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Spectroscopic characterization of chrysotile asbestos from different regions
Journal of Molecular Structure 993 (2011) 120–126
Contents lists available at ScienceDirect
Journal of Molecular Structure journal homepage: www.elsevier.com/locate/molstruc
Spectroscopic characterization of chrysotile asbestos from different regions Mira Ristic´ a,⇑, Ilona Czakó-Nagy b, Svetozar Music´ a, Attila Vértes b a b
- Boškovic´ Institute, P.O. Box 180, HR-10002 Zagreb, Croatia Department of Materials Chemistry, Ruder Department of Nuclear Chemistry, Loránd Eötvös University, H-1518 Budapest, Hungary
a r t i c l e
i n f o
Article history: Available online 29 October 2010 Keywords: Chrysotile asbestos 57 Fe Mössbauer FT-IR Electron microscopy
a b s t r a c t Chrysotile asbestos from different geographic regions was characterized by 57Fe Mössbauer and FT-IR spectroscopies, and FE-SEM coupled with EDS. Mössbauer spectra showed incorporation of Fe(II) and Fe(III) into the crystal structure of chrysotile. The Fe(II)/Fe(III) ratios were calculated. The Fe(II) occupied the octahedral Mg(II) positions. Broadening of Mössbauer lines corresponding to the Fe(III) doublet indicated a possible presence of Fe(III) in tetrahedral and octahedral positions. In all samples magnetite (Fe3xO4) was found as associated mineral. Changes in the FT-IR spectra of chrysotiles were assigned to structural incorporation of metal ions, such as iron or aluminium ions. The morphology of chrysotile asbestos was inspected by FE-SEM. Ó 2010 Elsevier B.V. All rights reserved.
1. Introduction Asbestos is a general term used to describe a group of minerals which can be separated into long silky fibers. They can be divided into serpentines (sheet silicates) and amphiboles (chain silicates). The chemical composition of asbestiform minerals may vary in dependence on the deposit locality. Asbestiform minerals are usually characterized by a close association with other minerals. Moreover, these minerals can incorporate small concentrations of other elements in the mineral crystal lattice. Fig. 1 shows the principal varieties of asbestiform minerals. Serpentine minerals are divided into three main groups – chrysotiles, lizardites and antigorites. All have the chemical composition Mg3(Si2O5)(OH)4, but varying adjustments of the misfit between the tetrahedral and octahedral sheets. Chrysotile has a tubular structure, and the tube-like fibers are the result of a curvature of the 0 0 1 crystal plane parallel to the a and b crystallographic axes. These asbestiform minerals used to play an important role in industry. However, in many countries the use of chrysotile asbestos is nowadays prohibited due to its toxicity. There are attempts to eliminate this toxicity by converting chrysotile asbestos to other phases, the glass state, for example. Minerals which are frequently associated with chrysotile asbestos are magnetite, chromite, brucite, calcite, dolomite and awaruite. Nickel and iron ions can replace magnesium ions by isomorphic substitution. Generally, the application of Mössbauer spectroscopy to iron containing silicate minerals is a well established practice [1–3].
⇑ Corresponding author. Tel.: +385 1 4680 107; fax: +385 1 4680 098. E-mail address: [email protected] (M. Ristic´). 0022-2860/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.molstruc.2010.10.005
The Mössbauer parameters, such as the isomer shift and quadrupole splitting, can be correlated with the oxidation state and the coordination environment of iron in silicates. In some cases it is possible to distinguish Fe2+ in structurally different positions as having the same environment [4]. The ratio of the intensities of appropriate Mössbauer lines can be used to determine the Fe3+/ Fe2+ ratio. Mössbauer spectroscopy finds its applications in geological studies of crystallization conditions and the subsequent thermal history of natural crystals and rock-forming minerals [5,6]. Darby Dyar and Schaefer [7] reported the list of potential Fe-bearing minerals on the Martian surface, including chrysotile. The authors discussed the application of Mössbauer spectroscopy in the analysis of minerals on the Mars. Extraterrestrial Mössbauer spectroscopy supplies us with important data about minerals compositions at the surface of Mars [8,9]. Infrared spectroscopy also provides useful data on chemical bonds in the structural units and the influence of impurities on asbestos properties. In the present work, chrysotile asbestos from various localities around the world has been collected and investigated by 57Fe Mössbauer and FT-IR spectroscopies as well as FE-SEM and EDS. The aim of this work was to obtain more data about the oxidation state of iron in chrysotiles and the dependence of Fe2+/Fe3+ ratio on the origin of chrysotile. Furthermore, the corresponding FT-IR spectra were monitored.
