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CS 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) OCCUPATIONAL SAFETY AND HEALTH ADMINISTRATION METHODS Willard C. Dixon Occupational Safety and Health Administration , Salt Lake City, Utah 84117 Abstract Occupational Safety and Health Administration (OSHA) uses the membrane filter method at 400 - 450X magnification (4 mn objective) with phase contrast illumination for the analysis of asbestos in air. This method is substantially the same as is used by NIOSH. In an atmosphere known to contain asbestos, all particulates with a length to diameter ratio of 3:1 or greater and a length greater than 5 micrometers are, in the absence of other information, considered to be asbestos fibers and counted as such. The equipment for optical analysis of asbestos in use at the OSHA Salt Lake City Laboratory includes Zeiss microscopes having 40K objec- tives and lOX eyepieces, rotating stages, phase contrast illumination, polarized light, and retardation plates. The transmission electron microscope equipment in use by OSHA at the Salt Lake Laboratory is a Jeol model JEM 100C with a side entry goniometer and ASID-45 Model EM-15 SPS-2 scanning image display unit. We also have an Ortec-Delphi x-ray energy dispersive system. X-ray diffraction, atomic absorption, and other instrumentation are also available. The techniques used for the identification of asbestos include sight recognition based on morphology, and optical tests including polarized light, index of refraction, angle of extinction, dispersion staining, and retardation. Electron microscopy tests include morphol- ogy, selected area diffraction, and a determination of elemental composition by x-ray energy dispersive analysis. A plan is presented for distinguishing between asbestos and other fibers which may be mistaken for asbestos. A system for differentiating between the various kinds of asbestos fibers is also presented. Key Words: Airborne fiber; asbestos; bulk samples; dispersion staining; membrane filter; optical microscope; phase contrast. OSHA performs all routine determinations of airborne concentrations of asbestos fibers by the membrane filter method at 400 to 450X magnification (4 millimeter objective) with phase contrast illumination. The optical microscopes in use at the Salt Lake City OSHA Laboratory are Zeiss phase contrast microscopes which are also equipped with a polarizer, analyzer, retardation plates, and rotating stage with degree markings at the edge. The objectives are 40X and the eyepieces are 1oX. We also have Wild Stereo microscopes equipped with a polarizer, analyzer, retardation plates, and rotating stage. 431 2063105220

The primary emphasis . in identification of asbestos fibers is optical microscopy. Back-up methods include electron microscopy, x-ray diffraction, atomic absorption, and wet chemistry. Other instrumentation or methods are available if needed. The Salt Lake City OSHA Laboratory has a Jeol transmission electron microscope, JEM 100C, with a side entry goniometer and ASID-4S model EM-15 SPS-2 scanning image display unit. The system includes an Ortec-Delphi x-ray energy dispersive unit with a POP 11/05 computer and Digital Decwriter II teletype with AED 3100 P dual drive for floppy disks. The JEM 100C is basically a transmission microscope, but is equipped to function as a scanner. We can obtain selected area diffraction patterns, or determine which elements are present in particles or fibers, provided that the atomic number is eleven or greater. The OSHA Salt Lake City Laboratory uses Philips Norelco XRG-3000 x-ray diffractometers. This Lab has eleven ganiometers. Analysis of Bulk Samples Bulk samples are examined on a reflected light stereo microscope for the presence of fibers. Fibers can be isolated from the matrix at this time for identification by optical methods, x-ray diffraction or electron microscopy, or further analysis can be performed with fibers still in the matrix. Small fibers not easily identified by optical techniques, samples subject to litigation, or samples which are not positively identified by other means are the most likely candidates for electron microscopy. Slides are prepared and examined with a Wild transmitted light stereomicroscope having crossed polars and a first order red retardation plate. Asbestos fibers and bundles are recognized by their appearance. Oftentimes plant fibers will curve a little at the edge. The slides are examined at 6X, then at higher magnifications up to 50X on the stereomicroscope. This is followed by optical examinations at various magnifications, including 400X with a phase contrast microscope. At this point, it would be known whether asbestos is present and in approximately what concentration. If organic materials interfere too greatly, the sample is ashed at 550 °C and re-examined. The percent ash places an upper limit on the possible concentration.of asbestos. X-ray diffraction scans are run between 6° and 60° 20. X-ray diffraction can determine the concentration of a mineral but not how fibrous the mineral is. Tremolite may be present in high concentration in a talc sample, for example, but an actual fiber count may show the tremolite asbestos concentration is much lower. For this reason, OSHA does not place its entire reliance on the results of x-ray diffraction without optically confirming the concentration of fibers present. The percent by number of asbestos in talc samples is determined by particle counting. If a scan of two slides shows no fibers present, the analyst will report no asbestos detected without counting the slide. If a preliminary scan shows the presence of asbestos fibers, 100 fields or 100 asbestos fibers will be counted with a minimum count of 20 fields. In order to randomize possible differences between slides, the counts will be divided between four slides taken from different parts of the bulk sample. 432

