J. Chem. SOC., Faraday Trans. I, 1982,78, 3561-3571 Characteristics of Asbestos Minerals Structural Aspects and Infrared Spectra BY MARIE-JOSE LUYS, GILBERT DE ROY, ETIENNE F. VANSANT* Department of Chemistry, University of Antwerp (U.I.A.), Universiteitsplein 1, B-26 10 Wilrijk, Belgium AND FRED ADAMS Received 8th March, 1982 The infrared spectra of U.I.C.C. standard asbestos have been investigated in relation to their mineral structure. A quantitative account is given of the surface hydroxy species. The effect on the spectra of pretreatments in dilute acid and basic solutions have been studied with reference to the carcinogenic activity of asbestos minerals. Asbestos is the name given to a group of naturally occurring silicate minerals in the serpentine and amphibole series, possessing a fibrous habit.They have been identified as biologically active agents capable of fatal lung scarring, asbestosis, pleural and peritonial mesothelioma, lung cancer and gastrointestinal cancer. The activity of the fibres is attributed to both morphological and chemical fact0rs.l The surface characteristics of asbestos minerals may be important in determining their adsorption of environmental carcinogenic species. The use of infrared spectroscopy in this field offers many opportunities for identification2 and structure elucidation3 of these minerals. The internal fibrous structure is reflected by the spectral shape, and the influence of pretreatment conditions can be evaluated therefrom. The spectra also contain information relating to the structural and surface hydroxy groups.In this work the infrared spectra of asbestos samples have been studied and the influence of structural aspects on biological activity is discussed. EXPERIMENTAL Amosite, anthophyllite, crocidolite, and Rhodesian and Canadian chrysotile samples were kindly supplied by the U.I.C.C. Pneumoconiosis Research Unit, Johannesburg. Their prepar- ation has been described at length* and their physical properties are well known.5 The samples were analysed using energy-dispersive X-ray emission spectrometry, according to a procedure published elsewhere. The chemical stability of the asbestos minerals was investigated by treatments at 75 OC in 0.1 mol dm-3 NaOH or 0.1 mol dm-3 HC1 solution. The contact time was 30 min in each case. Thereafter the samples were thoroughly washed and dried at 50 OC.Infrared spectra were obtained using a Beckmann 4240 double-beam grating spectrometer, equipped with a variable reference-beam attenuator. The scan range extended from 4000 to 250 cm-l, using a conventional slit programme with a setting of 3 mm at 3000 cm-l. The scan speed was selected at 150 cm-' min-', as a compromise between accuracy and hydration time. The specimens were obtained using 150 mg of a 0.5 % mixture of asbestos fibre (freshly dried) in KBr, which was pressed (9 ton cm-2)f into 1.3 cm2 dies. t 1 ton = lo3 kg. 356 13562 CHARACTERISTICS OF ASBESTOS MINERALS The positions of the absorption bands were determined by visual inspection of the spectra. The error on these readings was estimated to f 2 cm-l, except for shoulders, where the accuracy was worse.The relative intensities of the hydroxy bands were determined from the absorbances;' the background correction was performed manually. RESULTS AND DISCUSSION The chemical analysis data of the U.I.C.C. standard asbestos samples are collected in table 1. The compositions of these samples are very similar to those determined by other worker~.~~ 8-10 Approximate formula unit compositions were determined from TABLE I.--CHEMICAL COMPOSITION (Wt %) OF ASBESTOS MINERALS sample amosi te anthop hylli te crocidoli te RCa CCb Na A1 Si S c1 K Ca Ti V Cr Mn Fe c o Ni c u Zn As Rb Sr Pb Mg - 1.7 0.5 19.9 0.1 0.03 0.36 0.46 0.04 0.02 0.02 1.71 0.02 0.01 0.05 0.02 0.005 0.002 34.5 - - - 16.6 0.5 28.3 0.3 0.01 0.50 0.30 0.02 0.12 0.18 6.5 0.0 1 0.12 0.02 0.03 0.004 0.002 0.003 - - 3.3 I .o 20.8 0.1 0.01 0.15 1.09 0.04 0.01 0.01 0.1 1 0.02 0.01 34.7 - - 0.001 0.030 0.001 - 27.4 0.4 22.4 0.2 0.05 0.35 0.03 0.29 0.07 3 .O 0.01 0.28 0.02 0.003 0.00 1 0.002 0.008 - - - - 28.1 0.3 21.5 0.1 0.18 0.04 0.24 0.03 0.08 0.10 4.1 0.0 1 0.13 0.05 0.01 0.001 0.002 0.004 - - a Rhodesian chrysotile ; * Canadian chrysotile.these data. The calculations were performed on the basis of O,,(OH), for the amphiboles and 05(OH), for the sepiolites. All contributions < 0.1 atom per formula unit were neglected, and the results are collected in table 2, which also contains the idealized formulae as a basis for comparison. A fair agreement is observed, although the cation content of the sepiolites