2. Experimental Specimens of chrysotile asbestos were taken from several mining localities in Canada, Zimbabwe, Russia, Bosnia and Serbia and were characterized as received. Prior to the spectroscopic and electron microscopic characterization, chrysotiles were characterized
M. Ristic´ et al. / Journal of Molecular Structure 993 (2011) 120–126
121
Fig. 1. Principal varieties of asbestiform minerals.
Table 1 Chemical composition (%) of chrysotile asbestos. Sample (origin)
Moisture
Ignition loss at 1000 °C
SiO2
Fe2O3
MgO
CaO
Al2O3
Co
Ni
K
5A (Stragari, Serbia) 6A (Petrovo selo, Bosnia and Herzegovina) 7A (4D-Lake, Canada) 8A (BC-110-4D, Canada) 9A (98, Zimbabwe) 10A (4450, Zimbabwe) 11A (P-3-60, Russia) 12A (P-3-70, Russia)
2.14 0.90 0.73 0.84 1.13 0.79 0.95 0.83
16.33 14.89 14.31 14.50 15.57 16.01 14.70 15.53
39.24 39.48 39.11 38.89 36.71 36.31 39.82 38.32
5.74 5.21 5.41 6.61 6.01 8.24 6.65 7.14
39.23 39.04 39.90 44.84 40.29 39.42 40.43 37.96
0.29 0.20 0.62 0.20 0.22 1.20 0.24 1.33
1.47 1.62 1.80 1.50 1.59
0.39 0.38 0.26 0.29 0.35 0.30 0.37 0.31
0.15 0.19 0.16 0.15 0.19 0.22 0.23 0.22
0.57 0.50 0.36 0.35 0.46 3.14 0.37 0.36
by XRD. XRD showed that all chrysotiles contained a small amount of lizardite as an associated serpentine mineral. Chrysotile asbestos was decomposed, then quantitative analyses of specimens were performed using standard chemical analytical procedures. The results of chemical analyses are given in Table 2. 57 Fe Mössbauer spectra were recorded in the transmission mode at 20 °C using standard Wissel (Starnberg, Germany) instrumental configuration. 57Co in a rhodium matrix was used as the Mössbauer source. The velocity scale and all the data refer to the metallic a-Fe absorber at 20 °C. A quantitative analysis of the recorded spectra was made using the MossWin program. Fourier transform infrared (FT-IR) spectra were recorded at RT using a Perkin–Elmer spectrometer (model 2000). The FT-IR spectrometer was linked to a PC with an installed IRDM (IR data manager) program to process the recorded spectra. The specimens were pressed into small discs using a spectroscopically pure KBr matrix. A thermal field emission scanning electron microscope (FESEM, model JSM-7000F manufactured by Jeol Ltd.) was used. FESEM was linked to the EDS/INCA 350 (energy dispersive X-ray analyzer) manufactured by Oxford Instruments Ltd. The specimens were not coated with an electrically conductive surface layer.