, Analysis of Membrane Samples A fiber which has the correct size and aspect ratio for counting is not counted if other information is obtained which proves that the fiber is not asbestos. This infor- mation may be obtained by any scientifically valid method, including either optical or electron microscopy, x-ray diffraction, or wet chemical tests. In outlining some procedures which have been found useful at the Salt Lake City OSHA Laboratory, it is not intended to imply that other procedures cannot be used. Many textbooks on mineralogy will include identification tables and systematic outlines for mineral identification. "Optical Mineralogy" by Paul F. Kerr [1]i will be useful for the beginning analyst. "The Microscopic Determination of the Non-Opaque Minerals," Geological Survey Bulletin 848 [2] has extensive tables. "Gemstone and Mineral Data Book" by John Sinkankas [3] has specific gravity and other tables for mineral identification. The optical tests for asbestos or other fibers are divisible into two categories: A, those tests which can be performed while the fiber is still on the membrane, and 8, those tests which are performed after fibers have been removed from the membrane. A - Fibers on the Membrane In making the distinction between asbestos and non-asbestos fibers, it is highly desirable for the analyst to be familiar with the morphology of asbestos fibers and those fibers which are likely to be confused with asbestos. Thickness, pattern, and mbrphology will often be a clue that a fiber is fiberglass, fur, hair, plant fiber, or other non- asbestos fiber. If a fiber bundle is more than one or two micrometers thick, striations may be seen or fibrils may be splitting off in a way that is characteristic of 9sbestos. If long fibrils are seen and no bundles can be seen, the possible presence of fiberglass should indicate a need for further testing. With experience, the analyst will be able to distinguish between chrysotile asbestos, amphibole asbestos, and most non-asbestos fibers by recognizing the morphology as characteristic of one or the other. The fibers of chrysotile have a fine silky appearance. Sometimes a wavy pattern is seen in the bundles. When an interference Is expected, the industrial hygienist collecting the sample should also collect bulk samples of potentially interfering substances so that these can be studied separately and methods found to differentiate between asbestos and the interference. The analyst may have to delay his report until bulk samples are obtained for study in some circumstances. As bulk samples are received for analysis, there will be an opportunity to collect a small library of reference samples. Wards Natural Science Establishment, Inc. [4] sells mineral specimens, including asbestos. The International Union Against Cancer (UICC) asbestos standards can be obtained free from Pneumoconiosis Research Unit [5]. Polarized Light and Retardation Color Patterns Minerals having directional qualities yielding double refraction are anisotropic. Minerals lacking directional qualities yielding double refraction are dark between crossed polars and are isotropic. By crossing the polarizer and the analyzer, it is possible to determine whether fibers are isotropic or anisotropic. An isotropic fiber has only one index of refraction. Isotropic substances include minerals of the isometric system and amorphous substances, such as glass. By viewing fibers with crossed polars and noting that they remain at extinction (non-visible) at all positions of rotation, it is possible to eliminate interference from fiberglass, perlite veins, or diatomaceous earth. The latter substance may be crystalline, but since the difference between the high and low index of refraction is only 0.003 for cristobalite, this will not present a problem in small diameter particles. If crocidolite asbestos is present, the crossed polar test must be applied cautiously since crocidolite fibers may not be seen with crossed polars. This is due to the dark 'Figures in brackets indicate the literature references at the end of this paper. 433 2063105222