tends to be overestimated.THE AMPHIBOLE MINERALS The amphiboles belong to the inosilicate group. They occur as double chains of linked silica tetrahedra [fig. l(a)J which form the asbestos axis. These chains are cross-linked with bridging cations, which within the structure alter the interplanar spacings and the angle at which the adjacent units are stacked. The ions appear in four distinct sites [fig. 1 (b)]. The M, and M, sites are coordinated to four oxygen atoms (attached to one silicon) and two hydroxy groups, whereas the M, sites are surrounded by six oxygen atoms.l1? l2 Mossbauer spectroscopic data suggest that these sites haveM-J. LUYS, G . DE ROY, E. F. VANSANT AND F. ADAMS 3563 nearly perfect octahedral symrnetryl3 with mean bond distances M,-0 = 2.12 A, M,-0 = 2.1 I A and M,-0 = 2.1 1 A.14 The individual bond lengths deviate by no more than 0.02, 0.05 and 0.01 A, respectively.According to the Mossbauer parameters13 the M, site has an eight-coordination with lower symmetry. These ions are coordinated to four oxygen atoms (attached to one silicon atom) at a mean distance of 2.33 A, and to four oxygens (attached to two silicon atoms) at 2.7 A.14 TABLE 2.-FORMULA UNIT COMPOSITIONS sample approximate formula unit composition idealized formula unit Si, Al, ... . 0 0 8 OH 0 Mg, Fe, . . . FIG. 1 .-Amphibole structure, showing the silica double chain (a) and the sandwich structure (b). The cations occupying the M, positions determine the way in which the individual sandwiches agglomerate. In the monoclinic amphiboles (amosite and crocidolite) all chains are oriented in parallel and the oxygen atoms of two neighbouring sandwiches are nearly eclipsed.The packing density is determined by the mean M, cation radius, the larger cations yielding the closest packing.15 In this group a very wide range of isomorphous substitution can be achieved. Anthophyllite, however, displays orthorhombic structure. This phenomenon only occurs within a very limited substitution range. In the orthorhombic amphibole all chains lie parallel to the c-axis, but they are staggered in the other directions, yielding shorter bond distances. The densest packing is obtained when the M, cations are ~ma11.l~3564 CHARACTERISTICS OF ASBESTOS MINERALS The infrared spectra contain two regions of specific interest.In the 1300-250 cm-l range the lattice vibrations reflect the structural differences between the individual samples. Fig. 2 contains the lattice vibration spectrum of the amphibole samples, and the vibrational frequences are collected in table 3. The hydroxy stretching region (fig. 3) contains information concerning the cation distribution. I I I I I I I I O 3 1000 700 400 G1crn-l FIG. 2.-Infrared spectra of amphiboles in the lattice vibration region: A, amosite; B, anthophyllite; C, crocidolite. LATTICE VIBRATIONS The lattice vibrations consist of stretching and bending distortions in the lattice structure. In the 1200-900 cm-l region several major absorption peaks occur. The three spectra exhibit an intense and complex band near 1000 cm-l which is attributed to asymmetric stretching vibrations by species of the type X-0-Y (X,Y = Si, Al, Mg, Fe etc.).ls* l7 This absorption is considerably broadened in the anthophyllite spectrum as compared with the monoclinic samples. This observation is consistent with the closer stacking in the orthorhombic structure and the consequently larger number of vibrational contributions to this band.15 The bands at 1140 and 1085 cm-lM-J.LUYS, G . D E ROY, E. F. VANSANT A N D F. ADAMS 3565 are attributed to the symmetric stretching modes of tetrahedral and octahedral XOX species, respectively.l59 l7 The latter argument can be correlated with the chemical composition of the samples. In crocidolite a larger contribution by FelI1 ions is expected relative to amosite; this can be traced from the spectra as a larger line width and a 20 cm-l shift towards higher frequencies.In the three amphibole samples a shoulder is detected at 900 cm-l which may be attributed to Si-0-A1 or A1 0-H vibrations.18 The 800-600 cm-l region contains the stretching vibrations of X-0 species (X = Si, Al, Mg, Fe etc.) It contains several sharp peaks, but a detailed assignment could not be obtained. The considerable difference in this region between the anthophyllite spectrum and the other amphiboles should be related to differences in structure. The pattern in this region originates from five atomic species within an oxygen-hydroxy environment which can reside in various structurally inequivalent sites. TABLE 3 .