3. Results and discussion 3.1. Mössbauer spectroscopy Generally, asbestiform minerals contain various inorganic impurities either as associated minerals or located in the crystal structure of asbestos. The chemical compositions (%) of the chrysotiles investigated are given in Table 1. The total iron content varied
2.06
from 5.21% to 8.24 %, expressed as Fe2O3. Iron ions in chrysotiles can be incorporated into their crystal lattice, or can be present in associated minerals. Mössbauer spectra of chrysotiles from different localities are shown in Figs. 2–4, while the calculated Mössbauer parameters are given in Table 2. The main feature of all Mössbauer spectra of chrysotiles is the presence of Fe2+ and Fe3+ ions in the crystal lattice of chrysotile, as well as in magnetite as an associated mineral. Two magnetic splitting components were recorded in the Mössbauer spectra and assigned to magnetite, but not fully stoichiometric (Fe3xO4). Slightly decreased HMF values could have been influenced by the incorporation of other metal cation impurities into the crystal structure of magnetite. The inner part of the spectra was resolved for two quadrupole doublets. The quadrupole doublets with parameters d = 1.11–1.27 mm s1 and D = 2.41– 2.84 mm s1 can be assigned to Fe2+ ions in octahedral positions. Since Fe2+ and Mg2+ ions are isomorphous, it is obvious that Fe2+ is substituted for Mg in the brucite layer of chrysotile. The second doublet with parameters d = 0.30–0.40 mm s1 and D = 0.57– 0.70 mm s1 is due to the presence of Fe3+ ions in the crystal structure of chrysotile. The width C of Fe3+ lines was enlarged in relation to Fe2+ lines and varied between C = 0.50 and 0.74 mm s1. This C broadening suggests the presence of two or more Fe3+ doublets (superposition). In clay minerals (also layer silicates), Fe3+ ions in the tetrahedral position have Mössbauer parameters of d/ D = 0.22/0.33 [10]. The substitution of Fe3+ for Si4+ in tetrahedral positions can also be supposed in the case of chrysotile. If Fe3+ ions are substituted for Mg2+ + ions, an excess of charge (1+) appears and in that case Me3+ ions (generally, it will be Fe3+ ions) should be substituted for Si4+ ions due to the compensation of charges. The resulting Mössbauer spectrum should be characterized by Fe3+ in the tetrahedral position.
M. Ristic´ et al. / Journal of Molecular Structure 993 (2011) 120–126
122 Table 2 Calculated Mössbauer parameters at 20 °C. Sample
Spectral line
d (mm s1)
D or Eq. (mm s1)
HMF (T)
C (mm s1)
Area (%)
Fe2+/Fe3+
5A Serbia
Q1 Q2 M1 M2
1.12 0.40 0.24 0.69
2.73 0.65 0.05 0.06
– – 48.7 46.1
0.31 0.55 0.33 0.44
51 27 10 12
1.89
6A Bosnia and Herzegovina
Q1 Q2 M1 M2
1.13 0.35 0.24 0.74
2.84 0.68 0.00 0.00
– – 48.7 46.1
0.32 0.55 0.30 0.30
41 29 12 18
1.41
7A Canada
Q1 Q2 M1 M2
1.11 0.30 0.24 0.70
2.75 0.57 0.07 0.04
– – 48.6 46.0
0.31 0.55 0.30 0.37
26 24 22 28
1.08
8A Canada
Q1 Q2 M1 M2
1.13 0.32 0.24 0.69
2.74 0.70 0.05 0.06
– – 48.7 46.3
0.30 0.74 0.23 0.34
31 33 12 24
0.94
8Aa Canada
Q1 Q2 Q3 M1 M2
1.12 0.37 0.30 0.24 0.68
2.75 0.93 0.46 0.05 0.06
– – – 48.7 46.3
0.30 0.57 0.37 0.23 0.34
32 20 11 13 24
1.07
9A Zimbabwe
Q1 Q2 M1 M2
1.19 0.31 0.24 0.69
2.70 0.57 0.05 0.06
– – 48.7 46.3
0.48 0.55 0.30 0.40
15 49 14 22
0.31
10A Zimbabwe
Q1 Q2 M1 M2
1.27 0.32 0.31 0.70
2.41 0.62 0.05 0.06
– – 48.6 45.7
0.50 0.67 0.53 0.43
8 69 17 6
0.12
11A Russia
Q1 Q2 M1 M2
1.11 0.35 0.26 0.67
2.74 0.60 0.01 0.02
– – 48.9 45.7
0.34 0.50 0.23 0.48
31 32 12 25
0.97
12A Russia
Q1 Q2 M1 M2
1.13 0.31 0.30 0.65
2.74 0.60 0.01 0.02
– – 49.0 45.6
0.44 0.50 0.33 0.41
32 39 15 24
0.82
Errors: d = ± 0.01 mm s1, D or Eq = ± 0.01 mm s1; HMF = ± 0.2 T. Isomer shift, d, is given relative to a-Fe at 20 °C. a Fitted by using superposition of three inner doublets.
Fig. 2.
57
Fe Mössbauer spectra of chrysotiles from Serbia (5A), Bosnia and Herzegovina (6A) and Canada (7A, 8A), recorded at 20 °C.
M. Ristic´ et al. / Journal of Molecular Structure 993 (2011) 120–126
Fig. 3.
