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V National Bureau of Standards Special Publication 506. Proceedings of the Workshop on Asbestos:- Definitions and Measurement Methods held at NBS, Gaithersburg, MD, July 18-20, 1977. (Issued November 1978) IDENTIFICATION OF ASBESTOS BY POLARIZED LIGHT MICROSCOPY Walter C. McCrone (Paper presented by John G. Delly) McCrone Associates, Inc. Chicago, Illinois 60616 Abstract A number of analytical tools can be used to characterize and identify asbestos: infrared absorption, x-ray diffraction, DTA, SEM, TEN, and the light microscope. Each has advantages and limitations. The polarized light microscope (PLN) has many advantages, and the only disadvantages are 1) the asbestos particles must be at least a micrometer in largest dimension, and 2) considerable training in optical crystallography is needed. PLM, on the other hand, is very sensitive (ppm range), extremely rapid (1-5 minutes to identify all components of most samples) and, of all the methods, only PLM will identify the individual amphiboles. Key Words: Amphiboles; asbestos; dispersion staining; microscopy. There are a number of analytical methods useful for the identification of asbestos. These include infrared absorption (IR), x-ray diffraction (XRD), differential thermal analysis (DTA), scanning electron microscopy (SEM), transmission electron microscopy (TEM) with or without electron microprobe analyzer (EMA), and polarized light microscopy (PLM) with or without dispersion staining (OS). Each has advantages and disadvantages. Every analyst uses and should use the techniques in which he has the required training and with which he feels confident. At the same time, every sample should ideally be analyzed by the most suitable technique. Occasionally, of course, it may be wise to use two or more techniques and this is certainly true for asbestos. We would like to summarize our attitude toward the various techniques for asbestos and describe in more detail the technique we feel has many advantages and is under-utilized; this is polarized light microscopy, especially when supplemented by dispersion staining. First, however, the advantages and disadvantages of each technique: TEM is most useful for the detection and identification of asbestos fibers smaller than the resolving power limit of the PLM. This is usually the case for water or beverages in general. Quantitative procedures are available so that the number, size, and identity of asbestos fibers per unit volume can be accurately determined. Identification by TEM depends on selected area electron diffraction (SAED). Occasionally energy or wavelength dispersive detectors are fitted to the TEM to make possible elemental analysis of individual fibers. Nothing can compete with TEM for the analysis of samples containing subpicogram particles. SEM has no advantage over TEM except that it takes prettier pictures. It will also fail To- see the smallest fibers, and lacking SAED it cannot identify all fibers. The energy dispersive detector on most SEMs is not as effective as the wavelength dispersive detectors on some TEMs and SEMs. XRD is a useful method since it can be made quantitative. However, it cannot tell size or shape, is not very sensitive (about 1 percent or a bit better), and does not differentiate between most of the amphiboles. At best it supplements other techniques. 235 2063105030

IR and DTA can also be dismissed for all except routine samples containing high percentages oi-asbestos. This brings us to PLM and 5 on which we wish to spend more time because of our conviction that, of all the eicroanalytical techniques for asbestos, it is by far the most effective. It is the only method depending on the unique optical crystallographic properties of the various crystal phases in the sample. These properties - refractive indices, dispersion of refractive indices, birefringence, sign of elongation and extinc- tion angle - are unique to the crystalline state and therefore unequivocally identify chrysotile, anthophyllite, tremolite, actinolite, grunerite, cummingtonite, etc. The background for dispersion staining has been adequately covered elsewhere [13i. Very briefly, it imparts color to any transparent particle mounted in a liquid whose dispersion curve intersects the dispersion curve for the particle in the visible. The colors, related to this matching wavelength, characterize and identify any given sub- stance. With polarized light, isotropic substances show a single characteristic color, but anisotropic substances show different colors corresponding to the different refractive indices in different orientations. Chrysotile, for example, shows blue and blue-magenta colors, crosswise and lengthwise respectively, for each needle crystal when mounted in Cargille high dispersion liquid nps = 1.550. The colors shown by the various types of asbestos and a few other associated minerals are indicated in Figures 1-20 by the wavelengths on each crystal view. These are the wavelengths at which the liquid indicated and that direction in the crystal have the same refractive index. This matching wavelength, ao, determines the dispersion staining colors. TALC Mg8(S18020)(OI)4 In 1.55 (H.D.): n's (D, H, Z) * 360 497 y m 1.589-1.600 5990.05(-) This sample (Vermont): a II 1.546 0= 1.588 y= 1.589 8 = 0.045 (-) * Deer, Bovie and Zussman. Figure 1. Dispersion staining colors shown by talc crystals in Cargille high dispersion liquids nD = 1.550 and n0 = 1.580. 'Figures in brackets indicate the literature references at the end of this paper. p 9K 360 _, MQo_, «A a=645 nID 0 - 1.589-1.594 N O w 236 ~. 0 ~ 0 w ~