-VIBRATIONAL FREQUENCIES OF AMPHIBOLES (IN Cm-') sample X-0-Y stretch X-0 stretch amosite 1134 1084 1000 895 776 -- 730 704 660 640 crocidoli te 1150 1105 930 900,880 780 - 730 695 660 640 anthophyllite - 1101 1010" 918,900 775 755 730 694 670 - sample bending amosite 530 500 480 - 427 -I 338 304 250 anthophyllite 535 500 472 450 425 350 330 307 264 crocidolite 545 505 - 445 410 350 315 305 255 a Very broad.Italicised entries indicate a shoulder. The bending vibrations in the region 600-250 cm-l form a complex absorption band with a mineral-dependent shape. In the anthophyllite spectrum a broader range of frequencies is observed which confirms the structural characteristics of the monoclinic mineral. The difference we observe in the amosite and crocidolite spectra can only be attributed to compositional effects.The presence of a considerable amount of Na and FelI1 ions in the crocidolite mineral may be responsible for the different and more complicated pattern of the X-0-Y bending vibrations. The infrared spectra therefore reflect the compositional and structural differences between the amphibole minerals. Their overall shape, however, is characteristic of the mineral type and consists of well defined regions. CATION DISTRIBUTION Another region of interest is located in the hydroxy stretching-vibration range. In general four peaks are detected, with positions depending on the M, and M, occupancies which are bonded to the vibrating hydroxy g r o ~ p . l ~ - ~ ~ A tighter M-OH bond will result in a OH vibration absorbing at a lower frequency. Thus the presence of FelI1 and FeII cations should produce slightly different frequencies ( 5 cm-l apart) which are not resolvable in our spectra.We have therefore determined the overall iron content, irrespective of its oxidation state. The (M, M3M1) occupancies yield the frequencies collected in table 4.19-24 Fig. 3 contains the hydroxy stretching pattern of the amphibole minerals. From the absorbances the relative contributions of the peaks were determined, and the ion3566 CHARACTERISTICS OF ASBESTOS MINERALS TABLE 4.-HYDROXY STRETCHING FREQUENCIES IN AMPHIBOLES code (M, M, M,) populations frequency/cm-l A (MgMgMg) 3669 €3 3655 C 3639 D (FeFeFe) 3619 (Fe Mg Mg) (Mg Fe Mg) (Mg Mg Fe) (Fe Fe Mg) (Fe Mg Fe) (Mg Fe Fe) A B C IV 366" I I 3640 I 3625 3670 3660 I 3625 3640 3620 FIG. 3.-Hydroxy stretching patterns of amosite (A), anthophyllite (B) and crocidolite (C).TABLE 5 .-RELATIVE ABSORBANCES AND CATION DISTRIBUTION intensity cation number M,, cation number M,, sample A B C D Fe Mg Fe Mg amosite 0 6 19 1 1 2.1 0.9 3.8 0 anthophyllite 10 0 0 0 0 3.0 1 .o 2.7 crocidolite 0 0 10 15 2.6 0.4 3.6 0 distributions in the MI and M, positions were calculated according to table 4. The results are collected in table 5. The M,, populations were determined from the formula unit compositions by subtraction. In the amosite and crocidolite samples virtually all the Mg ions are located in the M, and M, positions between the paired chains, and the M, and M, positions are mainly occupied by Fe. This distribution fits the packing conditions for these minerals, and is in agreement with data published el~ewhere.~, In anthophyllite all the iron seems to occur in M, and M, sites, although the packing conditions should favour Mg.The distribution obtained in this work can, however, be accepted because of the low iron content of the sample. Moreover, it has been confirmed by X-ray and Mossbauer methods that Fe has a preference for the M, site in all the amphibole minerals. If the amount of Fe exceeds the number of M, positions (i.e. 2.0 atoms per formula unit)M-I. LUYS, G. D E ROY, E. F. VANSANT A N D F. ADAMS 3567 a symmetry transition occurs and the orthorhombic amphibole is converted to its monoclinic 25 THE SERPENTINE MINERALS The structure of chrysotile is the magnesium analogue of kaolin. Planar-linked silica tetrahedra face an adjoining brucite sheet composed of magnesium ions which are coordinated octahedrally with oxygens and hydroxy groups (fig.4). Two of the three apical oxygens in the silica sheet replace hydroxy groups in the brucite sheet to FIG. 4.