Fig. 4.
57
123
Fe Mössbauer spectra of chrysotiles from Zimbabwe (9A, 10A) and Russia (11A, 12A), recorded at 20 °C.
57
Fe Mössbauer spectrum of chrysotile from Canada (8A), fitted by superposition of three inner doublets and two sextets.
During the Mössbauer investigation of some Ural serpentines Malysheva et al. [11] have assumed Fe3+ to be in the tetrahedral position and they obtained d/D = 0.31/0.31 mm s1 values. Blaauw et al. [12] recorded the RT Mössbauer spectra of Canadian chrysotiles from four mining localities. The authors have concluded that Fe2+ occupies only the octahedral (Mg2+) position in chrysotile, whereas Fe3+ is distributed between the octahedral and tetrahedral (Si4+) positions. For the isomer shift (relative to a-Fe) and the quadrupole splitting of Fe2+ in the octahedral position the d/D = 1.12/ 2.65 values were calculated, whereas for Fe3+ the values in octahedral positions d/D = 0.34/0.75 and for Fe3+ the values in tetrahedral positions d/D = 0.20/0.34 were calculated. In the present work we have also fitted Canadian chrysotile (sample 8A) for three quadrupole doublets (Fig. 3). The parameters d/D = 1.12/2.75 can be assigned to Fe2+ in octahedral positions, the parameters d/D = 0.30/
0.46 can be assigned to Fe3+ in the tetrahedral position and the parameters d/D = 0.37/0.93 can be assigned to Fe3+ in octahedral positions. Due to the variation in the Fe3þ oct chemical environment in the chrysotile structure the corresponding D value may vary. Gonzáles-Mancera et al. [13] used Mössbauer spectroscopy to investigate the iron state in serpentine minerals from Southern Mexico. In all samples Fe2+ and Fe3+ ions were interpreted as located only in octahedral positions. O’Hanley and Darby Dyar [14] used Mössbauer spectroscopy to show that the different parageneses of lizardite and chrysotile result from differential partitioning of Fe3+ and Fe2+ between different positions in these two minerals. Gunter et al. [15] also applied Mössbauer spectroscopy P to study Fe3+/ Fe ratios of some non-asbestiform and asbestiform amphiboles from Montana (USA). The quadrupole doublets were fitted applying quadrupole splitting distributions (QSD).
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M. Ristic´ et al. / Journal of Molecular Structure 993 (2011) 120–126
On the basis of the areas under the Mössbauer peaks, the distribution of total iron between Fe(II) and Fe(III) states was calculated (Table 2). The Fe(II)/Fe(III) ratio in chrysotile asbestos strongly depends on the region of its origin. Generally, the Fe(II)/Fe(III) ratio in minerals provides important information about the partial pres-
sure of oxygen during crystallization, which is a parameter of geological significance [5]. The comparatively high Fe(II)/Fe(III) ratios, for example in the case of samples 5A (Serbia) and 6A (Bosnia), refer to the formation of chrysotile under high pressure and more reductive conditions. On the other hand, a low content of Fe(II)
Fig. 5. FT-IR spectra (RT) recorded at high wave numbers for chrysotiles from Serbia (5A), Bosnia and Herzegovina (6A), Canada (7A, 8A), Zimbabwe (9A, 10A) and Russia (11A, 12A).
Fig. 6. FT-IR spectra (RT) recorded in a lower IR region for chrysotiles from Serbia (5A), Bosnia and Herzegovina (6A), Canada (7A, 8A), Zimbabwe (9A, 10A) and Russia (11A, 12A).
M. Ristic´ et al. / Journal of Molecular Structure 993 (2011) 120–126
in samples 9A and 10A (Zimbabwe) permits us to conclude that these minerals have been formed under relatively low pressures and highly oxidative conditions.
3.2. FT-IR spectroscopy The FT-IR spectra of all chrysotile samples recorded at high wave numbers are shown in Fig. 5. Three IR bands are well visible, a strong one at 3688–3691 cm1, a weak one at 3644–3646 cm1, and a medium intensity but very broad band at 3450–3440 cm1. The IR band at 3688–3691 cm1 can be assigned to the surface
Fig. 7. FE-SEM images of selected chrysotiles from (a) Bosnia and Herzegovina (6A), (b) Zimbabwe (10A) and (c) Russia (12A).