blue color of crocidolite--and its birefringence, which may be as low as 0.004. However, the blue color of crocidolite is itself a clue that crocidolite asbestos may be present. If crocidolite has been heated above 200 °C, the fiber may be brown. An anisotropic substance has.more than one index of refraction, and can include plant and other fibers as well as asbestos fibers., The tetragonal and hexagonal mineral classes have two indexes of refraction, omega and epsilon. The orthorhombic, monoclinic, and triclinic minerals have three indexes of refraction; alpha, beta, and gamma. If an anisotropic fiber is examined with crossed polars, it will have four positions in which it goes to extinction, and four positions in which brightness will be a maximum as the stage is rotated. The positions of extinction will be 900 apart. If a first order red retardation plate is now added to the optical path, a retardation color can be added (or subtracted) to produce a second order blue or first order yellow color in asbestos fibers, depending upon the orientation of the fast or slow rays of the fibers with respect to the slow ray of the retardation plate. The quadrants can be numbered as follows: Upper left and lower right, one and three respectively; upper right and lower left, two and four respectively. Fibers can be described as aligned with quadrants one and three, or aligned with quadrants two and four if the fibers are at maximum brightness. Most asbestos fibers will be yellow if aligned with quadrants one and three, or blue if aligned with quadrants two and four. The exception is crocidolite, which sometimes gives a yellow to greenish color if the fibers are aligned with quadrants two and four, and a blue color if the fibers are aligned with quadrants one and three. If amorphous (isotropic) fiberglass is present, the first order red plate will make the fibers clearly visible, but they will have the red color of the background and will not change their color as the stage is rotated. Small asbestos fibers, less than about 1.5 micrometers in diameter, may appear as dark lines in which the yellow color is so faint that it is not recognized. It is characteristic of asbestos that the yellow or blue color developed in this way will be pure. A pure color is a single color or shade along the length of the fiber as long as the fiber does not bend or change orientation. Talc fibers may show a variation of color with blue shading slightly toward orange as the fiber varies slightly in thickness. This may be due to the high birefringence of talc, 0.030 to 0.050. Plant fibers will have a mottled appearance with a recognizable color pattern showing the complicated structure of the fibers. In rare cases, plant fibers will have pure colors like asbestos, and in such cases it will be necessary to pay close attention to the morphology, particularly the thickness of the fibers, the bluntness of the ends, and the way in which fibrils separate from the bundle. In such cases, it is possible to see structures which would not otherwise be visible by looking at the fibers at the extinction position without retardation plates. If morphology and color patterns provide insufficient clues to distinguish plant fibers, it will be necessary to ash the fibers at 500 to 550 °C and re-examine the sample after ashing. Birefringence has already been mentioned. Birefringence is n2-nl, the difference between the high index of refraction and the low index of refraction of a particle. The higher the birefringence or the thicker the particle, the higher the order of color seen when particles are examined with crossed polars. By the use of a Michel-Levy color chart, it is possible to determine the birefringence of particles if their thickness is known. This will help to limit the number of minerals which must be considered in determining what is present. 434

S 3 The.following table shows the birefringence of several minerals [61: crocidolite 0.004 chrysotile 0.011 to 0.014 anthophyllite 0.016 to 0.025 tremolite-actinolite 0.022 to 0.027 amosite (cunmingtonite) 0.025 to 0.029 (grunerite) 0.042 to 0.054 gypsum 0.009 wollastonite 0.014 anhydrite 0.044 talc 0.030 to 0.050 Angle of Extinction 4 Many minerals extinguish between crossed nicols when cleavages or crystal boundaries lie at oblique angles to the planes of vibration of the two nicols. These are said to have inclined extinction. By measurement of the angle of extinction, anthophyllite and chrysotile can be distinguished from other asbestos minerals. Anthophyllite has parallel extinction: that is, the angle of extinction is zero degrees. The extinction of chrysotile will be close to zero degrees. The angle of extinction of other asbestos minerals is as follows [7]: tremolite 15-20° actinolite 10-15" amosite 10-20° crocidolite 80-90° Wollastonite will have parallel or very nearly parallel extinction. If a mineral is known to be either anthophyllite or tremolite by dispersion staining tests, the angle of extinction can then be used to distinguish between the two. Caution: it is possible for a mineral which usually has inclined extinction to have a few fibers with parallel or close to parallel extinction, depending upon orientation. Measurement of the angle of extinction can be performed as follows: Line up the cross hairs (if the eyepiece does not have a cross hair, it is possible to use the lines of a Patterson Globe and Circle Reticle or a Porton Reticle) with a natrolite particle or fibers of an anthophyllite asbestos standard which is at extinction when the polars are crossed. The fiber should be parallel to the cross hair and displaced slightly to the side so as to be visible in bright field. Tape the eyepiece so that it is immobilized in this position. Check the alignment with several other fibers to be sure that it is exact. Line up an unknown fiber with the same cross hair line. Take a reading of the position of the stage. With the polars crossed, move the fiber by rotation to its position of maximum extinction. Take a reading of the position of the stage again. Repeat the measurement to be sure that it is accurate. If the difference between the two readings is close to zero, the fiber has parallel extinction. If the extinction angle is 15° to 20° and the index of refraction matches tremolite, it is probable that the fiber is tremolite. 435 2063105224

