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FILE NAME Talc TALC DATE 1974 Dec DOC TALC049 DOCUMENT DESCRIPTION Journal Article - Asbestos in Talc Environmental Health Perspectives Vol 9 pp 129 1321974 Asbestos in Talc by Arthur N. Rohl Tale deposits include asbestos minerals such as chrysotile and amphiboles that may be carried over into consumer products Optical microscopy and ray diffraction analyses may not reveal their presence Examples are given of electron microscopy procedures that permit detection and measurement The mineral tale is a hydrous magnesium sheet silicate that occurs in both platy and fibrous crystal forms Talc tends to occur in rock masses coexisting with a number of other hydrous magnesium silicate minerals Typically talc deposits consists of grained intergrown mixtures of minerals which may con- tain considerable amounts of asbestos In addi- tion talc deposits often show complex mineral zonation which adds to the difficulty of selective mining For example in the tale deposits of the Gouveneur District of New York State talc occurs with the asbestos minerals chrysotile tremolite and anthophyllite in addition to other silicate minerals Since the mining of talc rock almost in- variably includes the mining of asbestos as well the asbestos contaminant may be carried over into the consumer product and thus introduce the risk of asbestos disease This possibility leads to an important public health question is asbestos present in consumer tales and if pre- sent which mineral fibers in what concen- trations Among the standard mineralogical techniques which may be used for identification and quantitation of asbestos in talc are optical microscopy ray diffraction and electron . oo microscopy EM Optical microscopy employing polarized light optics is useful for determining the optical Environmental Sciences Laboratory Mount Sinai School of Medicine New York N.Y. 10029 properties of particles However in the instance of talc the extremely fine grained intergrowths of different minerals and the extensive overlap- ping and similarities of their optical properities limit this technique to a preliminary or screen- ing function Since large numbers of fibers may go undetected optical microscopy would not be capable of quantitative analysis routine ray powder diffraction is a routine tech- nique for analyzing crystalline materials It is relatively simple in principle but the results may be difficult to interpret The limitations of precision and accuracy must be given careful ' consideration quantitation The identification and quantitation of asbestos fibers in tale by ray diffraction may be achieved by comparison of known dilutions fiber type and quantity of asbestos in a tale matrix with unknowns The preparation of stan- dard dilutions of asbestos minerals in talfcor quantitative analysis requires 1 a talc matrix completely free of contaminating asbestos miner2alpusre asbestos fiber as the sought adventitious phase 3 a preparation method for insuring homogeneity and reproducibility of the standard dilution material and selection of ray reflections with no superimposed in- terferences Condition 1 requires the selection of pure talc matrix material In the first stage of screening material was first scanned by ray diffraction to identify the major mineral phases rapidly especially asbestos minerals If no asbestos phases were detected the material was December 1974 129 0001 examined in a more sensitive mode of ray diffraction called step scanning and finally by electron microscopy In this way a pure talc was selected pure with respect to asbestos small amounts of chlorite and phlogopite mica were tolerated In a similar way pure samples of anthophyllite tremolite and chrysotile were screened and selected for use in preparing the asbestos standard dilutions In ray diffraction the reproducibility of reflection intensities is strongly influenced by the degree of cleavage of crystalline powders The minerals under investigation exhibit a high degree of platy and fibrous cleavage A number of preparation techniques have been developed for reducing preferred orientation effects These were tested but none were found to give satisfactory reproducibility Accordingly a technique was developed and employed which gives a high degree of sensitivity for substances present in minute quantities and with a greater level of reproducibility of reflection intensities Binary systems of three asbestos minerals in talc were prepared at varying levels of dilution concentrations Standard weights of these mixtures were dispersed in water with ultrasonic energy to disperse the phases homogeneously This slurry was filtered through a membrane filter by use of a hypodermic syringe Fig 1 The residue forms a flat cake which is mounted for ray analysis This technique has the advantage of uniformly preparing mounting and measuring the asbestos dilutions under identical conditions Because of the structural similarities between some of the minerals there was considerable overlapping or interference in many reflections and this made it necessary to select reflections which could be unambiguously used as indices of the amount of each mineral present These diagnostic reflections were scanned at 0.01 in a count mode This permits the weak reflections produced at low dilution levels to be determined with precision From the fixed- count data a profile of the diagnostic reflection is obtained and the area above background is taken to be proportional to the reflection intensity The results of these analyses are given in Table 1 which shows that chrysotile at dilution levels less than 0.25 was not detected tremolite was detected down to 0.1 dilution level and anthophyllite was not detected at con- centrations below 2.0 diagnostic reflection is at 8.26 A I = 55 In order to determine the number of chrysotile fibers present at various dilution levels aliquots of the various dilutions were prepared for EM scanning A fairly standard technique called the rubout method was used For each dilution level 20 fields from three EM grids are photographed at constant magnification and the number of long unit fibrils per field are counted from printed enlargements Figs 2 and 3 These fiber counts show fairly good correlation with levels of chrysotile dilution By using the fiber count data it is possible to calculate the number of fibers in a unit weight of sample Thus at a % dilution level there would be about 40 ^ 108 fibers Even at the lowest level of detection by ray diffraction i.e. 0.25 there would be about fibers Cosmetic talcum powder for example which had been scanned and chrysotile not found might contain billions of fibers released during dusting with a gram dose : Thus very large numbers of asbestos fibers may be present in tale end products yet they remain undetected if only optical microscopy and ray diffraction are used On the other hand EM can be a very sensitive method for detecting extremely minute amounts of asbestos in tale Environmental Health Perspectives 130 0002 Table 1. Comparison of lower limits of detection of asbestos minerals in talc by step scanning and continuous scanning Asbestos mineral Diagnostic reflection A Detection limit concentratio%n Step scanning Continuous scanning " 0.01 28 1 min Chrysotile 3.66 0.25 1.0 Tremolite 8.38 0.10 2.0 Anthophyllite 8.26 2.0 4.0 a Operating conditions fixed 20 mA 4 Operating conditions 500 = 2.0 45 kV 20 mA count rate counts = 2000 45 kV time constant 4 Tum \ FIGURE 2. Electron photomicrograph of chrysotile 99 talc % chrysotile December 1974 131 0003 av FIGURE 3. Electron photomicrograph of chrysotile 95 talc % chrysotile Environmental Health Perspectives 132 0004