CHLORITE (Mg, Al, Fe)12((Si' AI) 8020) (0R)16 In 1.55 (H.D.): pale yellow to golden yellow In 1.580 (H.D.): 529 nm = 420 nm 513nm a= 529 nm ~+~-- 420 nm p = 513 nm Figure 3. Chrysotile. IL n's (D, H, Z) aa 1.57-1.66 S ~ 1.57-1.67 y ~ 1.57-1.67 5 = 0-0.01 Figure 2. Chlorite. The sample (Caiifornia): a=1.586 $=1.587 y = 1.598 5 = 0.010 (+) CHRYSOTILE Mg3[S12O51(O1i)4 In 1.550 (H.D.); a= 660 nm FEj ~ 530 ma , '(y p- 558nm n'e(D,$,Z) a- 1.5324.549 S = 1.540-1.553 y s 1. 545-1.558 5 = 0. 013-0. 007 (-) Kingte Mine, Quebec Sample -a= 1.5444 9-1.5525 y= 1. 5555 5 = 0. 0111 ANTIGORITE Mg3(Si205)(OH)4 In 1.550 (H.D.); F3; •Y$1W--r =3na...,.~~~ Figure 4. Antigorite. n's (D, H, Z) a~ 1.558-1.587 ~ ~ 1. 56 -1.57 y s 1. 562-1.574 5 = 0.004-0. 007 ( ) This sample: a=1.555 A= 1.559 y= 1.561 N ~ 5 = 0.008 (-) W 237 O S W N

LIZARDTTE Mg3(53205) (OH)4 In 1.55 (H.D.); n's (D, H, Z) 520 494 nm ~= 690 nm ~494 nm undulose extinction Figure 5. Lizardite. TREMOLTTE Ca2(Mg, Fe ~5(Sl 8022)(OH, F)2 (low Fe) y = 427 nm 530 nm 91460 nm Figure 6. Tremolite. a= 1.538-1.544 Q° y = 1. 546-1.560 6 = 0. 016-0. 008 This sample: a= 1.545 9911.555 y-1.557 6=0.012 (-) n'a (D, H, Z) a = 1.604-1.619 y = 1.627-1.642 8 = 0. 021-0.023 (-) This sample: a= 1.599 y= 1.621 8=0.022() 238

ACTINOLITE Ca2(MB, Fe)5(Si8022)(OH' F)2 (20-80% Fe & 80-20% Mg) In 1.605 (H.D.): In 1.640 (H.D.): n's (D, H, Z) o•= 436 nm a= 1.619-1.668 9= y = 1. 642-1.687 5 - 0. 023-0. 019 (-) This Sample (Virginia): a- 1.633 9= 1.841 y=1.847 5 = 0.014 (-) Figure 7. Actinolite. ANTHOPHYLLTTE (Mg, Fe2+)7(S18022)(OH, F)2 ffi 1.580 (H.D.): n'a (D, H. Z) In 1.605 (H.D.): y=395am p = 370 am y ~ 300 nm a= 421 nm a- 598 nm Figure 8. Anthophyllite. a= 1.596-1.694 S = 1.605-1. 710 y= 1.615-1.722 5 = 0.013-0.028 (+) (-) This sample (Pine Mt., Ga. ) o:= 1.601 p = 1.618 y=1. 628 5=0.027 (-) 239