-Chrysotile structure. complete the chrysotile structure.26* 27 The distance between these adjacent sheets is ca. 7.3 A, with symmetry repetitions every 14.6 A. The physical shape of chrysotile is determined by the bond lengths in the tetrahedral and octahedral sheets. Their ratio is expressed by the morphological index,2s yielding a value of 76-77 in the two samples we investigated. The calculation was performed assuming all iron to be present in the octahedral layer and an FeI1/Fe1I1 ratio of 1.0. The high index value is indicative of intense intracrystalline torsions, which are released by sheet curvature.In contrast to halloy~ite,~~ chrysotile is curved with the brucite sheet to the outside, forming tubular crystals with an internal free diameter of ca. 76 A. This morphology has been confirmed by electron micrograph^^^ and diffraction 31 The infrared spectra of the chrysotile samples contain a number of significant features in the lattice-vibration region as well as the hydroxy stretching frequencies. Because of the similarity of the spectra, fig. 5 and 6 only show the Rhodesian chrysotile patterns. LATTICE VIBRATIONS The lattice-vibration pattern (fig. 5) contains a number of major peaks. Their frequencies are listed in table 6 and agree with those obtained by Yariv et aL3 The3568 CHARACTERISTICS OF ASBESTOS MINERALS 1075 cm-l absorption can be attributed to symmetric stretching modes of SiO and (Mg, Fe) 0 species.According to polarization data3 these vibrations are identified as out-of-plane stretchings, and they appear at nearly identical frequencies because of the strong coupling between the tetrahedral and octahedral sheets. This coupling also induces a small desymmetrization and a consequent infrared activity of these vibrations. The 1025 cm-l absorption is also attributed to symmetric stretching modes, but these should occur in the curved plane. The 954 cm-l peak results from I I 1 1 - 1 1 I I I 1400 1100 800 500 200 F/cm-' FIG. 5.-Lattice-vibration spectra of Rhodesian chrysotile (A), Rhodesian chrysotile treated in dilute acid (B) and Canadian chrysotile (C).asymmetric stretching modes of both silica and brucite, with a strong coupling between the two. It probably also contains part of the in-plane stretching modes, whose degeneracy could be lifted by the curvature of the layers3 An attempt has been made to assign the vibrational bands in the 600-250 cm-l region. The bands are primarily caused by the more complicated vibrations of the tetrahedral (v, v,) and octahedral (v, v3 v, v5 v6)16 entities. It should be borne in mind that all vibrations influence each other in a very complicated way. The 630 and 600 cm-l vibrations are attributed to the vlv3 stretching modes of the magnesia octahedra occurring in the plane of the curved sheets. The splitting of this absorption is again determined by the lifting of degeneracy of the modes in the linear and curved directions.The 560 cm-l peak is attributed to v, out-of-plane bending vibrations of silica tetrahedra. The v, bendings of the Mg octahedra probably cause the 480 cm-l (out-of-plane) and the 435 and 400 cm-l (in-plane) absorptions. The 380 and 360 cm-l peaks cannot be identified clearly but it is suggested that in-plane v2 Si-0 and v5 Mg-0 bending vibrations might contribute to them. The 285 cm-l peak is probably caused by the v6 Mg-0 bending mode.M-J. LUYS, G. D E ROY, E. F. VANSANT A N D F. ADAMS 3569 TABLE 6.-LATTICE VIBRATIONS OF CHRYSOTILE ASBESTOS sample vibrational frequency/cm-l Rhodesian 1075 1034 964 630 605 565 490 435 392 380 360 285 Canadian 1084 1025 954 635 600 560 480 432 400 384 355 274 HYDROXY GROUPS In the hydroxy stretching region we observe two fundamentally different species.A very broad band was detected near 3440 cm-l and two sharp hydroxy peaks were obtained at 3645 and 3685 cm-l (fig. 6). The very broad 3440 cm-l band must contain mostly hydroxy species, since the H,O bending vibration region (near 1650 cm-l) contains no appreciable absorption. I c v I --r----- 0 3LOO 2800 1700 tL( 5lcrn-l FIG. 6.-Hydroxy patterns of chrysotile samples : (A) Rhodesian chrysotile, (B) Rhodesian chrysotile treated in dilute acid and (C) Canadian chrysotile. The assignment of the three bands has been performed on the basis of the chrysotile structural data. Three crystallographically inequivalent hydroxy species are present in the crystal, as indicated in fig.4. They appear in two planes: (OH), in the plane which is common to the tetrahedral silica and the octahedral brucite sheet, and (OH), and (OH), in the outer plane of the chrysotile tubes.26 The position of the magnesium ions, however, is not mid-way between these two planes, but is shifted to the 0-(OH), plane. Therefore the (OH), band strength is expected to be lower as compared with those of the other species. It may be assigned to the 3440 cm-l vibration, although the width indicates extensive hydrogen bonding. This is a situation comparable to the3570 CHARACTERISTICS OF ASBESTOS MINERALS behaviour of the inner hydroxy groups of silica.,, It is therefore suggested that the (OH), species form hydrogen bonds with their oxygen neighbours.The two absorption