125
Mg–OH stretch, whereas the IR band at 3644–3646 cm1 can be assigned to the inner Mg–OH stretch [16]. The IR band at 3450– 3440 cm1 can be assigned to the –OH stretch of nonstructural water (moisture). Fig. 6 shows the FT-IR spectra of all chrysotile samples recorded in the lower MID-IR region. Three IR bands recorded in the region 1088 to 955 cm1 are typical of the Si– O–Si stretches in the silica network. The IR band at 1088– 1081 cm1 can be assigned to the out-of-plane symmetric stretching vibration of the silica sheet, whereas the two IR bands at 1010 to 1022 cm1 and 964 to 955 cm1 can be assigned to the inplane Si–O stretching, mainly parallel to the b- and a-axes, respectively [17]. The positions of the three IR bands at 1088, 1010 and 964 cm1 (sample 5A) differ from other samples (6A–12A). The distinct differences in the shape of the FT-IR spectra of chrysotile asbestos were also observed in the IR region 700 to 500 cm1. For example, chrysotile 5A showed in this region a strong IR band at 606 cm1, a weak one at 562 cm1, and three shoulders at 650, 618 and 582 cm1. In accordance with Foresti et al. [17], the shoulder at 650 cm1 can be assigned to outer Mg–OH vibration, the IR band at 606 cm1 to inner Mg–OH vibration, the shoulder at 582 cm1 to antisymmetric vibration and the weak peak at 562 cm1 to perpendicular Mg–O/Si–O bending vibration. In chrysotiles from 6A to 11A only a strong band at 605 cm1 and two shoulders at 665–657 cm1 and 561– 546 cm1 were visible. The IR band at 606 cm1 and the shoulder at 558 cm1 were recorded for chrysotile 12A. The shoulder at 470 to 480 cm1 and a strong band at 435 cm1 were recorded for chrysotiles 6A to 11A, whereas in sample 12A the shoulder 470 to 480 cm1 was not visible due to a shift to lower wave numbers. The IR band at 435 cm1 and the shoulder at 470 to 480 cm1 can be assigned to the bending vibrations of Si–O–Si and Mg–O bonds. Generally, the effects observed (the shifts of IR bands in chrysotile samples) can be related primarily to the incor-
Fig. 8. EDS spectrum of chrysotile from Canada (8A).
126
M. Ristic´ et al. / Journal of Molecular Structure 993 (2011) 120–126
poration of Fe3+ and Al3+ impurities into the chrysotile crystal structure. The presence of a magnetite phase as associated with chrysotile could not be detected due to a small amount and strong overlapping with the bands of the dominant phase (chrysotile). Kloprogge et al. [18] used Raman spectroscopy to chrysotile mineral. The infrared and Raman spectra of different silicate minerals can be found in reference literature [19,20]. 3.3. FE-SEM + EDS FE-SEM inspection involved all chrysotile samples consisting of very elongated fibrils laterally arrayed in bundles. This is well visible in Fig. 7 which shows the FE-SEM images of selected samples from: (a) Bosnia and Herzegovina, (b) Zimbabwe, and (c) Russia. Fig. 8 shows EDS analysis of a selected area of sample 8A from Canada. Apart from Mg, Si and O, which are constitutive elements of the chrysotile crystal structure, the Fe and Al impurities in small amounts were detected. 4. Conclusions Chrysotile asbestos specimens from different geographic regions were characterized using 57Fe Mössbauer and FT-IR spectroscopies, FE-SEM and EDS. 57Fe Mössbauer spectra showed the presence of iron in the chrysotile crystal structure in oxidation states Fe2+ and Fe3+. Iron was also present in magnetite (Fe3xO4) as a mineral associated with chrysotile. Fe(II) occupied the octahedral Mg(II) positions in the chrysotile crystal structure. Fe(III) may occupy tetrahedral and octahedral positions. The Fe(II)/Fe(III) ratios obtained can be used to discuss the influence of reductive/oxidative conditions during the formation of chrysotiles. Differences
in the FT-IR spectra typical of silicate minerals were assigned to iron and aluminium impurities in the chrysotile crystal structure. FE-SEM showed very long laterally arrayed chrysotile fibers.