In making measurement of angles of extinction, measure the highest angle of extinction obtainable by rotating the fiber around its long axis. A binocular microscope which is adjustable for various interpupillary distances should always be used on the same interpupillary setting as was used for alignment of the cross hairs for zero extinction. Determination of the position of maximum extinction of some dark fibers may be difficult. The fibers may appear to be dark over a wide range of rotation of the stage. In such cases, it may be possible to locate the position of maximum brightness. If the position of maximum brightness is 45° from the cross hair, the angle of extinction is zero. Cleavage Some minerals which have lathlike cleavage, such as gypsum, may be confused with asbestos by inexperienced analysts. Such particles may have aspect ratios of five to one or greater. Gypsum will often have the appearance of small rectangles. The blocky appearance of gypsum is usually sufficient to make a distinction. The low indexes of refraction of gypsum (alpha = 1.520, gamma = 1.529) can be used to make a distinction if the analyst needs additional clues. Although wollastonite is similar to tremolite, careful attention to fine details of the cleavage patterns can make distinction between the two minerals. The cleavage lines of tremolite tend to be straight; the cleavage lines of wollastonite tend to curve slightly. The cleavage planes of tremolite tend to be uniform in thickness; wollastonite cleavage planes tend to feather to thin edges. Sides of tremolite particles will be straight or palisaded; wollastonite edges may be serrated. The ends of tremolite are square; wollastonite will be more smoothly rounded. If some fibers are still not recognized, other tests can be applied after removal of the fibers from the membrane. 9- Removal of Fibers from the Membrane Removal of fibers has the disadvantage that the count of fibers is difficult to relate to a known area and therefore to the concentration of fibers in air. However, it is possible to mark the position of fibers on the membrane and remove selected fibers for further analysis. This particle picking technique is described in "The Particle Atlas" [8]. When asbestos is in a mixture with other fibers, it is possible to bracket the asbestos concentration by determining the percent of asbestos fibers in the mixture removed from the membrane and applying this percentage to the total fiber count on the membrane. Ashing a Millipore membrane is difficult due to the tendency of the membrane to flash when it is heated. Low temperature ashing is a solution to this problem but low temperature ashing equipment will not be available in every laboratory. A Millipore membrane can be ashed by folding the membrane, sample side in, moistening with alcohol, then igniting the alcohol with a small flame. Instead of ashing, it is possible to dissolve the membrane in acetone and separate fibers and particles by centrifuging, followed by removal of excess acetone. After the third treatment, an aliquot can then be placed on a slide, and after evaporation of the acetone the particles can be blended into an index of refraction medium selected for identification of the particles present. A quick and simple separation procedure is to place one drop of the same index of refraction medium on each of three slides, then cut a small segment of the membrane and, holding it with fine tipped tweezers, dip the membrane sample side down successively into each drop of index of refraction medium. After placing a cover slip over the medium, the slides are ready for study. 436

el 3 " Dispersion Staining Dispersion staining is a convenient technique for determining the identity of fibers and particles. If the analyst is unfamiliar with this technique, McCrone Research Institute, Chicago, Illinois, teaches courses in dispersion staining. This training may also be obtained from a university if it has a department of geology or materials science. "The Microscope" [9] has an article entitled "Identification of Asbestos Fibers by Microscopical Dispersion Staining." Other articles on dispersion staining are in "The Microscope," and the techniques are also described in "The Particle Atlas" [10]. The Zeiss microscopes in use at Salt Lake City produce the equivalent of a central stop (dark field) dispersion stain by using a phase 2 16X phase contrast objective with the phase 3 ring in place. Leitz manufactures a phase contrast microscope which produces central stop dispersion colors at 400X. If the microscopes in use at other labs do not produce a central stop dispersion stain in this way, a "dispersion stainer" can be purchased from Walter C. McCrone.Associates [11]. For dispersion staining analysis, it is necessary to have quality high dispersion liquids. These are available from R.P. Cargille Laboratories, Inc. [12]. The Appendix of this paper gives directions for