Document VGDVRNpw1D5nej1x8mDbqgxL8

t * . ;-W :^4^t#r:'`^':w,..;.-; ' ^v-:`\^' ;v-4S; -r. . -'' -.- -".:: -i**:v"`. . .y : .. .r* :-.; . .;, i UNITED STATES DEPARTMENT OF THE INTERIOR 15190376 Information Circular 8751 Selected Silicate Minerals and Their Asbestiform Varieties Mineralogical Definitions and Identification-Characterization By W. J. Campbell, R. L. Blake, L. L. Brown, . E. Cather, and J. J. Sjoberg This current report on asbestos has been prepared by the Bureau of Mines, U.S. Department of the Interior to-- 1. Provide precise nomenclature and information on selected silicate minerals and their asbestiform varieties. 2. Invite comment, revisions, or additional information on the subject. Please direct communications to the author-- William J. Campbell Bureau of Mines College Park Metallurgy Research Center College Park, Md. 20740 UNITED STATES DEPARTMENT OF THE INTERIOR Cecil D. Andrus, Secretary BUREAU OF MINES As the Nation's principal conservation agency, the Department of the Interior has responsibility for most of our nationally owned public lands and natural resources. This includes fostering the wisest use of our land and water re sources, protecting our fish and wildlife, preserving the environmental and cultural values of our national parks and historical places, and providing for the enjoyment of life through outdoor recreation. The Department assesses our energy and mineral resources and works to assure that their development is in the best interests of all our people. The Department also has a major re sponsibility far American Indian reservation communities and far people who live in Island Territories under U.S. administration. This publication has been cataloged as follows: Selected silicate minerals and their asbestifarm varieties : mineralogieal definitions and identification-characterization / by W. J. Campbell ... [et al.] [Washington] : U.S. Dept, of the Interior, Bureau of Mines, 1977. 56 p. : ill. ; 27 cm. (Information circular - Bureau of Mines ; 8751) Bibliography: p. 54-56. 1. Silicate materials. 2. Asbestos. }. Asbestos fibers. I. Campbell, William Joseph, 1926- . II. United States. Bureau of Mines. III. Series: United States. Bureau of Mines. Informa tion circular - Bureau of Mines ; 8751. TN23.U71 no. 8751 622.06173 U.S. Dept, of the Int. Library CONTENTS Abstract.............................................................................. Introduction................................. The Particulate Mineralogy Unit......... ....................................................................... Scope of report..................................................................................................................... Acknowledgments............................................... Nomenclature of selected silicate minerals and their asbestiform varieties......................................................................................................................................... Background................................................................................................................................ De finitions.............................................................................................................................. Mineral terms.............................................................................................................. Asbestos-related terms.......................................................................................... Crystal terms.............................................................................................................. Breaking of minerals.............................................................................................. Mineral identification and characterization................................................................. Macroscopic samples............................................................................................................ Microscopic samples............................................................................................................ Applying mineral terminology to the identification and characterization of particulates............................................................................................................................ Applying morphological terminology........................................................................... Particulates from a known asbestiform serpentine or amphibole source Particulates from a known nonasbestiform serpentine or amphibole source..................................................................................................................................... Comparison of particulates from known serpentine and amphibole minerals and their asbestiform varieties........................................................ Aspect ratio.................................... Particulates from unknown sources............................................................................. Applications....................................................................................................................................... Ambient-air samples near serpentinite rock quarry......................................... Asbestos in ceiling and wall materials................................................................. Amphiboles and talc............................................................................................................ Research needs.................................................................................................................................. References........................................................................................................................................... 1 1 3 3 3 4 4 12 13 14 21 28 31 32 33 38 38 38 39 39 44 46 46 47 48 50 52 54 ILLUSTRATIONS 1. Regions of the United States reported by the Environmental Protec tion Agency to contain asbestiform minerals in the bedrocks.............. 2. Macrophotographs of serpentine and chrysotile................................................... 3. Macrophotographs of tremolite and tremolite asbestos................................... 4. Macrophotographs of anthophyllite and anthophyllite asbestos................. 5. Macrophotographs of actinolite and actinollte asbestos............................... 6. Macrophotographs of cummingtonite and cumnlngtonite-grunerite asbestos................................................................................................................................ 7. Macrophotographs of riebeckite and crocidolite............................................... 8. Four varieties of gypsum................................................................................................. 9. Macrophotograph of tremolite asbestos.................................................................... 10. Macrophotograph of anthophyllite asbestos............................................................ 11. Macrophotographs of two fibrous amphiboles showing asbestiform habit. 2 6 7 8 9 10 11 14 15 15 16 15190379 <1 ii ILLUSTRATIONS --Continued 12. Photomicrograph Illustrating the separation of a crocldolite fiber bundle Into fibers........................................................................................................ 17 13. Four fibrous nonasbestlform mineral varieties............................... 18 14. Light optical photomicrograph of fibers from tremollte asbestos.... 19 15. Fibers of epsomite...................................... 19 16. Crysotlle, showing individual fibrils, at two magnifications.............. 20 17. Crocidolite, showing a fiber bundle and fibers............................................. 21 18. Chrysotile by polarized light................................................................................... 21 19. Chrysotile.............................................................................................................................. 22 20. Chrysotile at two magnifications............................................................................... 23 21. Various shapes of single crystals, and patterns or arrangements of crystal aggregates.......................................................................................................... 24 22. Macrophotograph of spodumene showing prismatic shape................................... 25 23. Macrophotograph of tremollte showing prismatic and acicular crystal shapes................................................................................................................... 25 24. Riebeckite, showing prismatic shape........................................................................ 26 25. Actinolite, showing prismatic shape........................................................................ 26 26. Tremollte cleavage fragments, showing acicular, fibrous, and prismatic shapes.............................................................................................................. 27 27. Macrophotograph of columnar aggregates of coarse anthophylllte............ 28 28. Macrophotograph of radiating aggregates of acicular pyrophyllite.... 29 29. Macrophotograph of calcite rhombohedral cleavage fragments..................... 29 30. Macrophotograph of pyroxene showing good cleavage Interrupted by uneven fracture............................. 30 31. Tremollte, showing good prismatic cleavage...................................................... 30 32. Cleavage fragments of riebeckite............................................................................... 31 33. Quality of SAED pattern as a function of amphibolefiber diameter... 35 34. Intensity ratio of FeKar, MgXor, or CaKar relative to SlKhr as a function of fiber diameter........................................................................................ 36 35. Energy-dispersive X-ray spectra of chrysotile as a function of fiber diameter, BeO substrate................................................................................. 