References [1] N.N. Greenwood, T.C. Gibb, Mössbauer Spectroscopy, Chapman and Hall Ltd., London, 1971. [2] G.M. Bancroft, Mössbauer Spectroscopy, An Introduction for Inorganic Chemists and Geochemists, McGraw-Hill, London, 1973. [3] T.C. Gibb, Principles of Mössbauer Spectroscopy, Chapman and Hall Ltd., London, 1976. [4] G.M. Bancroft, Mössbauer spectroscopic studies of the chemical state of iron in silicate minerals, J. Phys. 40 (1979) C-2, 464. [5] S.S. Hafner, Mössbauer spectroscopy in lunar geology and mineralogy, in: U. Gosner (Ed.), Mössbauer Spectroscopy in Applied Physics, Springer-Verlag, Berlin, Heidelberg, 1975. [6] F.A. Seifert, D. Virgo, Science 188 (1975) 1107. [7] M. Darby Dyar, M.W. Schaefer, Earth Planet. Sci. Lett. 218 (2004) 243. [8] C. van Cromphaut, V.G. de Resende, E. De Grave, A. Van Alboom, R.E. Vandenberghe, G. Klingelhofer, Geochim. Cosmochim. Acta 71 (2007) 4814. [9] C. Schroder, G. Klingelhofer, R.V. Morris, D.S. Rodionov, I. Fleischer, M. Blumers, Hyperfine Interact. 182 (2008) 149. [10] T. Ericsson, R. Wappling, K. Punakivi, Geol. Foren. Stock. For. 99 (1977) 229. [11] T.V. Malysheva, V.I. Grachev, I.S. Chaschuhin, Geokhimiya 11 (1975) 621. [12] C.G. Blaauw, W. Leiper, M. Zentilli, Can. Miner. 17 (1979) 713. [13] G. Gonzáles-Mancera, F. Ortega-Gutierrez, N.E. Nava, H.S. Arriola, Hyperfine Interact. 148 (149) (2003) 61. [14] D.S. O’Hanley, M. Darby Dyar, Can. Miner. 36 (1998) 727. [15] M.E. Gunter, M. Darby Dyar, B. Twamley, F.F. Foit Jr., S. Cornelius, Am. Miner. 88 (2003) 1970. [16] R.L. Frost, E. Mendelovici, J. Colloid Interf. Sci. 294 (2006) 47. [17] E. Foresti, M.F. Hochella Jr., H. Kornishi, I.G. Lesci, A.S. Madden, N. Roveri, H. Xu, Adv. Funct. Mater. 15 (2005) 1009. [18] J.T. Kloprogge, R.L. Frost, L. Rintoul, Phys. Chem. Chem. Phys. 1 (1999) 2559. [19] G. Jovanovski, P. Makreski, B. Kaitner, B. Boev, Croat. Chem. Acta 82 (2009) 363. [20] G. Anbalagan, G. Sivakumar, A.R. Prabakaran, S. Gunasekaran, Vib. Spectrosc. 52 (2010) 122.
Contents lists available at ScienceDirect
Journal of Molecular Structure journal homepage: www.elsevier.com/locate/molstruc
Spectroscopic characterization of chrysotile asbestos from different regions Mira Ristic´ a,⇑, Ilona Czakó-Nagy b, Svetozar Music´ a, Attila Vértes b a b
- Boškovic´ Institute, P.O. Box 180, HR-10002 Zagreb, Croatia Department of Materials Chemistry, Ruder Department of Nuclear Chemistry, Loránd Eötvös University, H-1518 Budapest, Hungary
a r t i c l e
i n f o
Article history: Available online 29 October 2010 Keywords: Chrysotile asbestos 57 Fe Mössbauer FT-IR Electron microscopy
a b s t r a c t Chrysotile asbestos from different geographic regions was characterized by 57Fe Mössbauer and FT-IR spectroscopies, and FE-SEM coupled with EDS. Mössbauer spectra showed incorporation of Fe(II) and Fe(III) into the crystal structure of chrysotile. The Fe(II)/Fe(III) ratios were calculated. The Fe(II) occupied the octahedral Mg(II) positions. Broadening of Mössbauer lines corresponding to the Fe(III) doublet indicated a possible presence of Fe(III) in tetrahedral and octahedral positions. In all samples magnetite (Fe3xO4) was found as associated mineral. Changes in the FT-IR spectra of chrysotiles were assigned to structural incorporation of metal ions, such as iron or aluminium ions. The morphology of chrysotile asbestos was inspected by FE-SEM. Ó 2010 Elsevier B.V. All rights reserved.