the dispersion staining identification of asbestos minerals and wollastonite, a common interference. In distinguishing between fibers, as many clues as necessary to make the distinction should be used. In most cases, morphology, color patterns with crossed polars and retardation plates, angles of extinction, or central stop dispersion staining ;colors, especially if tests are made at more than one index of refraction, will give sufficient clues to identify fibers. , Some fibers may remain unidentified after this type of screening. A sample analyzed at the Salt Lake City OSHA Laboratory contained fibers very similar to asbestos. Optical tests, however, indicated that they were not asbestos. X-ray energy dispersive analysis showed a high concentration of silicon in the fibers. It was then suspected that the fibers might be one of the polymorphs of SiOz. The fibers were separated from other particles by treating the sample for twelve minutes with hot phosphoric acid. Central stop dispersion staining and x-ray diffraction showed that the fibers were quartz. Quartz fibers have been reported in the literature. However, it was unexpected to find quartz fibers in a sample taken from a vacuum cleaner bag. References [1] Kerr, Paul F., 0 tical Mineralogy, Third Edition (1959), McGraw Hill Co., NY. [2] Geological Survey Bulletin 848, 1934. [3] Sinkankas, John, Gemstone and Mineral Data Book, Collier Books, New York, N.Y. [4] Wards Natural Science Establishment, Inc., Rochester, N.Y. [5] Pneumoconiosis Research Unit, Penarth, Glamorgan, Wales. UICC standards can be purchased from Mccrone Accessories and Components, 2820 South Michigan Avenue, Chicago, IL 60616. [6] ibid. Reference 1. [7] Berkley, C., Langer, A., and Baden, V., Instrumental Analysis of Inspired Fibrous Pulmonary Particulates, Transactions, New York Academy of Sciences, pp. 333-350, Oec. 1967. • [8] The Particle Atlas, Edition Two, Vol. i, pp. 233-258, Ann Arbor Science Publishers. 437 2063105226

[9] The Microscope, Vol. 18, NO. 1, pp. 1-18, January-April 1970, Microscope Publications, 2820 South Michigan,Avenue., Chicago, IL 60616. [10] The Particle At1as, Ann Arbor Science Publishers. [11] Walter C. McCrone Associates, 2820 South Michigan Ave., Chicago, IL 60616. [12] R. P. Cargille Laboratories, Inc., 55 Commerce Road, Cedar Grove, NJ 07009. Appendix If chrysotile is mounted in 1.546 high dispersion medium, and viewed by a central stop dispersion staining technique with the polarizer in and analyzer out, the colors will be yellow to orange if the fibers are oriented parallel to the polarizer, and orange red to red blue if the fibers are oriented perpendicular to the polarizer, depending upon the index of refraction of the fibers. (In all asbestos fibers except crocidolite, the high or gamma index of refraction will be seen when the fibers are oriented parallel to the polarizer.) If chrysotile is mounted in 1.560 high dispersion medium, the central stop color will be blue if the fibers are oriented parallel to the polarizer and blue white if the fibers are oriented perpendicular to the polarizer. Amphibole asbestos minerals will not give dispersion colors in 1.560 high dispersion medium, or the color will be straw yellow and easily distinguishable from the colors given by chrysotile asbestos. It is possible for fiberglass to have the same index as one of the indexes of chrysotile or other asbestos minerals. Fiberglass and other amorphous substances have only one index of refraction. By rotating the stage, this type of interference can easily be detected since fibers oriented parallel or perpendicular to the polarizer will have the same central stop dispersion color. Synthetic polymers may show birefringence, which is due to the orientation of molecules in the drawing process. These fibers will generally be too thick to be confused with asbestos. Talc fibers can be very similar to anthophyllite asbestos in appearance. Inter- mediate forms may occur which are between talc and anthophyllite in physical and optical characteristics. In 1.550 high dispersion medium, talc fibers which are oriented parallel to the polarizer will be yellow, indicating that the index of refraction is higher than 1.550 (gamma and beta). Talc fibers oriented perpendicular to the polarizer will be blue, indicating an alpha index of about 1.550. In 1.585 high dispersion medium, talc fibers which are oriented parallel to the polarizer will have a blue central stop dispersion color, indicating a gamma index of about 1.585. Fibers which are oriented perpendicular to the polarizer will have a blue white central stop dispersion color, indicating an alpha index less than 1.585. In 1.585 high dispersion medium, the central stop dispersion colors for tremolite or anthophyllite fibers will be yellow if the fibers are oriented parallel to the polarizer, and orange yellow if the fibers are oriented perpendicular to the polarizer. In 1.585 high dispersion medium, the central stop dispersion colors for chrysotile will be similar to talc fibers. In 1.550 medium, however, the orange red color of chrysotile fibers oriented parallel to the polarizer compared to a yellow color of talc fibers similarly oriented will serve to make a distinction. In 1.620 high dispersion medium, actinolite will be yellow if the fibers are oriented parallel to the polarizer and the central stop color for fibers oriented perpendicular will be orange yellow. Large particles will have a natural greenish color which may influence the central stop dispersion color. 438