37 36. Energy-dispersive X-ray spectra of chrysotile as a function of fiber diameter. Be substrate.................................................................................... 38 37. Light optical photomicrographs of chrysotile andantigorite- lizardite at three magnifications........................................................................ 40 38. Light optical photomicrographs of crocidolite and riebeckite at three magnifications...................................................................................................... 41 39. Light optical photomicrographs of tremollte asbestos and tremollte at three magnifications............................................................................................... 42 40. SEM photomicrographs of crocidolite and riebeckite at three magnifications................................................................................................................... 43 41. Frequency polygons for the aspect ratios of anthophylllte and anthophylllte asbestos................................................................................................. 44 42. Frequency polygons for the aspect ratios of tremollte and tremollte asbestos..................................... 44 43. Frequency polygons for the aspect ratio ofhornblende.................................. 45 44. Frequency polygons for the aspect ratios of commercial-grade chrysotile and chrysotile in ambient air......................................................... 45 15190380 ill ILLUSTRATIONS --Continued Page 45. Macrophotograph showing chrysotile veins in serpentine rock.................. 47 46. Chrysotile bundle................................................................................................................ 47 47. Mixture of nonasbestiform serpentine and chrysotile at five magnifications................................................................... 49 48. Differential thermal analysis of sample from school ceiling.................. 50 49. X-ray diffractometer scan of sample from school ceiling........................... 50 50. Sample from university building, showing a mixture of chrysotile and fiberglass................................................................................................................... 51 51. Typical platy morphology of talc................. 51 52. Platy talc and tremolite cleavage fragment........................................................ 52 53. Platy talc, tremolite cleavage fragments, and a fibrous tremolite particle................................................................................................................................ 52 TABLES 1. Selected silicate minerals and their asbestiform varieties.................... 4 2. Refractive indices for the serpentine group and selected amphibole minerals................................................................................................................................ 34 3. Frequency distribution of the width of chrysotile fibers for ambient-air samples.......................................................................................................... 37 l5l90382 SELECTED SILICATE MINERALS AND THEIR ASBESTIFORM VARIETIES Mineralogical Definitions and Identification-Characterization by W. J. Campbell, 1 R. !_ Blake,2 L. L. Brown,3 E. E. Cather,4 and J. J. Sjoberg5 ABSTRACT This report by the Federal Bureau of Mines Particulate Mineralogy Unit recommends mineralogical definitions and identification-characterization con cepts for selected silicate minerals and their asbestiform varieties. Precise definitions acceptable to mineral analysts, regulatory personnel, and medical scientists are essential because of the present lack of conformity in termi nology concerned with measuring and controlling asbestiform particulates and their related health effects. Because of the complexity and variability of. crystal morphology in different mineral groups, the descriptive terms are gen erally explained by illustration rather than by numerical values. Applica tions and limitations of several analytical techniques for particulate identification and characterization are discussed. INTRODUCTION Concurrent with concerns within the Federal Government over future avail ability of minerals to meet our expanding needs is the requirement that miner als and mineral commodities be mined and processed with minimum environmental impact. Traditionally, matters related to "... inquiries and scientific and technologic investigations concerning mining, and the preparation, treatment, and utilization of mineral substances with a view to improving health condi tions and increasing safety...." have been within the province of the Bureau of Mines as authorized in the amended Organic Act of 1913 (Public Law 62-386). Since its establishment by Congress, the Bureau of Mines has long been deeply involved in investigating the explosive characteristics of dusts in the min eral Industries in its mining and metallurgy research centers, and has estab lished analytical and mineralogical laboratories in seven metallurgy research centers. These laboratories are essential to solving the increasingly complex 1Program coordinator. Particulate Mineralogy Unit, College Park Metallurgy Research Center, College Park, Md. 3Supervisory geologist. Twin Cities Metallurgy Research Center, Twin Cities, Minn. 3Geologist, Albany Metallurgy Research Center, Albany, Oreg. 4Geologist, Salt Lake City Metallurgy Research Center, Salt Lake City, Utah. sGeologist, Reno Metallurgy Research Center, Reno, Nev. 15190382 jsl .mrst-si 2 safety, health, and environmental problems posed by dusts within the mineral system. Various legislative actions and public concerns within the past decade have had, and will continue to have, an impact upon the mineral industry. As a result, control of mineral particulates is becoming increasingly important, with much recent attention focused on asbestiform particulates in both air and water. Figure 1, from an Environmental Protection Agency report, shows the widespread occurrence of common amphlbole and serpentine minerals that, accord ing to existing regulatory definitions, may be classified as asbestiform min erals (16).6 With such possibly overwhelming Implications to both mineral producer and mineral consumer, it is essential that existing ambiguities regarding silicate minerals and their asbestiform varieties be resolved. Until recently, adverse health effects associated with asbestos were focused on occupational exposure in asbestos-related industries. Now there is inter national concern regarding the effect on health from long-term low-level, or short-term high-level, exposure to mineral particulates by the general public (.5, 28, 36) . These particulates may include both the common and the asbesti form varieties of certain silicate minerals. In many instances, cleavage fragments of common amphibole minerals have been mistakenly Identified as FIGURE 1. - Regions of the United States (shaded area) reported by the Environmental Protec tion Agency to contain asbestiform minerals in the bedrocks (16), Underlined numbers in parentheses refer to items in the list of references at the end of this report. 15190383 3 microscopic fibers of the related asbestlform variety. Such lack of precision in identifying these particulates is a handicap to scientific decisionmaking by regulatory agencies and medical researchers. The Particulate Mineralogy Unit was created to work on problems such as this. The Particulate Mineralogy Unit Hie Bureau of Mines established the Particulate Mineralogy Unit in September 1976. The College Park Metallurgy Research Center in College Park, Md.,7 is the focal point for this unit, but substantial support will be sup plied by the other Bureau of Mines metallurgy and mining research centers, which are located throughout the United States. The unit is to assist local. State, and Federal agencies in establishing precise and workable mineral defi nitions and to improve or develop methods of particulate identification and quantitative measurement. The unit is also providing characterized serpentine and amphibole minerals for use by Federal health agencies in their asbestos related research programs. Scope of Report This Bureau of Mines report is intended to clarify some of the terminol ogy used in identification and characterization of asbestlform minerals, and to sharpen the distinction between common rock minerals and their asbestlform varieties. It defines certain mineral terms related to asbestlform minerals and discusses mineral-characterization techniques on a strictly mineralogical basis. The report then discusses the identification of silicate particulates and suggests how to apply this Information to asbestos-related problems. Sug gested areas for further research are sunmarized at the end of this report. ACKNOWLEDGMENTS The following College Park Metallurgy Research Center personnel are acknowledged for their invaluable contributions. Photographic assistance was provided by Garrett Hyde, research physicist, and Lawrence Johnson, geologist (mineralogy). Application data were provided by Raymond Brown, physical science technician; Charles W. Huggins, research chemist; and Eric Steel and Robert Vlrta, geologists (mineralogy). 7The research center is scheduled for relocation to Avondale, Md., in June 1978. 