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QUARTZ In 1.550 (H.D.): W= 682 = 682 nm 510 2 n's (D, H, Z) m=1.544 e = 1.553 682 5 = 0.009 (+) = 553 nm This eamplee m=1.544 [=1.553 Figure 19. Quartz. ORGANIC FIBERS (1. 550 liquid) ny- 630 nm Paper fiber figure 20. Organic fibers. Although we speak of dispersion staining colors as specific for a given substance in a given liquid (at a given temperature) we sometimes observe closely similar colors for other substances. We must, especially when this possibility exists, make sure that we observe enough data to be able to state with certainty that the substance is, say, chrysotile. It is not sufficient to observe the proper color in one direction - both chrysotile and paper fibers can show the same blue color perpendicular to their lengths. Nor is it sufficient to observe the two colors on a single view of a crystal - both quartz aad chrysotile can have two colors in common. If all colors shown by the crystal in all orientations correspond to the known data for a given substance, and if the crystal morphology shows the colors to be oriented properly, there is then very little chance of misidentification. 245 2063105040

Another serious complication, especially with minerals, is the eff-ect of substi+ tutionat solid s9lution op the optical properties. The substitution of F for OH , Fe2 for Mg2 , or Ca2 for 2Na can drastically change the optical properties of many minerals. One of the most serious in this respect is anthophyllite. Nominally Mg75is0Y2(OH)Z, anthophyllite forms a continuous series of solid solutions with iron replacing magnesium (Table 1) with corresponding changes in the refractive indices and dispersion staining colors. Anthophyllite can also have up to 14 percent MnO, 10 percent ZnO, or 15 percent A1203 with corresponding variations in the optical properties. Figures 8 and 9 show the dispersion staining properties for two different anthophyllites, one from Connecticut and the other from Georgia. In spite of the wide differences between these two anthophyllites, both samples show parallel extinction, a unique characteristic among the asbestos minerals, and the birefringence values, y-p, S-a, and y-a, as well as the optic axial angle remain quite uniform or change progressively and uniformly as the composition changes. If, for example, one observes refractive indices in the anthophyllite range, the possibility of tremolite, actinolite, ferroactinolite, or cummingtonite should be considered. The index range will tell which is present, and all of the latter differ from anthophyllite in that they show oblique extinction, usually about 20° rather than parallel extinction. In other words, anthophyllite is orthorhombic; all other amphiboles (and chrysatile) are monoclinic. a Table 1. Optical properties in Jhe anthaphyllite solid solution series. - - - - Refractive indices - - - - X Fe a A y 2V 0 1.596 1.608 1.615 120(-) 20 1.622 1.632 1.642 91(-) 40 1.641 1.650 1.665 685+) From Deer, Howie, and Zussman, "An Introduction to the Rock-Forming Minerals," Longmans, London (1966), pages 156-7. Many interfering substances are just not fibrous, hence they can be ignored if only asbestos is the target. Quartz has only two refractive indices, 1.544 (w) and 1.553 (s), but these fall within the range of chrysotile, a = 1.544 and y= 1.558. However, chrysotile is very fibrous whereas quartz is usually flakes or chips. Chrysotile shows three refrac- tive indices a, S, and y and a low 2V = 30-35° (+) and always shows nearly the maximum birefringence, 0.014 or 0.012. Quartz can show any birefringence value between 0.000 (w-w) and 0.009 (s-w) depending on orientation. Even a thin sliver of quartz oriented to show e and w (and therefore chrysotile colors) can be bounced into other more nearly isotropic orientations by tapping on the coverslip with a needle. Organic fibers are not generally confused with asbestos because they have obvious morphological differences, e. , pits, twists, central lumens, nodes, cross-over marks, etc. However, if mechanically broken down into tiny fibrils they lose this obvious morphology and some, e. , wool and other animal hairs, may closely resemble chrysotile in optical properties. A carefui exa.ination of such fibers morphologically and optically will usually, however, end any confusion and permit certain identification. Glass or mineral wool may happen to show a color near the chrysotile range but these, of course, are isotropic and morphologically quite distinctive. With careful application, dispersion staining is capable of rapid certain identifica- tion of any transparent substance whose optical and morphological properties are known. It also quickly differentiates between fibrous and nonfibrous minerals and detects traces of any substance in extraneous mixtures. 246 I

