic (lung hardening) and frequently cancer-producing hazards of asbestos are finally fully acknowledged. Hence, there is need of a method of determining qualitatively and quantitatively environmental pollution by respirable asbestos dust (1-7 pm). Asbestos is a fibrous form of silicate rock. There are six types: chrysotilc, amosite, crocidolite, actinolite, tremolite and anthophyliite; only the first three are of commercial importance. All are amphiboles (minerals with chains of silica tetrahedra as their basic structure) except chrysotile which is a serpentine (mineral made up of layers of silica tetrahedra). The property of asbestos that best lends itself to identification is refractive index. The fibers are so fine that electron microscopy would be necessary if morphology were to be used. There are no dependable differences in absorption color (except possibly for crocidolite) nor specific gravity (other than chrysotile) (Table I). If we wish to use chemical composition for identification of single asbestos fibers, we require highly sophisticated instruments and techniques--the electron microscope or an electron (or ion) microprobe. X-ray diffraction requires considerable sample and is not sensitive to small percentages in any sample (/.e., < 5-10%). Very small asbestos fibers can, however, be identified simply and quickly using dispersion staining. Experimental The McCrone dispersion staining objective, based on the focal screening method of Cherkasov4, was used in this study of asbestos (Figure 2). Figure J. Dispersion staining data for a typical sample of crocidolite. 2 MICROSCOPE (1970) IS TABLE I i ASBESTOS SERPENTINES AMPHIBOLES Composition Chrysotile 3Mg0.2Si02. 2Hj0 Spec. Grav. Cryst. Syst. Extinction Sign (elong.) 2.36-2.5 Monoclinic Y A L*=Q + Actinolite 2Ca0.4Mg0. FcO.8SiOi.HjO Tremolite 2Ca0.SMg0. SSiOj.HjO Anthophyllite 7Mg0.8Si0j. HjO Amosite 5-SFcO. l-5Me0. fiSiO.I+n 3.03-3.5 2.9-3.2 2.85-3.4+ 2.6-3.0 Monoclinic Monodinic Orthorhombic Monoclinic taL= 10-15" YAL~ 10-21" Y A L=0" YAL= 14-21" ++ ++ Crocidolite NojO.FejOj. 3FcO.8S.Oj HjO 3.0-3.45 Monodinic oaL=3-I5" -- *L=long direction of fibers i V. JULIAN & W. C. McCRONE Bfl 0015626 1 -- 1 ANNULAR STOP CENTRAL STOP Nr /, PREPARATION CONDENSER SUBSTAGE iris Figure 2. The arrangement for focal screening: the annular stop permits passage of the matching wavelength and the central stop white light minus the matching wavelength. Figures 4-7. Central stop colors for chrysolite (Figure 4), anthophyllite (2). amosile (6) and crocidolite (7) each with the appropriate Cargille refractive index liquid, 1.560 (Figured). 1.610 (Figure J), 1.670 (Figure 6), 1.700 (Figure 7). The vibration direction of the polar is east-west in each figure, 4 MICROSCOPE (1970) 18 Number of Samples 2 1.550 1 ANTHOPHYLLITE 5 TABLE II Wavelengths Corresponding to Observed Dispersion Staining Colors Matching Wavelength Vo. nm <420 420-440 440-470 470-500 500-540 540-580 580-610 610-640 640-680 >680 Colors observed Central Annular stop stop light yellow yellow golden yellow golden magenta reddish-magenta magenta blue-magenta blue blue-green blue-white dark-blue blue-violet blue blue-green green yellow-green yellow orange orange-red brownish-red Though phase contrast and dark-ground illumination are often used for dispersion staining, the data obtained by these methods are empirical. Pure colors are not obtained and the mixture of colors observed is dependent upon the particular optical system used. With the Cherkasov procedures, axial illumination is utilized and hence essentially pure reproducible colors are obtained. A graph of matching wavelength, was determined as a function of immersion liquid refractive index for each of 26 asbestos samples. The central stop in conjunction with Table II was found to be most useful in estimating because it provides better resolution and contrast; hence higher sensitivity for small single fibers. The procedure involved: 1. mounting each asbestos successively in