15190384 4 NOMENCLATURE OF SELECTED SILICATE MINERALS AND THEIR ASBESTIFORM VARIETIES8 Background In August 1976, a preliminary paper (1) was presented from which the Bureau of Mines hoped to elicit suggestions from the mineralogical community for critically defining silicate minerals and their asbestiform varieties. Significant terms such as "asbestos," "asbestiform," and "fibers" have dif ferent meanings to the various concerned groups--minerslogists, regulatory agencies, medical scientists, and industry. This has resulted in situations where erroneous conclusions have been drawn. Thus, precise definitions are required that will be uniformly accepted and applied by all personnel involved with silicate minerals that have asbestiform varieties (4, 11^, 24-25, 31, 39). Mineralogical nomenclature recognizes the historical origin of terms, and changes are made only when they are proven incorrect by new information (10, 32). The science of mineralogy is constantly being advanced with newly examined mineral occurrences and with new and improved instruments and tech niques that provide more details on chemical composition, crystal structure, and morphology. To prevent constant revision, nomenclature has to have a certain flexibility, yet must be definite enough to be scientifically useful. The objective of this part of the report is to summarize mineralogically acceptable terms that relate to asbestos. There is no "group" of asbestos minerals. "Asbestos" is a general term applied to certain minerals (that are themselves classified under crystalstructure -based groups) when these minerals crystallize as the asbestiform variety. Table 1 lists some common silicate minerals and their asbestiform varieties, together with their relationships and formulas. Although discus sion in this report is limited to these minerals, appropriate terms and state ments also apply to other silicate minerals that have rare fibrous varieties such as talc, some clay minerals such as attapulgite, and other amphlboles such as arfvedsonite, eckermannite, and richterlte. TABLE 1. - Selected silicate minerals and their asbestiform varieties Mineral______ ______________ Asbestiform variety AMPHIBOLE GROUP Anthophyllite: (Mg,Fe+2)7 Sia022(0H,F)2... Anthophyllite asbestos. Cunmingtonite-grunerite: Cummingtonite-grunerite asbestos. (Mg,Fe+a), Sie0sam)2- Tremolite-actinolite: Tremolite-actinolite asbestos. Ca3 (Mg,Fe+s )s Sis022 (0H,F)a . Riebeckite: Na2Fe+3 Fe23 SigOgg(0H,F)s,... Crocldollte. SERPENTINE GROUP Serpentine: MggSi^Q (0H)g.................................. Chrysotile. BTibor Zoltai, Professor, Department of Geology and Geophysics, University of Minnesota, made significant contributions to this section of the report. 15190385 5 Amphibole minerals and,' to a lesser degree, serpentine minerals occur widely distributed in the earth's crust In many igneous or metamorphlc rocks. In some rare Instances, the mineraloglcal occurrences contain sufficient quantities of usable asbestlform minerals to be economically minable for comaercial asbestos.9 The soft, silky fibers of asbestos (sometimes called mineral silk) are so flexible that they can be spun into threads from which cloth can be woven. The resulting material is fireproof, is a good thermal and electrical insulator, and has moderate to good resistance to acids. It has been used from Roman times, and is most familiar in daily use in brake linings for automobiles and as the "asbestos" siding used in resi dential construction. Only a very small quantity of the amphibole and serpentine minerals under par ticular geologic circumstances occur as the asbestlform variety of the mineral. The asbestlform varieties occur in veins or small veinlets within rock containing or com posed of the common (nonasbestlform) variety of the same mineral. Macrophotographs of the minerals of table 1, both asbestlform and nonasbestlform varieties, are shown in figures 2 through 7. The serpentine group of minerals is limited to serpentine as the cotmnon variety and to chrysotile as the asbestlform variety. Antigorite and lizardite are not listed as separate varieties, but are understood to be Included in the term "serpentine" because they represent 2 frequently named polytypes of about 10 recognized polytypes of serpentine (20). The polytypes differ only in minor struc tural stacking of components and are not sufficiently different to have a separate mineral status. Chrysotile generally occurs segregated as parallel fibers in veins or veinlets, although a recent study (6J has shown fine chrysotile Intimately intergrown with the lizardite polytype. The minerals and mineral series of the amphibole group in table 1 have variable compositions with extensive elemental substitutions and are found in forms ranging from massive to blocky to very fibrous. Crocldolite is the varietal name given to the fibrous habit of the mineral riebeckite (fig. 7), as shown by at least one study (35). It is retained here as a useful and correctly identified term. Cummingtonite-grunerite asbestos is the mlneraloglcally proper terminology for the commercial material conmonly known as "amoslte." Amosite, implied to be a min eral variety, is really an acronym--Asbestos Hines of South Africa--for a fibrous mixture of minerals; namely, cummingtonite-grunerite with variable tremoliteactlnollte. Amosite has been discredited as a mineral species (22), and its use as a mineraloglcal term should be discontinued; however, it is still useful as a com mercial term. Mineral compositional series such as cumningtonite-grunerite involve replacing one cation for another in a crystal structure without significantly altering the structure. There may be a gradation in the structure in some series, and minor changes in physical characteristics may occur with elemental substitution. Usually a series involves two named compositional end members with intermediate substitu tional compounds being separately named (if the members were recognized by early mineralogists), given a varietal name (for similar reasons), or just qualified by being referred to as members of the series. Members of the tremollte-actlnoliteferroactlnolite series are hydroxylated calcium-magnesium, magnesium-iron, and iron silicates, respectively. Their series is named in table 1 for two of its members, and when its composition is known, it should be called by the specific name, such as 9Clifton, R. A. Asbestos. BuMines MCP-6, in preparation, 1977. FIGURE 2. Macrophotographs of serpentine (top, XI) and chrysotile (bottom, X 3). 15190387 1 7 i i i t 15190388 ? 8 FIGURE 4'. - Macrophotographs (X 3) of anthophyllite (top) and anthophyllite asbestos (bottom)^ l5l90389 9 15190390 FIGURE 6; - Macrophotographs (X 3) of cumming* tonite (top) and cummingtonite* grunerite asbestos (bottom). 15190391 11 15190392 3 tremolite. The following nomenclatare for this series (7) is based on the mole Fe ratio (in parentheses) of Fe + in the formula Ca3(Mg,Fe)sSl808S (0H)a: tremolite (0 to 0.2), actlnolite (0.2 to 0.8), and ferroactinolite (0.8 to 1.0). Definitions Many mineraloglcal definitions apply to the entire mineral field, but the defi nitions in this report are restricted to those necessary to characterize the fibers and cleavage fragments related to asbestos identification-characterization. The terms to be defined are outlined in the following list: Mineral terms: Mineral Mineral groups Mineral series Mineral varieties Asbestos-related terms: Asbestos Fibrous Mineral fiber Fibril Fibril structure Asbestiform Crystal terms: Crystal structure Crystalline Crystal Single crystal Twinned crystal Polycrystalline Polymorphs Polytypes Crystal form Crystal morphology: Single crystals: Equant Prismatic Acicular Fiber Fibril Filiform Bladed Platy Lamellar Crystal aggregates: Asbestiform Columnar Fibrous Lamellar Massive Radiating Reticulated Breaking of minerals: Cleavage: Types: Pinacoidal Prismatic Rhombohedral Cubic Octahedral Quality: Perfect Good Fair Imperfect Cleavage fragment Fracture: Types: Even Uneven Splintery Conchoidal Hackly Parting 13 Mineral Terms Mineral.--A homogeneous, naturally occurring, usually Inorganic and crys talline substance. Minerals with distinct crystal structure types (including lack of crystal structure) and limited variation in chemical composition are given individual names. The early concept of a mineral as a natural-history species was gradually abandoned and changed into a chemical and structural definition around the middle of the 19th century, long before crystal structures were understood. The term "species" persisted for a long time with this newer definition, but is now seldom used in mineralogy. Mineral Group.--Minerals that have essentially the same (or comparable) structures, but have different cations in secondary structural position (for example, pyrites, feldspars, and amphiboles). In silicates, most of the mineral groups have the same linkage patterns of the silica tetrahedra, like the characteristic double chains of aaqihiboles. However, the secondary atomic sites may be occupied by a relatively wide vari ety of cations or some may even be vacant in the actual structures of the mem bers. In some silicate-mineral groups, the identity of the silica-tetrahedral frame is less restricted and may be limited to the similarities of some basic characteristics. For example, the silica-tetrahedral frames of the zeolite minerals are variable, but they are all characterized by large open channels. All mineral groups have names. In some instances, this name is the uni versal name of a common or important member of the group (for exaiqile, ser pentine group). Mineral Series.--Two or more members of a mineral group in which the cations in secondary structural position are similar in properties and can be present in variable, although frequently limited, ratios (for example, cummingtonite-grunerite). Also known as an isomorphic series. Some mineral series such as the plagioclases have unique names, but most are identified by the combined names of the end-member minerals, such as tremolite-actinolite. The current trend is to simplify long series names by using the mineral name of only one (end or intermediate) member. Individual minerals in the series are either identified by the names given to compositional ranges (for example, bytownite or oligoclase in the plagioclases), or by the name of the series followed by a symbol expressing the mineral's position in the series or the ratios of the variable cations (Ab^AnjQo -x% fr plagioclases, where Ab and An designate the two end members, albite and anorthite). Mineral Variety.--Minerals that are conspicuously different from those considered normal or common in crystallization habits, polytypes, and other structural variants, or other physical properties such as color. Varieties are named by mineralogists, miners, gemologists, manufacturers of industrial products, and mineral collectors. 