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C3 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. (November 1978) IDENTIFICATION OF SELECTED SILICATE MINERALS AND THEIR ASBESTIFORM VARIETIES William J. Campbell Particulate Mineralogy Unit, Bureau of Mines College Park Metallurgy Research Center College Park, Maryland 20740 Abstract The problem of asbestiform particulates with its environmental and health implications has been compounded by the lack of precision with which the term "asbestos" has been used. In many instances, non- asbestiform mineral particles have been identified as microscopic fibers of asbestos-related minerals. This lack of precision in identifying these particulates not only works to the disadvantage of the minerals industry, but is also a handicap to rational science-based decision making by regulatory agencies. This presentation summarizes methods and terminology suggested by the Bureau of Mines for the identification and characterization of asbestiform minerals and also sharpens the distinction between common serpentine and amphibole minerals and their relatively rare asbestiform varieties.l The continuing effort of the Bureau's Particulate Mineral- ogy Unit is to characterize mineral particles by morphological, compositional, and structural data using various instrumental analytical techniques and by developing new methods for identification and characterization. Key Words: Asbestos; cleavage fragments; fibers; silicate minerals. Introduction The objective of this paper is to present a general introduction on the identification and characterization of asbestos-related minerals. Detailed discussions of specific analytical techniques are given in other papers presented at this workshop. At present there are three types of identification-characterization to supply the needs of regulatory agencies, medical researchers, and mineral scientists. It is hoped that through inter- actions such as this workshop a common mineralogical-based procedure can be developed that meets the needs of all concerned groups. Until recently, emphasis in the United States was placed on occupational exposure of employees manufacturing or using asbestos products for insulation and other applications. Regulatory procedures were adopted from those used in Great Britain. The industrial- hygiene identification procedures were acceptable to industry, health, and regulatory organizations because the concern was restricted to several mineral products known collectively as asbestos. Although light optical microscopic procedures counted only the larger particles collected on the air filters, the procedure was adequate for correlating IThis paper is an abbreviated version of the sections on mineral identification and charac- terization in Bureau of Mines Information Circular 8751 - Selected Silicate Minerals and Their Asbestiform Varieties: Definitions and Identification-Characterization, 1977, 56 pp, authored by W. J. Campbell, R. L. Blake, L. L. Brown, E. E. Cather, and J. J. Sjoberg. Copies of IC 8751 are available upon request to W. J. Campbell. 201 2063104996

health effects to the number of fibers observed. Exact definitions for asbestos-related mineralogical terms were essential since all three groups (industry, health, and regulatory) clearly understood what was being counted and regulated. The light optical microscopic procedures used by industrial hygienists were designed for control of asbestos-processing operations in which the chrysotile and asbestiform amphiboles are present as bundles of fibers as well as individual fibers [1]2. These bundles may have an average diameter of 0.75 to 1.5 pm for chrysotile and 1.5 to 4.0 pm for the amphibole asbestos [2]. Particulates of these sizes can be readily observed at a magnification of X 450 to X 500. In contrast, samples from ambient air and personnel air monitors may consist of individual fibrils or small bundles of chrysotile 0.02 to 0.1 pm in diameter, and/or amphiboles 0.1 to 0.2 pm in diameter [3]. Fibrils and small fibers in this size range are not visible with the conventional light optical microscopic procedures. Therefore, the identificaiton procedures currently used for regulating the U. S. mineral producing and consuming industries must be reexamined to insure that they are both mineralogically correct and applicable to the size range of the particles being regulated. This discussion will be limited to the selected silicate minerals and their asbestiform varieties listed in Table 1. The objective is to point out the particle size at which the minerals can be identified and characterized by various analytical techniques [4]. Oetailed descriptions of the various analytical and characterization techniques are available in numerous publications and textbooks. Table 1. Selected silicate minerals and their asbestiform varieties. Mineral Asbestiform variety AMPHIBOLE GROUP Anthophyllite: Anthophyllite asbestos. (Mg, Fe+2)7 Sia022(OH,F)2 Cunmingtonite-grunerite: Cummingtonite-grunerite asbestos. (Mg,Fe+2)7 SiB022(OH)2 Tremolite-actinolite: Tremolite-actinoiite asbestos. Ca2(Mg,Fe+2)5 SiB022(OH,F)2 Riebeckite: Crocidolite. Na2Fe32 FeZ3 Sia022(OH,F)2 SERPENTINE GROUP Serpentine: Chrysotile. M96S14070(OH)8 2Figures in brackets indicate the literature references at the end of this paper. 202