SS In 1.620 high dispersion medium, the central stop dispersion color for anthophyllite, tremolite, and wollastonite will be yellow orange to orange if the fibers are oriented parallel to the polarizer. If the fibers are oriented perpendicular to the polarizer, the colors will range from yellow orange to blue depending upon how the fiber is lying. By rotating the fiber around its own long axis, the fiber can be brought to a position in which it will be blue. Tremolite may be blue green. The fibers can be caused to rotate about their long axis by gently tapping the coverslip with a dissection needle. Amosite asbestos can be expected in samples of insulation from steam lines and boilers, especially from ships. If amosite is mounted in 1.670 high dispersion medium, the central stop dispersion color will be yellow if the fibers are oriented parallel to the polarizer, and red violet if the fibers are oriented perpendicular to the polarizer. Other asbestos minerals, except crocidolite, have an index of refraction far enough from amosite that no dispersion color will be developed in 1.670 medium. The central stop dispersion color for crocidolite will be yellow orange if the fibers are oriented parallel to the polarizer, and yellow if the fibers are oriented perpendicular to the polarizer. Crocidolite will show the low (alpha) index parallel to the polarizer. If the dispersion staining tests or cleavage patterns show that wollastonite may be present and a test other than cleavage or the angle of extinction is needed to distinguish between wollastonite and tremolite, the following method may be useful. This method can be used in the absence of chrysotile asbestos to distinguish between fairly acid resistant amphibole minerals and wollastonite. ~ Wash the fibers into a drop of concentrated hydrochloric acid on a slide by; dipping a membrane segment sample side down as previously described. Place a coverslip over the drop of hydrochloric acid and heat the slide on a hot plate which is warm to the touch but not hot enough to be uncomfortable. The slides will be dry in one hour. The coverslip will tend to prevent the particles from migrating as the acid evaporates. Let the slide cool, and add a drop of 1.620 high dispersion medium at the edge of the coverslip. Capillary action will immerse the particles in the medium. When the slide is examined, tremolite or anthophyllite will still show central stop dispersion colors; wollastonite will not. Wollastonite will have been decomposed by the hydrochloric acid or partially decomposed with separation of silica, but without formation of a jelly. The wollastonite fibers will still have their original shape, but larger fibers will show a crosshatching pattern, and the anisotropy of the fibers will be greatly reduced. Fibers which were not previously present in the sample will result from the treat- ment of wollastonite with hydrochloric acid, followed by evaporation of the acid. These fibers will be needlelike and often form radiating patterns. The highest concentration of these fibers will be in areas in which the hydrochloric acid evaporated last. In 1.620 high dispersion medium, these artifact fibers will not give a central stop color like that obtained from woilastonite, tremolite, or anthophyllite, and are distinguishable from the fibers which were originally present. N O W Lr O 439 ~

Discussion W. ROFF: On your Zeiss microscope, maybe I misunderstood, you have the combination phase as well as the optical mineralogy incorporated in one microscope? W. DIXON: Yes. ROFF: You do? DIXON: We have a Zeiss universal; with this microscope we can make the switch back and forth between the two techniques (phase contrast or polarized light) very quickly because of its fingertip control. ROFF: You mention something about ashing between 500 and 550 °C; well for chrysotile, you have to be very, very careful ..... DIXON: Right, at 650 °C its going to be converted to forsterite. ROFF: And possibly a little enstatite will keep its fiber form. We really have to do it at a much lower temperature for a longer period of time. DIXON: What temperature do you use, may I ask? ROFF: We use 400 °C overnight, or a plasma asher. With respect to the nitric acid for wetting your Millipore, we would rather fold the Millipores carefully and then wick with alcohol and ignite and then put that into the furnace; I think you would find it quite successful. Incidentally, I think your paper was very well done and I think should be commended. There are many people here from the various mining companies, especially from the western part of the U.S. that are concerning themselves with zeolite fibers, and may I suggest that perhaps in your final text you might incorporate a sentence or two on zeolites; how to differentiate the zeolite fibers from the other fibers you are talking about. DIXON: I can't answer that question at the moment. What I would have to do would be to look up the index of refractions of the zeolites and I would probably find a dispersion staining technique from that which would help me to make a distinction between the two. 440 c4s