14 FIGURE 8. - Four varieties of gypsum: (A) Selenite; (B) satin . spar, fine fibrous; (C) satin spar, coarse fibrous; and (D) alabaster. Although mineral names are controlled by national and inter national mineralogical organizations, variety names are not. In practice, any variety name that becomes suf ficiently popular is eventually recognized by these organizations as distinct enough to be used as a mineralvariety name. Typical of these are the vari ous varieties of gypsum shown in fig ure 8. Figures 2 through 7 illustrate the massive and asbestiform varieties of serpentine and amphiboles. Asbestos-Related Terms In the following discussion, asbestiform refers only to asbestos. The other terms, "fibrous," "mineral fiber," "fibril," and "fibril structure," apply to both asbestiform and nonasbestiform varieties. Asbestos.--(1) A collective mineralogical term encompassing the asbesti form varieties of various minerals; (2) an Industrial product obtained by min ing and processing primarily asbestiform minerals. The quality of asbestos depends on the mineralogy of the asbestiform variety, the degree of asbestiform development of the fibers, the ratio of asbestiform fibers to acicular crystals or other impurities, and the length and flexibility of the fibers. The major asbestiform varieties of minerals used for asbestos are chrysotile, tremolite-actinolite asbestos, cummingtonitegrunerite asbestos, anthophyllite asbestos, and crocidolite. Asbestos may be marketed by its mineral name such as anthophyllite asbestos, its variety name such as chrysotile or crocidolite, or a trade name such as Amosite or Montasite. Two types of commercial asbestos are shown in figures 9 and 10. The term "asbestos" was first Introduced by Plinlus Secundus in 77 A.D. The term "amiant" was previously used for the same mineral by Dioscorides in 50 A.D., and this term was the more common one until the middle of the 17th century. After that until the 20th century, "asbestos" was more common, and "amiant" was reserved for the more silky and flexible asbestos. In the 18th century, asbestos was classified into five different species. Currently, all 15190395 15 FIGURE 9. - Macrophotograph of tremolite asbestos (X 3). asbestos Is recognized as varieties of sev eral individual miner als. Thus, in mineralogy, "asbestos" became a collective term, somewhat like "clays" or "gems." During the 20th cen tury, asbestos devel oped into an important Industrial material. Some asbestos products contain nonasbestiform minerals (for example, asbestos-cement and asbestos-magnesia); consequently, the mlneraloglcal and the Industrial definitions of asbestos do not always coincide. Fibrous.--The occurrence of a min eral in bundles of fibers, resembling organic fibers in tex ture, from which the fibers can usually be separated (for exanq>le, satin-spar and chrysotlle). The term "fibrous" has been used during the last 200 years to describe all kinds of minerals that crystal lized in habits resembling organic fibers, including FIGURE 10. Macrophotograph of anthophyllite asbestos (X 3). asbestos minerals. However, the related term "asbestiform" was never used for fibrous mineral habits other than asbestos. Accordingly, "fibrous" is the more general term, and asbestiform is a specific type of fibrosity. Figures 11, 12, and 13 show various types of fibrous mineral habits. Examples of fibrous minerals, both silicates and non- silicates, that are not classified as asbestiform are shown in figure 13. 15190396 FIGURE 11. - Macrophotographs (X 3) of two fibrous amphiboles showing asbestiform habit: Byssolite (top) and richterite (bottom); 15190397 17 Mineral Fiber.--The smallest elongated crys talline unit that can be separated rom a bundle or appears to have grown Individually in that shape, and that exhibits a resemblance to organic fibers. (Examples: fiber bundles, chrysotlle and crocidolite; individual fibers, epsomite and millerite.) The term "fiber" is not limited to asbestos. However, it is distinct from "acicular" because it requires the resem FIGURE 12. * Photomicrograph illustrating the separation of blance to organic fibers. Figures 14 and 15 illus a crocidolite fiber bundle into fibers (X 600). trate mineral fiber habits.. Excellent photomicrographs of organic fibers are illustrated in The Particle Atlas (18). Fibril.--A single fiber, which cannot be separated into smaller compo nents without losing its fibrous properties or appearances. Most fibers are single structural entitles, such as millerite and nickel sulfide, and some may be called fibrils. However, some fibers are conq>osed of two or more fibrils that are less readily separable from each other than fibers are from bundles (for example, chrysotile and crocidolite). Figure 16 shows the high magnification necessary to resolve a fibril. Fibril Structure.--A systematically deformed and/or defective crystal structure of a fibril. A defect structure would involve various types of dis location. The fibril structure may be exhibited by a single crystal, a group of single crystals, or a twinned single crystal. 15190393 18 FIGURE 13. - Four fibrous nonasbestiform mineral varieties: A, Fibrous talc (X 500); B, fibrous brucite (X 50); C, palygorskite (X 30,000); and D, attapulgite (X 30,000). 15190399 19 The scroll-like fibril structure of chrysotile (38). the twinned single-crystal fibrils of chrysotile (6), and the incompletely resolved fibril structure of an amphibole (9) are all examples illustrated in the literature. Some acicular single crystals may have the appearance of fibers and fibrils, yet there is nothing unusual about their crystal structures. Other acicular single crystals may have signif icant structural devia tions in addition to FIGURE 14. - Light optical photomicrograph of fibers from tremolite asbestos (X 115). A few particles appearance that result in the display of certain ~ properties usually found are seen to be bundles of fibers. in fibers such as high tensile strength along the fiber axis. Thus, fibril structure is not limited to asbestiform structures, but may occur in a minor form in non- asbestiform structures. Asbestiform.--A specific type of mineral fibrosity in which the fibers and fibrils possess high tensile strength and flexibility. "Asbestiform" and "asbestos" are essentially synonymous in current usage. Some special properties of asbestiform vari FIGURE 15. - Fibers of epsomite (magnesium eties, including optical extinc sulfate hydrate) (X 13). tion and surface charge, are either not fully understood or are not uniformly applicable to all asbestiform fibers; consequently, they cannot be considered fundamental characteristics at this time. The prototype of the expression "asbestiform" was introduced by Werner in 1774. He recognized three subspecies of actino- lite and of tremolite. One of these subspecies had the prefix "asbestartiger." Thus, the restriction of "asbestiform" to certain mineral varieties appears to 15190400 20 FIGURE 16. - Chrysotile, showing individual fibrils, at two magnifications: X 18,000 (top) and X 35,000 (bottom). The hoi low-tube structure is visible at the higher magnification, (TEM micro photographs .) 15190401 21 be justified on a historical as well as physical basis. Fig ures 17 to 20 show several asbestiform varieties of minerals. Crystal Terms FIGURE 17. Crocidolite, showing fiber bundle and fibers (X 300). Crystal Struc ture. --The pattern of the regular arrange ment of atoms and ions in space. Usually refers to the basic (or average) structure of solids (and the ordered portions of liquids) without ref erence to minor, local* lzed deviations. Crystalline.--A substance possessing a reasonably well devel oped and long-range ordered crystal structure. Crystal.--(1) Any single crystal; (2) a single crystal termi nated by planar or nearly planar surfaces called crystal faces. Single Crystal.-- A crystal containing an uninterrrupted crystal structure in a single orientation. FIGURE 18. - Chrysotile by polarized light (X 100). Twinned Crystal.-- A crystal composed of two or more single crystals where adjacent crystals share a plane that is an integral part of both orientations of the crystal structure. Polycrystalline.--A substance composed of two or more single crystals. Polymorphs.--Two crystals with identical chemical composition but dif ferent crystal structures. 15190402 22 Polytypes.--Polymorphs that contain essentially Identical structural components, like layers, which are arranged in various patterns, like stacking of layers. Crystal Form. --A set of sym metrically equivalent faces in a single crystal. A crystal may display several crystal forms. Crystal Habit.--The actual shape assumed by a crystal or group of crystals as a result of the growth of dominant crystal forms (faces). Also known as crystal morphology. Crystal Aggregate.--A cohe sive mass of individual crystals or grains. FIGURE 19. - Chrysotile (X 300). Single crystals exhibit growth shapes, while crystal aggregates exhibit growth patterns or arrange ments (fig. 21). Some habits imply minor deviation of the crystal structure (deformation of the structure, presence of defects, inclusions, or impurities). Most habits are limited to the appearance of single crystals (crystal forms, irregular terminations, dimensional development, texture) or the pattern of aggregation of single crystals (dimension, orientation of the crystals, and cohesion between them). Under the following heading of "Single Crystals," the growth shapes are arranged first as equant, followed by other shapes that can be thought of as being derived from equant by suppressing or extending one or two of the three space dimensions, thus conveying the gradational nature of various defined crystal shapes. Single Crystals Equant.--The shape of a single crystal or grain with three approximately equal space dimensions. 3.5190403 23 FIGURE 20. - Chrysotile at two magnifications: X 1,960 (top) and X 2,800 (bottom). 