(C I A crystalline mineral is defined primarily by its crystal structure and by its definite composition or range of compositions. Therefore, any system of mineral identification should be based principally on crystal structure and chemical criteria. Additional characteristics have to be determined to distinguish varieties. These varieties have similar basic. crystal structures and composition, but are usually differentiated macroscopically by the characteristic habits and/or other specific features of the varieties. The objective is to summarize the methodology for identifying the mineral first by mineral group (such as serpentine and amphibole), then by mineral (actinolite, anthophyllite, or chrysotile), and finally by mineral variety. Macroscopic Samples At the macroscopic level (easily visible by the unaided eye), the obvious feature of the asbestiform varieties is the presence of fibers that can be easily separated, while the nonasbestiform varieties have a massive, blocky, bladed, or columnar appearance. Although chrysotile does occur very rarely in a nonasbestiform habit, in general the distinction between chrysotile and serpentine can be based on the presence or absence of separable fibers. In some serpentine samples where an obvious asbestos texture is not displayed, the distinction between serpentine varieties may require more specialized techniques [5,6]. The distinction between serpentine and amphibole minerals at the macroscopic level can be made by elemental analysis, differential thermal analysis, and x-ray diffraction techniques. For essentially pure samples, these techniques should also be sufficient to identify the individual amphibole minerals based on the elemental composi- tion corresponding to the various members of the solid solution series. Many macroscopic samples of interest to the occupational and environmental health personnel may contain low percentages of asbestiform minerals (for example, chryfotile in serpentine and tremolite asbestos in talc). As a supplement to optical microscopy, the presence or absence of serpentine or amphibole minerals can be determined in 10- ~0 100-mg samples by instrumental techniques such as x-ray diffraction, differential' thermal analysis, or infrared spectrophotometry. In general, the sensitivity of these instrumental methods is approximately 1.0 weight-percent. Sensitivity is significantly affected by the presence of other minerals that give a response at or near the response peak of the serpentine and amphibole minerals. It is important to note that these methods usually only distinguish between mineral groups; light optical or electron optical microscopy is required to obtain morphological characteristics necessary to identify varieties of the same material. Chemical characterization is generally necessary to assign a specific mineral name to an amphibole whose structure is known. The amphiboles have been described [7] using ~he structural formula Wo_ix2YsZa022(OH,O,F)2. Generally, W = Na, K; X = Na, Ca, Mg, Fe 2, Mn; Y= Al, Fe 3, Ti; and Z = Si, Al. In addition to the variation implied by the structural formula, a chemical analysis must take into account inclusions of other minerals that may be present. In contrast to the more formidable task of chemical characterization of amphiboles, the serpentine minerals generally show little deviation from the formula Mg3SiZ0s(OH).. For either structural or chemical characterization of a macroscopic sample, sufficient time must be spent in sample preparation to insure that relatively pure minerals are being examined. Microscopic Samples The petrographic microscope provides a general method by which particles larger than 5 pm can be characterized. By observing the optical properties characteristic of the structure and chemistry of a mineral, an experienced microscopist can distinguish amphiboles from serpentines and, in some cases, distinguish individual minerals within these groups [8]. The refractive indices are sufficiently different for the serpentine and amphibole groups to make a distinction between groups by using the appropriate index oil (Table 2). There is significant overlap in the range of the three refractive indices among the amphiboles, but a specific index (for example, a, P, or y) can be determined to aid in identifying the amphibole species. Optical relationships can be confused, however, if the particle consists of fiber bundles or is some other form of crystalline aggregate. 203 2063104998