those Cargille refractive index liquids imparting dispersion colors to it. 2. noting >.c for the sample using both polarized and unpolarized light. (a) fibers oriented parallel to the vibration direction of the polar show a (crocidolite) or y (all other asbestos fibers). (b) fibers oriented crosswise show p and y (crocidolite) or a and p (all other asbestos fibers). Random orientations under these conditions show crocidolite in p < n y positions and all other asbestos fibers in P ? n a positions. The two extreme colors noted for crosswise vibrations are designated a (highest X,,) and P (lowest Xe) for all asbestos fibers other than crocidolite or p (highest X,,) and a (lowest }.,,) for crocidolite. The small oblique extinction angle often observed with asbestos causes no significant variation in )... Figure 1 shows the data for one sample of crocidolite determined in this way. When the polarizer is removed, asbestos fibers show nominally a single color that is a mixture of all the wavelengths observed during rotation of the polarizer. In practice, due to polarization by reflection 6 MICROSCOPE (1970) 18 Y. JULIAN & W. C. McCRONE from the microscope mirror, there is a slight change in color when the stage is rotated. All our data were taken with the fibers normal to the mirror reflection vibration direction. Figures 4-7 show the dispersion staining colors of representative samples of chrysotile, anthophyliice, amosite and crocidolite res pectively. Each is shown in the various orientations (relative to the polarizer vibration direction) necessary for determination of a, fl and y. Careful study of these figures may indicate the simplified procedure used to orient the fibers at precise 90 degree angles to each other. To determine if the simpler application of dispersion staining without a polarizer is adequate for identifying asbestos, the fields of view of Figures 4-7 were also taken with no polar (Figures 8-11). The curves obtained without polar were plotted in a single graph (Figure 12). The bunching of the curves of each different type of asbestos indicates that for identification purposes a liquid can be chosen that will impart a characteristic color to one species of asbestos only. A polarizing microscope is not, therefore, required. However, we can anticipate that with additional samples of crocidolite and amosite there might be some overlap. For this reason, the distinction between the signs of elongation (Figures 6 and 7) and the crocidolite blue absorption color are mentioned as special aids for differentiation. In this case a polarizer is useful. 7 The characteristic color will vary somewhat for different samples of the same type of asbestos. So, the problem arose of determining the ranges of colors for each asbestos in the chosen liquid (the liquid in which most samples of any asbestos type give a color of wavelength 580-620 nanometers). To determine this range of colors for any type of asbestos, the matching wavelengths observed in the chosen liquid are plotted against the number of different samples showing that The area under the resulting curve (colored in Figure 3 as per Table II) shows all the colors shown by our samples of anthophyllite and chrysotile. Too few samples of amosite or crocidolite have been studied to justify their inclusion in Figure 3. Conclusion The limited results obtained indicate that asbestos can be identified by dispersion staining. More work on many more samples will establish the full range of k, values for each asbestos. Relating X0 data to asbestos source may then also pinpoint the mine source of unknown asbestos samples. Acknowledgement The authors are grateful to Dr. Bertram G. Woodland, Curator of Igneous and Metamorphic Petrology of the Field Museum of Natural History in Chicago for the samples of asbestos from various sources. 12 REFERENCES 1 H.M. Factory Inspectorate, (1968). Problem arising from the use of asbestos. Memorandum of the Senior Medical Inspector's Advisory Panel, Department of Employment and Productivity, London, Her Majesty's Stationery Office. 