15190404 24 SINGLE-CRYSTAL SHAPES CRYSTAL-AGGREGATE PATTERNS OR ARRANGEMENTS \l- Equant Prismatic Acicular Rber Fibril Columnar See "Asbestiform" Fibrous above. Lamellar Bladed Massive Radiating Lamellar Reticulated FIGURE 21. Various shapes of single crystals, and patterns or arrangements of crystal aggregates. I5l9045 25 \ Prismatic.--The shape of a single crystal with one elongated dimension and two shorter, approximately equal, dimensions. Prismatic shapes of various single crystals are shown in figures 22 to 25. Acicular.--The shape shown by an extremely slender crystal with small cross-sectional dimen sions (a special case of prismatic form). Acicular crystals may FIGURE 22. - Macrophotograph of spodumene (LiAISi206) be blunt-ended or pointed. The term showing prismatic shape (X 1). "needlelike" refers to an acicular crystal with pointed termina tion at one or both ends. Figures 23 and 26 show acicular crystals or crystal shapes. Fiber.--See definition under "Asbestos-Related Terms." Fibril.--See definition under "Asbestos-Related Terms." Filiform.--The shape of threadlike mineral fibers. FIGURE 23. * Macrophotograph of tremolite showing prismatic and acicular crystal shapes (X 10). Bladed.--The bladelike shape of a crystal with one longer dimension and two unequal, much shorter, dimensions. 15190406 26 FIGURE 24. - Riebeckite, showing prismatic shape (SEM photomicrograph at X 800). FIGURE 25. Actinolite, showing prismatic shape (SEM photo* micrograph at X 20,000). 15190407 27 Figure 25 shows the bladed prismatic habit of actinolite. Platy.--The shape of a crys tal with one short dimension and two longer, approximately equal, dimensions. Chlorite, micas, and talc usually crystallize into platy shapes. FIGURE 26. - Tremolite cleavage fragments (X 130), showing acicular (A), fibrous (F), and prismatic (P) shapes. Crystal Aggregates The serpentines, although possessing layered structure simi lar to micas and talc, rarely grow in platy shapes. Talc is of interest here because some talc deposits have associated asbestiform and nonasbestlform minerals. Platy talc, when seen in thin sections or as fragments in oil, may occur in various orientations. Plates lying flat look like plates but, if standing on edge, could appear fibrous. Lamellar.--The shape of a very thin platy crystal. Asbestiform.--See definition under "Asbestos-Related Terms." Columnar.--The arrangement of a group of approximately parallel, pris matic, acicular, or bladed crystals. Figure 27 shows columnar aggregates of coarse anthophyllite. Fibrous.--See definition under "Asbestos-Related Terms." \ 15190408 28 Lamellar.--The pattern exhib ited by aggregates of very thin platy minerals. Massive.--Homogeneous s true ture without stratification, flowbanding, foliation, or schistosity. Also, crystals or crystalline grains that are tightly packed and scarcely distinguishable. Figure 2 (top) is a good example of massive serpentine. Radiating.--An arrangement of prismatic, acicular, or bladed crystals that appear to be diverg ing from a conmon center. Figure 28 shows radiating aggregates of pyrophyllite. FIGURE 27. - Macrophotograph of columnar ag gregates of coarse anthophyllite (X 1). Reticulated.--The pattern of a crisscross network of acicular, prismatic, or bladed crystals. Breaking of Minerals When a mineral crystal or grain is strained beyond its elastic and plastic limit, it will break in one of several characteristic modes described as cleavage, fracture, or parting. Cleavage.--The tendency of a crystal to break in definite directions that are related to the crystal structure and are always parallel to possible crys tal faces. Cleavage Types Pinacoidal Cleavage.--A crystal with only one cleavage plane that yields platy or lamellar fragments (for example, talc and the mica minerals). Also called platy, basal, or lamellar cleavage. Prismatic Cleavage.--A crystal with two distinct cleavage planes that yield prismatic fragments (for example, the amphiboles and pyroxenes). ! f 15190409 29 FIGURE 28. - Macrophotograph of radiating aggregates of acicular pyrophyllite (X 1). Rhombohedral Cleavage. --A hexagonal crystal with three dis tinct cleavage planes that yield rhombohedral fragments (such as calcite). Figure 29 shows the excellent rhombo hedral cleavage of calcite. Cubic Cleavage.-An isometric crystal with three distinct cleavage planes that yield cubic fragments (for exanple, halite). Octrahedral Cleavage.--An isometric crystal with four dis tinct cleavage planes that yield octahedral cleavage fragments (for example, magnetite and fluorite). Cleavage Quality The quality or persistence of a cleavage is described as follows: Perfect.--Diffi cult to break in any other direction; cleavage surfaces are extensive and smooth. The cleavage of calcite in figure 29 is perfect. Good.--Breaks readily along one direction, but can be broken in other direc tions; cleavage 15190410 30 surfaces are smooth but Interrupted by other fractures. Figures 30 and 31 show good cleavage of pyroxene and tremolite. Fair.--Breaks most readily along cleavage but also fractures easily In other direc tions; cleavage sur faces are seldom large. Imperfect.--Breaks about as easily by fracture as by cleavage; cleavage surfaces tend to be small and fre quently interrupted. FIGURE 30. - Macrophotograph of pyroxene showing good cleav age interrupted by uneven fracture (X 1). Cleavage Fragment Cleavage Frag ment.--A fragment pro duced by the breaking of crystals in direc tions that are related to the crystal struc ture and are always parallel to possible crystal faces. Minerals with per fect cleavage can pro duce perfect regular fragments. Amphiboles with prismatic cleavage will produce prismatic fragments (fig. 31). These fragments can be elongated and on super ficial observation may resemble fibers (figs. 26 and 32). However, because they did not grow as fibers, they cannot have the characteristics of fibers. Consequently, 15190411 31 cleavage fragments can not be called fibers. Minerals do not always break Into the same shapes as their growth habits. For example, calclte has many growth habits, but usually breaks into rhombohedral cleavage fragments (fig. 29). Fracture.--The tendency of a crystal or grain to break in an irregular manner appar ently unrelated to crys ta1lographic directions. >, FIGURE 32. - Cleavage fragments of riebeckite (X 500). Particle F has a fibrous appearance. Fracture types are as follows: Even.--Breaking along relatively smooth planes. Uneven.--Breaking along irregular planes. Splintery.--Breaking into irregular, elongated fragments. Conchoidal.--Breaking along spherical or conical surfaces. Hackly.--Breaking with a jagged irregular surface. Parting.--The tendency of a crystal or grain to break along crystallo graphic planes weakened by inclusions or structural defects. Different speci mens of the same mineral may or may not exhibit parting. Twinned crystals often part along composition planes, which are lattice planes and, therefore, potential crystal faces. Parting is similar to cleavage. MINERAL IDENTIFICATION AND CHARACTERIZATION Until recently, emphasis in the United States was placed on occupational exposure of employees manufacturing or using asbestos products for insulation and other applications (29, 33). 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 15190412 1 32 correlating health effects to the number of fibers observed. Exact defini tions for asbestos-related mineralogical terms were not 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 (15). These bundles may have an average diameter of 0.75 to 1.5 um for chrysotile and 1.5 to 4.0 um for the amphibole asbestos (3). 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 moni tors may consist of individual fibrils or small bundles of chrysotile 0.02 to 0.1 um in diameter, and/or amphiboles 0.1 to 0.2 um in diameter (3). Fibrils and small fibers in this size range are not visible using the conventional light optical microscopic procedures (13. 34). Therefore, the identification procedures currently used for regulating the U.S. mineral producing and con suming 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 (17). Detailed descriptions of the various ana lytical and characterization techniques are available in numerous publications and textbooks and are therefore not Included in this report. 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 dis tinguish varieties. These varieties have similar basic crystal structures and composition, but are usually differentiated macroscopically by the character istic habits and/or other specific features of the varieties. The objective for this section of the report is to summarize the methodology for identifying the mineral first by mineral group (such as serpentine and amphibole), then by mineral (actlnolite, 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 nonasbestlform varieties have a massive, blocky, bladed, or columnar appearance (figs. 2-7). Although chrysotile does occur very rarely in a nonasbestlform 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 (6, 19). The distinction between serpentine and 15190413 33 amphibole minerals at the macroscopic level can be made by optical microscopy, elemental analysis, differential thermal analysis, and X-ray diffraction tech niques. For essentially5pure samples, these techniques should also be suffi cient to identify the individual amphibole minerals based on the elemental composition corresponding to the various members of the solid solution series. Many macroscopic samples of interest to the occupational and environ mental health personnel may contain low percentages of asbestiform minerals (for example, chrysotile 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- to 100-mg samples by instrumental techniques such as X-ray diffraction, differential thermal analysis, or infra red 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 amphlboles have been described (8) using the structural formula W0_jX2Y5ZeOas(0H,0,F)2. Gen erally, W = Na, K; X = Na, Ca, Mg, Fe+a , Mn; Y = Al, Fe+a , 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 amphibole chemical char acterization, the serpentine minerals generally show little deviation from the formula Mg3Si20s(0H)4. For either structural or chemical characterization of a macroscopic sample, sufficient time must be spent in sanple 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 urn can be characterized. By observing the optical properties characteristic of the structure and chemistry of a mineral, an experienced microscopist can distinguish amphlboles from serpentines and, in some cases, distinguish individual minerals within these groups (7). 