ANTHOPHYLLITE (Mg. Fe2+)7(Si8 22)(OH"~2 (Mg> Fe) In 1.640 (H.D.): In 1.67: ,, , p = 633 nm 557 am n's (D, H, Z) ~= 1.596-1.694 ~ = 1. 605-1.710 y = 1.615-1.722 6 = 0.013-0.028 (+) (-) This sample (Connecticut): n= 1.659 p = 1.666 y=1.674 5a0.015(+) Figure 9. Anthophyllite. GRUNERITE (Fe}2, Mg)7(Bi8022) (OH)2 (high Fe) ffi 1.670: In 1.700: a = 627 nm Figure 10. Grunerite. n'e (D. H, Z) a= 1.663-1.686 S 21 1.681-1.707 y= 1. 697-1.729 5 = 0.034-0.043 (-) Thie sample: a=1.669 5=1.684 y=1.697 b = 0. 026 (-) 240

CROCIDOLITE Na2Fe32Fe23(Si8022)(OH)2 (no Mg, contains Fe+2) In 1.670: 0 =386nm 0=418 = 380 nm nm In 1. 700: w= 408 nm E= 1. 624-1.666 a=1.688 0= 1.703 y=1.708 6 = 0.010 (-) Figure 11. Crocidolite. APATTTE Ca5(P04)3(OH, Cl, F) ln 1.606 (H.D.): w = 408 = 448 nm Tn 1.64 E=715nm Onm n's (D, H, Z) Figure 12. Apatite. n's (D, H, Z) tt= 1.654-1.701 P - 1.662-1.711 y - 1.668-1.717 6 = 0.006-0.016 (-) This sample (Orange River, South Africa): w - 1.628-1.667 w= 408 nm 6= 0. 001-0. 007 (-) This sample: E = 1. 6285 w = 1.6357 6 = 0.0062 (-) 241

FORSTERITE Mg2SLO4 ln 1. 640 (H.D. ): n's (D, H, Z) r ** a= 1.635 -1. 827 ~ = 1.651 -1.869 y=1.670 -1.879 6 = 0. 035 -0.052 (+) (-) *pure forsterite **plns Fe2t replacing Mg giving fayalite, Fe2Si04 This sample: a= 1.643 0 = 1.663 y= 1.682 6 = 0.039 (-) Figure 13. Forsterite. HORNBLENDE )(OH,F7 (Na, K)0.1-0.7Os2(Mg, F 2{, F 3+, Al) 5 (Si 6_7, At 2-1O22 2 ffi 1.605 (H.D): , _ _ ,,,, -,,, n's (D, H, Z) a= 570 nm Is. 1.640 (H.D): , _ 518nm 475 Figure 14. Hornblende. a= 1.615-L 705 ~ a 1.618-1.714 y a 1.632-1.730 6 - 0.014-0.026 (+) This sample: a= 1.843 ~ = 1.650 y= 1.660 6=0.017 (+) 242

es In 1.580 (H.D.): In 1.605: WOLLASTONITE Ca(8103) n's (D, H, 4 a a= 412 am a= 1.616-1.640 = 1 628-1 650 p~340nm S . . y = 1.631-1.653 y s< 300 nm 6 = 0. 015-0.013 (-) This sample: a= 532 am Figure 15. Wollastonite. CALCITE CaCO3 a= 1. 612 9=1. 628 y=1. 632 6 = 0. 020 (-) In 1.64 (H.D.): n's (D, H, Z) s - 1.486-1.550 m - 1.658-1.74 6 - 0.172-0.190 (-) Tbie sample: s'= 1.525 c=1.486 ~=1.653 6=0.167(-) Figure 16. Calcite. 243

DOLOMITE (Ca, Mg)CO3 In 1.64 (H.D.): n'e (D, H, Z) In 1.67: Figure 17. Dolomite. MAGNESTTE MgCO3 In 1.670: * ** e = 1.500*-1.520 ~ = 1.679 -1.703** 6 = 0.179 -0.185 (-) *pure dolomite **plus Fe2+ for Mg This eamplea w - 1.677 a's (D, H, Z) * e s 1.509-1.563* w = 1. 700-1. 782 6 = 0.190-0. 218 (-) *wlth Fe replacing Mg This sample: In 1.700: w=1.694