2 Smith, K. W., (1955). Pulmonary disability in asbestos workers. Arch. Ind. Health, 12 198-203. 3 Brodeur. P., (1968). The Magic Mineral, New Yorker Magazine, 12 October. 4 Brown, K. M., and McCronc, W. C., (1963). Dispersion Staining, Parts I and II, Microscope, 13 311 and 14 39. MICROSCOPE (1970) 18 j BB 0015631 I Y. JULIAN & W. C. McCRONE APPENDIX Dispersion Staining Data, n"'C of Cargille refractive index liquid Chrysolites Sample Orientation 450 Thetford Mines, Quebec y 1.539 1.535 0 1.533 a 1.528 Matching wavelength, >-o, nm 486(F) 520 590(D) 656(C) 1.542 1.546 1.553 1.559 1.539 1.543 1.550 1.556 1.539 1.543 1.547 1.556 1-531 1.534 1.541 1.546 Eden, Vermont E19043** Y 1.549 1.553 1.558 1.566 1.574 1.543 1.547 1.551 1.559 1.566 0 1.543 1.547 1.551 1.559 1.566 a 1.534 1.539 1.543 1.550 1.557 Thetford Mines, Quebec E2Q436 Y 0 a 1.545 1.540 1.538 1.536 1.549 1.546 ` 1.541 1.539 1.552 1.547 1.544 1.542 1.559 1.553 1.550 1.548 1.565 1.558 1.554 1.552 Thetford Mines, Quebec EI2185 Y P a 1.543 1.540 1.536 1.531 1.547 1.543 1.540 1.534 1.550 1.546 1.543 1.537 1.556 1.552 1.549 1.543 1.563 1.558 1.555 1.548 King's Mine, Quebec Y 1.543 1.547 1.551 1.559 1.566 1.539 1.543 1.547 1.554 1.561 P 1.535 1.539 1.543 1.550 1.557 a 1.529 1.532 1.535 1.542 1.547 Barquisimento, Venezuela .. Y* p a 1.536 1.530 1.530 1.524 1.541 1.537 1.534 1.528 1.546 1.541 1.539 1.532 1.556 1.550 1.547 1.540 1.566 1.559 1.555 1.548 * no polars were used. ** sample identification number of the Field Museum of Natural History, Chicago, Illinois. U.S.A. Minas Gerais, Brazil E16696 Maryland E3713 New Mexico E3709 Macon Co., N. Carolina E3700 Anthophyllitcs Y 1.604 1.613 1.602 1.608 0 1.599 1.606 a 1.582 1.588 Y 1.617 1.623 1.613 1.618 P 1.609 1.614 a 1.599 1.604 Y# 1.620 1.625 1.616 1.621 0 1.609 1.612 a 1.601 1.604 Y 1.614 1.618 1.610 1.614 p 1.606 1.610 a 1.594 1.597 1.620 1.614 1.612 1.594 1.630 1.624 1.619 1.609 1.629 1.625 1.616 1.607 1.623 1.619 1.614 1.604 1.633 1.628 1.625 1.606 1.642 1.636 1.630 1.620 1.638 1.634 1.620 1.613 1.632 1.627 1.622 1.607 1.645 1.638 1.636 1.616 1.658 1.649 1.642 1.630 1.648 1.642 1.628 1.618 1.641 1.635 1.629 1.613 I ii 1 i r Lincoln Co., Nevada El 101 APPENDIX (Continued) y* 1.609 1.611 1.616 1.602 1.605 1.609 6 1.596 1.599 1.602 a 1.589 1.592 1.595 Rio Grande de Sul, Brazil E14494 y B a 1.610 1.605 1.603 1.596 1.615 1.610 1.607 1.600 1.620 1.615 1.611 1.604 California El4464 y 1.619 1.624 1.630 1.614 1.619 1.624 B 1.612 1.616 1.622 a 1.601 1.605 1.610 Pine Mt., Georgia y 1.607 1.613 1.619 1.602 1.608 1.614 P 1.597 1.602 1.607 a 1.589 1.593 1.597 Encampment, Wyoming yft B a 1.619 1.613 1.609 1.598 1.620 1.616 1.612 1.600 1.626 1.619 1.615 1.603 Bedford, Virginia E12790 y 1.614 1.619 1.622 1.609 1.613 1.618 B 1.606 1.610 1.615 a 1.599 1.603 1.607 1.623 1.615 1.609 1.600 1.630 1.621 1.615 1.606 1.630 1.641 1.624 1.635 1.620 1.628 1.611 . 1.618 1.642 1.635 1.632 1.618 1.656 1.646 1.642 1.627 1.632 1.625 1.617 1.606 1.647 1.638 1.628 1.614 1.630 1.625 1.621 1.607 1.638 1.630 1.626 1.611 1.634 1.628 1.624 1.614 1.644 1.63B 1.633 1.622 Orange River, South Africa T 3 a Westerburg, South Africa r# 3 a South Africa Yft B a Crocidolites 1.688 1.684 1.680 1.674 1.694 1.690 1.686 1.678 1.686 1.682 1.679 1.674 1.692 1.687 1.683 1.678 1.691 1.683 1.680 1.674 1.696 1.690 1.685 1.679 1.700 1.696 1.692 1.682 1.697 1.692 1.688 1.683 1.701 1.695 1.690 1.683 1.712 1.707 1.702 1.690 1.708 1.701 1.697 1.691 1.712 1.703 1.700 1.692 1.726 1.721 1.713 1.698 1.718 1.710 1.706 1.699 1.723 1.714 1.710 1.700 Lydenburg Dist., Transvaal Penge, Transvaal 10 Amosites Y 1.676 1.682 1.687 1.699 1.712 1.663 1.670 1.676 1.687 1.698 3 1.658 1.663 1.668 1.678 1.689 a 1.652 1.657 1.662 1.672 1.682 Yft 1.672 1.677 1.682 1.692 1.701 1.665 1.669 1.674 1.682 1.691 B 1.660 1.664 1.668 1.676 1.684 0 1.653 1.657 1.661 1.669 1.676 MICROSCOPE (1970) IS J 1 ! ift I l- |J 5 I*4 ! I F j j I | BB 0015633 i