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 amphlboles, but a specific index (for example, a, 0, 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. The well-known parallel extinction of the commercial asbestos known as Amoslte can be used to distinguish that vari ety from the nonasbestiform varieties of cummingtonite and actinollte. A method of using extinction angles and cleavage directions to distinguish spe cific asbestiform and nonasbestiform amphiboles has been described (37); how ever, this technique is limited to particles with diameters greater than about 15190414 34 5 urn 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. TABLE 2. - Refractive indices for the serpentine group and selected amphibole minerals Chrysotile............................................................. Refractive index O' 0 V Range of values 1.493-1.560 1.504-1.550 1.517-1.562 Antigorite-lizardite...................................... a Y 1.538-1.564 1.546-1.573 Anthophyllite.................. .................................... O' 3 Y 1.596-1.652 1.605-1.662 1.615-1.676 Actinolite-tremolite...................................... a 0 Y 1.599-1.668 1.612-1.680 1.622-1.688 Cummingtonite-grunerite................................ a 0 Y 1.635-1.696 1.644-1.709 1.655-1.729 Riebeckite............................................................. O' 0 Y 1.654-1.701 1.662-1.711 1.668-1.717 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 fea tures 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-um 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 magnifi cation 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 X5190415 35 be used to characterize a wide range of sizes extending down to a few angstroms. Morphologic 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 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 che min eral, 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 (fig. 33). The larger diameter fibers (>0.5 um) strongly absorb the 60- to 100-kev elec trons used in a conventional TEM, while the very small-diameter fibers (<0.2 um) 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 largediameter 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 (d001 ~ 5.3A), the chrysotile pattern has very prominent streaks on these layer lines compared with the spot pattern for the amphiboles (27). Researchers indicate the ability to distinguish between the fibrous and non- fibrous variety of amphiboles by SAED is still to be resolved. FIGURE 33. - Quality of SAED pattern as a function of amphibole fiber diameter. The values indicated on the graph are the number of fibers examined at each diameter. (Reprinted with permission of D. R. Beaman and D. M. File from Analytical Chemistry Q). Copyright by the American Chemical Society.) At the very high magnification available with a TEM, chrysotile's hollow-tube (scroll-like) structure, approximately 50 A in diameter, is visible (fig. 16). This hollowtube 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 15190416 36 several fibrils) will not display this char acteristic because of stacking of the fibrils. The elemental com position of micro scopic grains is determined by either wavelength or energy- dispersive X-ray spectrography in con junction with scan ning or transmission electron microscopy. Extreme care must be taken in the calcu lation of elemental concentrations from X-ray spectral intensities because the spectral line intensities (FeKa, FIGURE 34. - Intensity ratio of FeKa, MgKa, or CaKa relative to Si Ka as afunctionof fiber diameter. (Reprinted with permission of D. R. Beaman and D. M. File from Analytical Chemistry (2). Copyright by the American Chemical Society.) MgKa, CaKa, relative to SiKa) are depen dent on particle diameter for small fibers (2). The particle diameter becomes a significant variable in the spectral intensity- composition relationship below 0.2 urn (fig. 34). Carbon contamination from diffusion pump oils must also be considered when analyzing small particles because the longer measuring times required to count sufficient numbers of X-ray photons allows time to build up a contamination layer. This carbon layer preferentially absorbs the lower energy X-ray photons. 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 character istic 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 beams diameters of approximately 0.1 urn; 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 (26) 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 um for amphiboles and less than 0.1 urn for chrysotile. A good example is the diameter size distribution of 15190417 37 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 um 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 diam eter below 0.2 um. TABLE 3. - Frequency distribution of the width of chrysotile fibers in ambient-air samples,1 percent Diameter of chrysotile Sample fibers, um 1 2345 6 0.02-<0.04 10 70 57 17 15 17 .04- <.06 47 24 28 29 33 49 .06- <.08 24 5 8 28 20 15 .08- <.10 14 1 2 12 26 6 .10- <.12 201736 .12- <.14 0023 1 1 .14- <.16 10121 1 .16- <.18 000 1 0 1 .18- <.20 00001 1 .20- <.22 10000 1 .22- .24 0 0 1 0 0 1 >.24 1 0010 1 Samples were collected 1-2 miles from a ser pentine rock quarry. ENERGY -- FIGURE 35. - Energy-dispersive X-ray spectra of chryso tile as a function of fiber diameter, BeO substrate (21). Another problem with the elemental characteriza tion of very small particles is the poor signal-tobackground 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 interac tion of the electron beam and the same substrate. Figures 35 and 36 show the energy-dispersive X-ray spectra from chrysotile fibers mounted on beryllium oxide (BeO) and beryllium (Be) substrates, respec tively. The lower effective atomic number of Be compared with that of BeO results in a reduced continuum, therefore 15190418 38 giving an improved signalto-background ratio. APPLYING MINERAL TERMINOLOGY TO THE IDENTIFICATION AND CHARACTERIZATION OF PARTICULATES This section addresses the practical considerations and limitations encountered when applying nomenclature and identificationcharacterization procedures to regulatory and environ mental samples. ENERGY Applying Morphological Terminology One of the obvious fea FIGURE 36. - Energy-dispersive X-ray spectra of chryso- tures of minerals and their tileasa function of fiber diameter, Be sub particulates is their mor strate (21). phology or shape. The need for precise definitions of terms such as "asbestiform," "fiber," "cleavage fragment," and "fibril" was explained earlier. These defi nitions 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 classifying the mineral particle as the asbestiform or nonasbestiform variety. The classification should with stand the test of mineralogical logic and proof. 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: morphol ogy, 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 this report and in common practice, to the serpentine and amphibole mineral groups. Chemically, these minerals are all hydroxylated silicates; the term "hydroxylated" is preferred over "hydrated" because these minerals contain OH ions rather than water of crystallization. The serpentines contain approximately 13 weight-percent water; the amphiboles, approximately 2.5 weight-percent water. 39 For Che 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, as in figures 14 and 17 to 19. These elon gated 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. (Several short fiber segments are visible in figures 14 and 16.) These very short fiber seg ments 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. Figures 26 and 32 for tremolite and riebeckite, respectively, show cleavage fragments with fibrous shapes that could be incor rectly identified as fibers. What can be stated morphologically about particles derived from crushing a known nonasbestiform mineral is that most of the particles are cleavage fragments with nonasbestiform texture; a few are fibrous in appearance, par ticularly at low magnification; and all of the particles are known to be derived from a nonasbestiform source. 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 37 to 40. The samples shown in figures 37 to 39 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 par ticles from the riebeckite and tremolite. Elongated particles of this type 15190420 40 FIGURE 37. 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 IJ, X 100; E, X 500; and F, X 950. 15190422 41 FIGURE 38. * Light optical photomicrographs of crocidolite and riebeckite at three magnifica tions; Croc idol ite (left) at A, X 100; B, X 500; and C, X 950. Riebeckite (right) at D, X 100; E, X 500; and F, X 950. 15190422 42 t - .- Vw- v- v : . yj *4 cZ&rj> ` ^ O7' ^: * ^ I # > ; n ?' >v; ^^ * 4> $ '^ w0 0* ' FIGURE 39. - Light optical photomicrographs 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. 15190423 43 FIGURE 40. - SEM photomicrographs of crocidolite and riebeckite at three magnifications: Crocidolite (left) at A, X 500; B, X 2,500; and C, X 10,000. Riebeckite (right) at D, X 500; E, X 2,500; and F, X 10,000. Rectangles indicate the area shown at the next higher magnification. 