bands at 3645 and 3685 cm-l are assigned to the (OH,) and (OH), stretching vibrations. The ratio (OH), : (OH), = 2 is confirmed by the infrared spectrum, where a value of 2.2 is observed. It is, however, remarkable that the (OH), and (OH), surface hydroxy groups are not involved in hydrogen-bonding interactions. The curvature of the sheets can explain the lack of mutual interaction, because of the longer in-plane distances and the unfavourable geometry involved. However, we would expect some interaction between adjacent tubes, or between these species and the water of hydration. Experiment clearly demonstrates that these phenomena did not occur in our samples. CHEMICAL STABILITY OF ASBESTOS MINERALS Spectra were recorded of asbestos samples which had been treated with dilute NaOH solution.In each case the characteristic lattice pattern was identical to that of the original samples. This observation clearly demonstrates the stability of these minerals in dilute basic solutions. Moreover, the hydroxy stretching region was not modified by the pretreatment. The base-treated minerals therefore have identical bulk and surface structures. The spectra of the amphibole minerals treated in dilute HCl solution did not change with respect to the original patterns. The pretreatment conditions used are insufficient to change the amphibole structure. Literature data1 indicate that after very long exposure times a slight degradation can occur. However, chrysotile samples treated in acid media yield spectra with characteristics which differ from those of the original samples.Fig. 5 and 6 contain the spectra of the acid-treated Rhodesian chrysotile sample (the spectrum of Canadian chrysotile is similar). In the lattice vibration region several absorptions are enhanced and new ones appear. The out-of-plane vibrations at 1075, 560 and 285 cm-l increase in intensity, while the in-plane 1025 cm-l stretching vibration intensity is weakened. Moreover, the occurrence of free silica stretching vibrations at 1200 cm-l and out-of-plane SOH and MgOH deformations at 800 cm-l confirm the overall degra- dation of symmetry and the structural breakdown of the chrysolite lattice. This trend is sustained by the increasing amount of water of hydration which is detected at 3410 and 1640 cm-l.We therefore suggest that as a consequence of the acid treatment the chrysolite lattice has been partially degraded probably even into amorphous, hydrated silica and magnesium (hydr)oxide. However, under the conditions of this test a considerable amount of chrysotile remains intact, and the general shape of the spectra is conserved. CONCLUSIONS The biological activity of asbestos minerals is determined by their surface structure and their reactions in uiuo. It has been suggested that the activity of chrysotile is much higher than that of the amphiboles, at least in the initial stages following exposure.lY 33 From the structural characteristics of the minerals it is known that the chrysotile surface acquires a positive net charge on contact with a solution of near-neutral pH.,, This type of charge centre is very effective in causing membrane functions to deteriorate, and its activity is not very susceptible to neutralization by surfactants.Moreover, the outer surface, composed of hydroxy species, offers considerable possibilities for interactions with tissue. This is in accord with our findings that the hydroxide species at the surface of the chrysotile tubes are free under normal conditions, and that the surface is not hydrated.3571 M-J. LUYS, G. D E ROY, E. F. V A N S A N T A N D F . ADAMS The chrysotile structure is decomposed in a low-pH medium, as opposed to the amphibole minerals which are stable in dilute acid, at least for the short interval of time used in our experiments. The breakdown of chrysotile fibres is a process which ultimately might decrease the mineral's interactive possibilities and consequently lower its biological activity.During the breakdown period, however, major alterations may occur in the vicinity of the fibres, including oxidation-reduction reactions of organic compounds contained in cells. The surface of the fibres in all cases could act as a catalyst for the denaturation of proteins and the alteration of macromolecules. Amphibole fibres are therefore thought to be the more active species in chronic diseases. A. M. Langer and M. S. Wolf€, Inorganic and Nutritional Aspects of Cancer (Plenum Press, London, 1977), vol. 91, chap. 3. R. W. Luce, U S . , Geol. Suru., Prof. Pap., 1971, 750B, 199. S. Yariv and L. 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