f• a Table 2. Refractive indices for the serpentine group and selected amphibole minerals. Refractive index Range of values Chrysotile a 1.493 - 1.560 0 1.504 - 1.550 y 1.517 - 1.562 Antigorite-lizardite a 1.538 - 1.564 y 1.546 - 1.573 Anthophyllite a 1.596 - 1.652 S 1.605 - 1.662 y 1.615 - 1.676 Actinolite-tremolite a 1.599 - 1.668 0 1.612 - 1.680 y 1.622 - 1.688 Cummingtonite-grunerite a 1.635 - 1.696 p 1.644 - 1.709 y 1.655 - 1.729 Riebeckite a 1.654 - 1.701 ~ 1.662 - 1.711 y 1.668 - 1.717 The well-known parallel extinction of the commerciai asbestos known as Amosite can be used to distinguish that variety from the nonasbestiform varieties of cummingtonite and actinolite. A method of using extinction angles and cleavage directions to distinguish specific asbestiform and nonasbestiform amphiboles has been described [97; however, this technique is limited to particles with diameters greater than about 5 Nm and cannot be universally applied to all amphiboles. There are many other optical parameters such as pleochroism, sign of the elongation, and color that are easy to obtain. Other parameters such as optic axial angle, optical orientation, and optic sign are relatively more difficult to obtain. Except for the asbestiform variety, serpentines are usually massive, while amphiboles range from fine-grained massive to columnar or radiating aggregates of prismatic or acicular crystals. Amphiboles in acicular habit may appear to grade into the asbestiform varieties. The characteristic features of this habit may still be seen by electron microscopy. Terms such as "acicular" or "prismatic" may still be applied when seen, but the term "asbestiform" begins to lose its usefulness. For example, how may flexibility be demonstrated in a 2-pm bundle of fibers? As particle size decreases, the inability to manipulate the mineral grains restricts the use of the term "asbestiform" without altering the original sense of the word. High magnification necessitates the use of strictly dimensional terms such as size and aspect ratios to accurately describe the morphology of the amphiboles and serpentines. The degree of morphologic characterization possibly will depend on the magnification being used. An asbestos particle being described as a single fiber at low magnification may be seen to be a bundle of fibers at some high magnification. Therefore, the magnification must be stated in the description. Morphologic characterization using light microscopy can be accomplished on particles as small as a few micrometers. Electron optics can be used to characterize a wide range of sizes extending down to a few angstroms. Morpholo4ic characterization alone will not identify a mineral without supplemental structural or chemical data. Structural information on individual particulates can be obtained by use of a transmission electron microscope (TEM) in the selective area electron diffraction mode (SAED). The inclination of the single crystal fragments to the electron beam is very critical since a slight tilt of the crystal may change a relatively simple reciprocal 204

lattice pattern into a very complex one. Consequently, a special goniometer or tilting stage•-..is necessary to obtain easily interpretable diffraction patterns. For the identification of the mineral, a goniometer or tilting stage is even more essential since dependable conclusions cannot be made from measurements on one reciprocal lattice plane. The quality of the SAED pattern is a function of fiber diameter. The larger diameter fibers (>0.5 pm) strongly absorb the 60- to 100-keV electrons used in a conventional TEM, while the very small-diameter fibers (<0.2 pm) do not give sufficient electron-diffraction intensity. A second problem with small-diameter fibers is the degradation of the single- crystal pattern by diffraction lines from nearby particles. A higher energy TEM, with the resultant greater penetration of the electron beam, can be utilized for large-diameter particles. However, these costly instruments are not widely available. Although the magnitude of the characteristic C, the distance between the conspicuous layer lines for chrysotile and the amphiboles, is similar in direct space (dool - 5.3A), the chrysotile pattern has very prominent streaks on these layer lines compared with the spot pattern for the amphiboles [10]. Researchers indicate that the ability to distinguish between the fibrous and nonfibrous variety of amphibotes by SAED is still to be resolved. At the very high magnification available with a TEM, chrysotile's hollow-tube (scroll-like) structure, approximately 5 nm in diameter, is visible (fig. 1). This hollow-tube structure, together with chemical and structural data regarding the sample, is sufficient to identify the mineral variety. However, the hollow-tube structure is only visible for individual fibrils; fibers (composed of several fibrils) will not display this characteristic because of stacking of the fibrils. Figure 1. Chrysotile, showing individual fibrils, at two magnifications: X 18,000 (left) and X 35,000 (right). The hollow-tube structure is visible at the higher magnification. tTEM microphotographs.) 205 N O a w ... 0 w 0 0 0