15190* I I I 44 are typical of the prismatic cleavage of amphiboles. To increase optical con trast, the serpentine group samples were dispersed in an immersion oil consid erably below the refractive indices for the serpentine. Riebeckite and crocidolite particles are compared at higher magnifica tions in figure 40. The outlined areas in the scanning electron micrographs indicate the area displayed at the next higher magnification. Again, note the presence of a few elongated cleavage fragments of riebeckite visible at the higher magnification. In contrast, the aspect ratio of the crocidolite will decrease with decreasing particle size'because the individual fibers cannot cleave further along the fiber axis; they can only break into shorter segments. Aspect Ratio Existing regulatory standards are based on counting specific mineral par ticulates with aspect ratios of 3 to 1 or greater. This report emphasizes that the aspect ratio has little mlneralogical significance for individual particulates but is applicable to a large number of particles. A few rela tively long thin particles are produced as cleavage fragments from the crush ing and grinding of many nonasbestiform minerals. Conversely, similar milling treatment will result in a few short segments of true fibers from the asbestiform varieties. However, statistically, the length-to-width characteristics of the milled amphiboles and serpentine and their asbestiform varieties are significantly distinct, as shown by the data in figures 41-44. Figures 41, 42, and 43 show the frequency polygons of the aspect ratio distribution for milled samples of the normal nonasbestiform variety of three amphiboles--anthophylllte, tremolite, and hornblende, respectively. Note that in all three examples, approximately 70 percent of the particles have an ASPECT RATIO FIGURE 41, - Frequency polygons for the aspect ratios of anthophyllite and anthophyllite asbestos. ASPECT RATIO 200:1 FIGURE 42. - Frequency polygons for the aspect ratios of tremolite and tremolite asbestos. 15190425 FREQUENCY, percent 45 aspect ratio of less than 3 to 1, and 95 percent of the particles have a length to-width ratio of less than 10 to 1. The frequency dis tribution maximums of the aspect ratios for milled anthophyllite asbestos and tremolite asbestos are sig nificantly higher than those for the normal, nonasbestiform variety. Thirty to forty percent of the asbestiform particulates are in the 10-to-1-or-longer class, with a significant number of particles having an aspect ratio greater than 20 to 1. 1:1 3:1 5:1 10:1 20:1 50:1 100:1 ASPECT RATIO H 10.w01 CP :IGURE 43. - Frequency polygons for the aspect ratio of hornblende. Figure 44 shows the distribution frequencies for a milled commercial grade of chrysotile asbestos and for chrysotile particulates col lected on ambient air fil ters in the vicinity of a serpentine rock quarry. For the commercial-grade chryso tile, over 50 percent of the particles have an aspect ratio greater than 50 to 1, whereas the frequency dis tribution for the ambient air sample has a maximum between 10 to 1 and 20 to 1. These results are antici pated because the higher aspect ratios for the commercial-grade chrysotile are characteristic of the significantly longer start ing material. 1:1 3:1 5:1 10:1 20:1 50:1 100:1 200:1 500:1 1000:1 ASPECT RATIO HIIMC* FIGURE 44. - Frequency polygons for the aspect ratios of commercial-grade chrysotile and chrysotile in ambient air. All of the aforemen tioned samples except the ambient air were milled, then dispersed in water for collection on a suitable substrate. The samples were then measured using electron microscopy at magnifications fr e q u e n c y , percent 15190426 46 of 5,000 to 10,000. The ambient air sample, collected near a serpentine rock quarry, was measured using a TEM with magnifications of X 5,000 to X 32,000. Based on these data, one test for distinguishing the presence or absence of the asbestiform variety of a mineral could be an examination of the fre quency distribution of the aspect ratio for that mineral. Assuming positive identification of the mineral type, then the designation of variety would be based both on particle morphology and the frequency maximum of the aspect ratio. Cleavage fragments will generally have a frequency maximum less than 3 to 1, whereas the asbestiform varieties will fall between 10 to 1 and 20 to 1 or higher, depending on the characteristics of the mineral and the history of the sample, particularly the type and degree of milling. If any shape or size limits are placed on characterizing mineral particulates, such limits should be based on medical evidence or on some limitation of the characteriz ing technique and so stated. Particulates From Unknown Sources Samples such as environmental airborne or waterborne mineral particulates collected at a considerable distance from a possible source are examples of particulates from an unknown source. The samples could have been collected at a location so distant from a known source that other mineral particulates originating from other sources compose most of the sample. The source of the particulates in an environmental sample may be located by taking additional samples at selected Intervals in the direction of, and closer to, the suspected source. However, several factors must be considered: The direction of air and water currents with respect to the suspected source, and the proximity to and direction of other sources with regard to the sus pected source. One study found very low concentration of airborne chrysotile upwind from a source compared with a concentration two orders of magnitude greater downwind (14). Another important consideration is the level of natu ral or human disturbances of particulates; for example, strong versus weak winds, or heavy versus light vehicle traffic. In some Instances, it may be possible to identify the source if the mineral particulates of Interest have unique trace, elements or combinations of elements that are specific to the probable mining or milling operation emitting the particulates. Detailed ele mental analysis using the X-ray spectral capabilities of an SEM or TEM is required on both the suspected source and the particulates. APPLICATIONS The following examples Illustrate the application of mineral terminology and identification-characterization procedures to three types of problems: (1) chrysotile determination in ambient-air samples collected near a serpen tine rock quarry, (2) identification of asbestiform minerals in ceilings and walls of public buildings, and (3) characterization of a mineral product. These examples illustrate, in order, the need for higher magnification than available with the light optical microscope, the use of various characteriza tion techniques to screen and identify asbestiform minerals, and the judgment of the analyst in distinguishing cleavage fragments and asbestiform particles. 15190427 47 Ambient-Air Samples Collected Near Ser- pentinlte Rock Quarry m; 1 .y, ' - v: A'U - > . \ * -x. . 7s* *__ The Bureau of Mines Is working with State and Federal officials to measure mineral par ticulates in ambient- air san^les collected in the vicinity of a serpentine rock quarry. SftrV' vlfe" Optical microscopic procedures at about v? , .rv>-.-4 , -' i* >.<*_- X 500 are limited to the identification of mineral particulates longer than 5 urn with an aspect ratio of 3 to 1 or larger (criteria set by the Mining Enforcement and Safety Administration and the FIGURE 45; - Macrophotograph showing chrysotile veins in serpentine rock (X 1); Occupational Safety and Health Administration). The mineralogist can further identify the particles as belonging to the serpentine, amphibole, or other mineral group with index oils (table 2). FIGURE 46; - Chrysotile bundle (SEM photomicrograph at X 5,000); The serpentine rock in the quarry is interlaced with small veins of chrysotile (fig. 45). Optical microscopic procedures used for industrial hygiene are adequate for the detection of large chrysotile fiber bundles. These fiber bundles of commercial-grade chrysotile can be several micrometers or larger in diameter (fig. 46). In contrast, the mining and crushing operations in the quarry plus transport of par ticulates over a distance breaks bundles of fibers down to fibers or fibrils with diam eters of 250 to 1,000 A (table 3). 15190428 48 Figure 47 is a series of SEM photomicrographs of a mixture of chrysotile and nonasbestiform serpentine handpicked from a small vein in the serpentine rock quarry. Note that at X 450 (corresponding to the optical microscope mag nification) , only one or two bundles of chrysotile are faintly visible; the predominant particles are the nonasbestiform serpentine. As the magnification is increased, the high concentration of chrysotile fibers becomes readily visible. The fiber diameter size data in table 3 indicate that more than 95 percent of the chrysotile fibers in these ambient air samples are below the limit of resolution of the optical microscope. Although many other scientists have pointed out the limitation of the optical procedures for chrysotile in ambient air, there is need for continuous emphasis that higher magnification techniques are necessary for environmental and regulatory samples. Asbestos in Ceiling and Wall Materials A possible environmental hazard is the release of asbestos from ceilings and walls in homes, churches, schools, and various other public and commercial buildings. Because of the very high number of potential samples to be exam ined by various State or Federal agencies, a rapid and reliable screening pro cedure is necessary to identify those samples that warrant further tests. Three complementary analytical methods for screening, identification, and semiquantitative estimate of the asbestiform mineral concentration are X-ray diffractometry, differential thermal analysis, and microscopy (light optical and scanning electron). The screening identification procedures can be relatively simple because chrysotile is the principal asbestos mineral used for building insulation mate rials, with Amosite used to a much lesser extent. In 18 sanples from a midwestern municipal health department, chrysotile was a major constituent (>50 weight-percent) in 2 samples, a minor constituent (1 to 10 weight-percent) in 12 samples, and not detected in 4 samples. Other minerals present in various concentrations in these samples were calcite, quartz, gypsum, and mica. Amosite was found as a major