Reference [1] Brown, K.M., McCrone, W.C., Kuhn, R. and Forlini, L., Microscope 13 311; 14 39 (1963). Discussion J. ZUSSMAN: I enjoyed this very beautiful demonstration of the method. This is an academic question, but I think I remember a phenomenon called "form birefringence" which is supposed to be effective in giving peculiar results for very fine particles of small dimensions. If you have a very fine piece of an isotopic material, there is a shape factor which can make it appear to be anisotropic. I wonder if you get any anomalies with this method coming up, particularly with chrysotile, with fine fibrils, because of form. I think it is called form birefringence. J. DELLY: To answer your question, yes, there is an effect, but we don't apply this technique to a single isolated fiber, so there is not really much chance of being wrong on that. I agree with you, it is extremely fascinating academically, but in a practical sense with a bulk sample there are so many fascinating things associated with it that one spends actually a great deal of time with any one sample playing with colors. R. DRAFTZ: We have been using some of the techniques, and run into a problem with paper fibrils, especially with parenteral contaminants. I wonder if you tried the dispersion technique with chrysotile and with paper fibrils and perhaps found some similarities in color since the refractive index range is about the same. DELLY: You will see that the highest reported value of y of chrysotile (Deer; Howie, Zussman) is 1.556. ,1, in 1.550 HO refractive index liquid is about 515'nm. Figure 20 of the article shows that paper fibers in liquid 1.550 will show a>to of 450 nm parallel to the fiber. This wide difference in wavelengths should be easily discerned by most people. In any case, the microscopist is in the enviable position of settling the matter finally by resorting to the familiar cuoxam test to detemine whether a given fibril is cellulose or not. J. LEINEWEBER: I appreciate your very elegant description of the technique, and it has aroused a lot of questions in my mind about how the dispersion staining really works, but I would also appreciate a comment or two on the advantages of this technique over ordinary petrographic techniques for fiber identification, and also the size limits that you are confined to in working with particular particles. DELLY: Those are a couple of very good questions. First one: The major advantage is speed. For somebody who does primarily dispersion staining, he can complete an analysis in, probably, under five minutes. It is cheap and it is fast. It is a very quick survey type of thing, a very quick confirmation. I think that is probably the primary advantage of the technique. But the lower limit is a bit tricky. The abstract says that the major dimension should be one micrometer, which, if you are going to use 3:1, makes it about 0.3 pm or 0.25 Nm for the minimum. This technique does not depend on resolving power. It could not; otherwise you would not put all these stops in the back focal plane that deliberately destroy the resolving power. But, the spread of the light is all you are really looking for. You don't want to see the particle. So, that the lower limit is probably nominally around 0.3x1 pm. The reason I say nominally is, as with any other technique, when you go to the limits of any instrumental technique, the art starts coming in as well as the science. There is no reason though, why you could not apply this technique with higher-aperture objectives as well and still carry it further down. I have not personally done it. V. WOLK000FF: I just cannot see the advantage of this particular technique compared to classical techniques. For example, even if crocidolite does or does not show the blue color, you can pick it up immediately under crossed polars. We have no difficulty whatever using classical methods for the time element or whatsoever the case may be. And as one gentleman pointed out, for paper fibers or textile fibers we can pick that up instantaneously. Also, we are looking for the resolution, and, as you well know, materials 247 2063105042

containing asbestos fibers contain other materials as well. I must agree that the slides are extremely glamorous and picturesque, but I really believe that there is just no substitute for the classical petrographic or optical mineralogy when it comes to solid solutions that exist in several of these asbestos series. I just want to go on record on that. DELLY: Dispersion staining methods do not exclude classical methods; indeed, they are used simultaneously. The commercial form of the dispersion staining objective has three positions of use: a central stop, an annular stop for dispersion staining, and a position free of any stops which is used for classical methods in conjunction with dispersion staining. 248