The elemental composition of microscopic grains is determined by either wavelength or energy-dispersive x-ray spectrography in conjunction with scanning or transmission electron microscopy. Extreme care must be taken in the calculation of elemental con- centrations from x-ray spectral intensities because the spectral line intensities (FeKa, MgKa, CaKu, relative to SiKn) are dependent on particle diameter for small fibers [3]. Energy-dispersive x-ray spectral calibration data for each scanning or transmission electron microscope must be made using relatively pure standard minerals analyzed by accepted chemical-instrumental techniques. The analyst should be aware that other nearby grains may be contributing to the characteristic x-ray lines because of either penetration of the electron beam through the particles or secondary excitation of nearby particles from primary x-rays generated in the particle being measured. Modern electron optical instruments have electron beam diameters of 0.1 to 0.01 pm; however, the sphere of excitation can be several micrometers in diameter as a result of scattered electrons and primary x-rays generated in this particle. Conversion of intensity into concentration using accepted computer programs such as "MAGIC" is limited in accuracy because these programs are designed for use with grains or particles several micrometers in diameter or larger, whereas the average mineral fiber diameter is less than 0.5 pm for chrysotile. A good example is the diameter size distribution of chrysotile fibers in ambient air samples (Table 3). The important point to note is that approximately 95 percent of these chrysotile fibers are 0.12 pm or less in diameter. Therefore, quantitative correction procedures applicable to large particles will be of limited value in mineral-fiber identification because the relative x-ray spectral intensities are dependent on fiber diameter below 0.2 pm. Table 3. Frequency distribution of the lidth of chrysotile fibers in ambient-air samples, percent. Diameter of chrysotile - - - - - Sample----- fibers, pm 1 2 3 4 5 6 0.02 -<0.04 10 70 57 17 15 17 0.04 -<0.06 47 24 28 29 33 49 0.06 -<0.08 24 5 8 28 20 15 0.08 -<0.10 14 1 2 12 26 6 0.10 -<0.12 2 0 1 7 3 6 0.12 -<0.14 0 0 2 3 1 1 0.14 -<0.15 1 0 1 2 1 1 0.16 -<0.18 0 0 0 1 0 1 0.18 -<0.20 0 0 0 0 1 1 0.20 -<0.22 1 0 0 0 0 1 0.22 - 0.24 0 0 1 0 0 1 >0.24 1 0 0 1 0 1 a Samples were collected 1-2 miles from a serpentine rock quarry. Another problem with the elemental characterization of very small particles is the poor signal-to-background ratio. Longer counting times will help to improve the reliability of the measurement, but the best approach is to minimize the continuum background resulting from the interaction of the electron beam and the sample substrate. 206