constituent in the ceiling of an older building located on a university campus. The presence of either serpentine or amphibole minerals in the insulation materials can be used as a probable indication of asbestos. Therefore, screen ing tests are based on the presence or absence of characteristic differential thermal analysis or X-ray diffraction peaks of either serpentine or amphibole minerals. For the positive samples, confirmation of the presence of the asbestiform variety requires some type of microscopic examination because the thermal and X-ray diffraction methods do not identify the mineral variety. Differential thermal analysis provides a detectable signal from chryso tile at 0.5 to 1.0 weight-percent, as indicated by the curves shown in fig ure 48. Note the increase in the endothermic (A) and exothermic peaks (B) upon addition of about 5 weight-percent chrysotile to a sample taken from a school celling. The sensitivity of differential thermal analysis for the amphibole minerals is significantly poorer because the Ha0 content of amphiboles is approximately 2.5 percent compared with about 13 percent for chryso tile. The sensitivity of the X-ray diffraction method also ranges from 0.5 to 15190429 49 FIGURE 47. Mixture of nonasbestiform serpentine and chrysotile at five magnifications: A, X 450; B, 2,250; C, X 9,000; D, 1,800; and E, X 18,000. Rectangles in dicate the area shown in the next panel. > I I 15190430 50 ENDOTHERMIC -- AT -- EXOTHERMIC *TEMPERATURE, C 300 400 500 800 I'll 700 I 800 I 800 FIGURE 48. - Differential thermal analysis of sampTewfrom school ceiling, showing endothermic (A) and exothermic (B) peaks of serpentine. Run 1 is the sample as received; run 2 is a mixture of 95 pet of the as-received sample and 5 pet chrysotile. 1.0 we ight-perc ent. An X-ray diffractometer scan of the 29 range for major ser pentine peaks is shown in figure 49. The magnitude of the characteristic peaks for chrysotile are a function of several factors, including degree of fiber orientation and the type of milling or crushing used to process the sample. Also, the sensitiv ity of both methods is affected by the presence of other minerals that have characteristic thermal or diffraction peaks in the same region as those of the minerals of interest. Some samples will be composed of a mixture of synthetic and natural fibers, such as the mixture of fiber glass and chrysotile shown in figure 50. Generally, it is not difficult to Identify the synthetic fibers based on their larger diameter and the more uniform appearance. Amphiboles and Talc 30 25 20 15 10 5 DEGREES, 20 ---- FIGURE 49. - X-ray diffractometer scan of sample from school ceiling, showing the presence of calcite(C), mica (M), and serpentine (S). Asbestos-related health regulations are having a significant Impact on the domestic talc industry from occupational exposure at the mines and mills and at vari ous manufacturing plants that use talcs in their operations. Certification that the talc does or does not contain asbestiform min erals is important because the occupational health requirements are much more restrictive if the talc is designated as containing asbestiform serpentine or amphibole minerals. 15190431 * 51 Talc is both the name of a spe cific mineral, MggSi40lo (0H)a, and a commercial term for a mixture of min erals ranging from essentially 100 percent talc to blends where the min-, eral talc is a minor constituent (12. 23). Semiquantitative estimation of the serpentine and/or amphlbole mineral concentration, if present, can be obtained by X-ray diffraction and differential thermal analysis. Sev eral talc deposits contain a variable amount of tremollte. Therefore, the essential question faced by the ana lyst is whether or not the tremollte is fibrous. Figure 51 shows the typical platy morphology of talc; no tremollte (aoqphlbole) was detected in this sample by X-ray diffraction. Figure 52 illustrates the type of FIGURE 50. Sample from university building, showing a mixture of chrysotile and fiberglass (X 140). particles obtained from a mixture of tremollte and platy talc. The cleav age fragments of tremollte are typical of the nonasbe8tiform variety. Better judgment is required of the analyst with the type of sample illustrated in figure 53. This sample consists of platy talc, cleavage fragments of tremollte, and minor to trace amounts of fibrous tremollte. For this latter sasq>le, the 3-to-l aspect-ratio criteria would greatly overestimate the number of fibrous tremollte particles collected on air filters or other monitors. FIGURE 51. Typical platy mor phology of talc (X 600). 15190432 52 RESEARCH NEEDS There are several areas In particulate Identificationcharacterization where further research Is warranted. Areas of research that are immediately applicable to asbestos are briefly summarized. FIGURE 52. - Platy talc (A) and tremolite cleavage fragment (B) (X 640). Commercially available elec tron optical instruments are generally limited to morphologi cal characterization for mineral particles with diameters less than 0.2 um. As pointed out in the identificationcharacterization section, both the signal-to-background ratio for energy dispersive X-ray spectra and the SAED pattern are significantly degraded for elon- gated particles less than 0.2 um in diameter. Field emis sion electron optical micro scopes with their higher vacuums and smaller beam diameter may have some advantages over conventional SEM instruments. Also, other microprobe techniques, in particular ion microprobe mass spectrographs and laser Raman microprobes, should be evaluated for particulate characterization. Although electron micro scopic methods can generally positively identify chryso- tile in air and water samples, the quantitative aspects of the measurements need sub FIGURE 53. - Platy talc, tremolite cleavage fragments, and a fibrous tremolite particle (A) (X 400). stantial improvement. Sample treatment and measurement errors need to be isolated from sampling variance. Because of the ambiguity of fiber counts, results should be reported both in mass equivalents and in fibers per unit volume. 15190433 53 Fundamental studies should be conducted to determine If there are unique chemical and physical characteristics of a mineral fiber as compared to elon gated cleavage fragments. For;jexample, the surface properties of chrysotile are similar to those of magnesium hydroxide, whereas the nonasbestlform vari eties of serpentine have the surface characteristics of a silicate (30). Variations in surface properties, if any, between asbestiform particles and cleavage fragments of amphiboles should be investigated. Surface characteri zation techniques to be considered should include Auger electron spectrography and low-energy X-ray spectrography. Research at the University of Minnesota indicates that asbestos fibers have an extensive surface charge over the whole surface, whereas cleavage fragments have a significantly lower surface charge (40). Extinction angle measurements are another possible approach to dis tinguishing asbestiform from the nonasbestlform varieties of amphiboles (37). There is a critical need to reexamine the 3-to-l aspect ratio as a cri terion for a mineral fiber. The aspect ratio for fibers from asbestiform min erals were as much as 200 to 1 or higher, whereas the ratio for cleavage frag ments is about 3 to 1, as illustrated by the data in this report. The 3-to-l aspect ratio may be valid for the industrial hygiene control of asbestos processing plants; however, its applicability to existing nonasbestos mining and ore processing plants requires critical evaluation. There is also need to evaluate the restriction of the mineral particulate measurments to light opti cal microscopy because many particulates of interest, especially chrysotile _ fibrils, are not visible by this technique. Low-cost scanning electron micro scopes are in the same price range as research-grade petrographic microscopes, and the skill requirements for the operator are comparable for both instruments. Particulate measurements by microscopic procedures are time consuming and expensive. Two possible approaches to reducing the time and cost are (1) auto mation of the particulate identification-characterization measurements using computerized image analyzers and (2) development of chemical reagents that give a specific response with either chrysotile or the various asbestiform amphibole minerals. Using the chemical-reagent approach, mass-concentration values could be obtained by measurement of some response such as color, ultra violet fluorescence. X-ray spectral Intensity, etc. Health studies related to Inhalation and ingestion of fibers have been essentially limited to well-defined commercial types of asbestos. The funda mental question to be resolved is the biological effects of cleavage fragments compared with those of true mineral fibers. If shape and size are the criti cal parameters, the analyst could establish and measure suitable analytical parameters to monitor the particulates of interest. Likewise, if the health scientists find a correlation between health and the amount of trace metals, adsorbed organics, surface area, etc., the analyst can respond accordingly. Therefore, there is a primary need for an adequate quantity of wellcharacterized amphiboles and serpentine of both the normal and asbestiform varieties for use in health-related studies. 15190434 54 REFERENCES 1. Ampian, S. 6. Asbestos Minerals and Their Nonasbestos Analogs. Proc. Electron Microscopy of Microfibers, Pennsylvania State University, University Park, Pa., Aug. 23-25, 1976, 11 pp.; available from S. Ampian, Bureau of Mines, Washington, D.C. 2. Beaman, 0. R., and D. M. File. Quantitative Determination of Asbestos Fiber Concentrations. Anal. Chem., v. 48, January 1976, pp. 101-110. 3. Berger, H. Asbestos Fundamentals. Chemical Publishing Co., New York, 1963, 171 pp. 4. Champness, P. E., G. Cliff, and G. W. Lorlmer. The Identification of Asbestos. J. Microscopy, v. 108, December 1976, pp. 231-249. 5. 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