Applying Mineral Terminology to the Identification and Characterization of Particulates This section addresses the practical considerations and limitations encountered when applying nomenclature and identification-characterization procedures to regulatory and environmental samples. Applying Morphological Terminology One of the obvious features of minerals and their particulates is their morphology or shape. The need for precise definitions of terms such as "asbestiform," "fiber," "cleavage fragment," and "fibril" was explained in IC 8751. These definitions were carefully structured to eliminate ambiguity and to be technically correct. Applying the definitions to samples requires careful thought as to what limits must be placed on interpretations resulting from the use of these terms and other mineralogical concepts. The underlying problem, recognized by both medical and regulatory personnel, is clas- sifying the mineral particle as the asbestiform or nonasbestiform variety. In a mineralogical sense, the source of the mineral particulates must be considered, as explained in the following discussion. Particulates From A Known Asbestiform Serpentine or Amphibole Source The definition of asbestiform minerals includes three aspects: morphology, structure, and chemistry. Morphologically, asbestiform mineral varieties separate into flexible fibers or flexible bundles of fibers. Flexible fibers bend readily and only break across the fibers into distinct pieces with some difficulty. Structurally, the asbestiform minerals are limited, in common practice, to the serpentine and amphibole mineral groups. Chemically, these minerals are all hydroxylated silicates; the term "hydroxylited" is preferred over "hydrated" because these minerals contain OH ions rather than water of crystallization. The serpentines contain approximately 13 weight-percent waCer; the amphiboles, approximately 2.5 weight-percent water. For the purpose of this discussion, assume that a hand specimen meeting these requirements is correctly identified as an asbestiform mineral. If this sample is crushed and its fragments examined at various magnifications, its fibrous nature would be apparent. These elongated fragments would be termed "fibers" and "bundles of fibers," and with the other available information would be called "asbestiform." As these asbestiform particles are examined at increasing magnification, smaller particles become visible, while the image of large fibers and fiber bundles may exceed the field of the microscope. At increasingly smaller sizes, while fibers or bundles of fibers are still the predominant shape, a few of the fibers are observed to have broken into shorter and shorter segments. These very short fiber segments are no longer described as fibers, but would be classified as fragments of fibers, or cleavage fragments if one or more cleavage planes govern their shape. Therefore, a known asbestiform sample would show an increase in the ratio of fiber fragments to fibers with a decrease.in particle size. Particulates From A Known Nonasbestiform Serpentine or Amphibole Source If the hand specimen discussed previously does not separate into flexible fibers or bundles of fibers, the mineral would not be considered asbestiform. However, the specimen would be classified as serpentine or amphibole if the specific mineral is identified on the basis of optical properties, chemistry, and structure. If crushed fragments of this known nonasbestiform mineral are examined at various magnifications, the particles would be primarily cleavage fragments, or irregularly broken fragments if cleavage does not govern breakage. However, a few elongated particles may resemble a fiber in appearance to the degree that they may be indistinguishable morphologically from fibers derived from an asbestiform mineral sample. What can be stated morphologically about particles derived from crushing a known nonasbestiform mineral is that most of the particles are cleavage fragments with non- asbestiform texture; a few are fibrous in appearance, particularly at low magnification; and all of the particles are known to be derived from a nonasbestiform source. 207 2063105002

Comparison of Particulates From Known Serpentine and Amphibole Minerals and Their Asbestiform Varieties The appearance of particles generated by milling known serpentine and amphibole minerals and their asbestiform varieties is shown in figures 2 to 5. The samples shown in figures 2 to 4 were photographed using light optical microscopy at three magnifications to show that, at decreasing size (depicted by increasing magnification), the original habit generally persists. For the nonasbestiform amphibole minerals, there were a few elongated particles from the riebeckite and tremolite. Elongated particles of this type are typical of the prismatic cleavage of amphiboles. To increase optical contrast, the serpentine group samples were dispersed in an immersion oil considerably below the refractive indices for the serpentine. Figure 2. Light optical photomicrographs of chrysotile and antigorite-lizardite at three magnifications. Chrysotile (left) at A, X 100; B, X 500; and C. X 950. Antigorite-lizardite (right) at 0, X-100; E, X 500; and F. X 950. 208

.4u Figure 3. Light optical photomicrographs of crocidolite and riebeckite at three magnifications; Crocidolite (left) at A, X 100; 8, N 500; and C, X 950. Riebeckite (right) at D, X 100; E, X 500; and F, X 950. N 209 w r 0 ~ ~ ~ C 3

Figure 4. Light optical photamicrographs of tremolite asbestos and tremolite at three magnifications. Tremolite asbestos left) at A, X 100; B, X 500; and C, X 950. Tremolite right) at D, X 100: E, X 500; and F, X 950. 210