1. Introduction Asbestos is a general term used to describe a group of minerals which can be separated into long silky fibers. They can be divided into serpentines (sheet silicates) and amphiboles (chain silicates). The chemical composition of asbestiform minerals may vary in dependence on the deposit locality. Asbestiform minerals are usually characterized by a close association with other minerals. Moreover, these minerals can incorporate small concentrations of other elements in the mineral crystal lattice. Fig. 1 shows the principal varieties of asbestiform minerals. Serpentine minerals are divided into three main groups – chrysotiles, lizardites and antigorites. All have the chemical composition Mg3(Si2O5)(OH)4, but varying adjustments of the misfit between the tetrahedral and octahedral sheets. Chrysotile has a tubular structure, and the tube-like fibers are the result of a curvature of the 0 0 1 crystal plane parallel to the a and b crystallographic axes. These asbestiform minerals used to play an important role in industry. However, in many countries the use of chrysotile asbestos is nowadays prohibited due to its toxicity. There are attempts to eliminate this toxicity by converting chrysotile asbestos to other phases, the glass state, for example. Minerals which are frequently associated with chrysotile asbestos are magnetite, chromite, brucite, calcite, dolomite and awaruite. Nickel and iron ions can replace magnesium ions by isomorphic substitution. Generally, the application of Mössbauer spectroscopy to iron containing silicate minerals is a well established practice [1–3].
⇑ Corresponding author. Tel.: +385 1 4680 107; fax: +385 1 4680 098. E-mail address: [email protected] (M. Ristic´). 0022-2860/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.molstruc.2010.10.005
The Mössbauer parameters, such as the isomer shift and quadrupole splitting, can be correlated with the oxidation state and the coordination environment of iron in silicates. In some cases it is possible to distinguish Fe2+ in structurally different positions as having the same environment [4]. The ratio of the intensities of appropriate Mössbauer lines can be used to determine the Fe3+/ Fe2+ ratio. Mössbauer spectroscopy finds its applications in geological studies of crystallization conditions and the subsequent thermal history of natural crystals and rock-forming minerals [5,6]. Darby Dyar and Schaefer [7] reported the list of potential Fe-bearing minerals on the Martian surface, including chrysotile. The authors discussed the application of Mössbauer spectroscopy in the analysis of minerals on the Mars. Extraterrestrial Mössbauer spectroscopy supplies us with important data about minerals compositions at the surface of Mars [8,9]. Infrared spectroscopy also provides useful data on chemical bonds in the structural units and the influence of impurities on asbestos properties. In the present work, chrysotile asbestos from various localities around the world has been collected and investigated by 57Fe Mössbauer and FT-IR spectroscopies as well as FE-SEM and EDS. The aim of this work was to obtain more data about the oxidation state of iron in chrysotiles and the dependence of Fe2+/Fe3+ ratio on the origin of chrysotile. Furthermore, the corresponding FT-IR spectra were monitored.
2. Experimental Specimens of chrysotile asbestos were taken from several mining localities in Canada, Zimbabwe, Russia, Bosnia and Serbia and were characterized as received. Prior to the spectroscopic and electron microscopic characterization, chrysotiles were characterized
M. Ristic´ et al. / Journal of Molecular Structure 993 (2011) 120–126
121
Fig. 1. Principal varieties of asbestiform minerals.
Table 1 Chemical composition (%) of chrysotile asbestos. Sample (origin)
Moisture
Ignition loss at 1000 °C
SiO2
Fe2O3
MgO
CaO
Al2O3
Co
Ni
K
5A (Stragari, Serbia) 6A (Petrovo selo, Bosnia and Herzegovina) 7A (4D-Lake, Canada) 8A (BC-110-4D, Canada) 9A (98, Zimbabwe) 10A (4450, Zimbabwe) 11A (P-3-60, Russia) 12A (P-3-70, Russia)
2.14 0.90 0.73 0.84 1.13 0.79 0.95 0.83
16.33 14.89 14.31 14.50 15.57 16.01 14.70 15.53
39.24 39.48 39.11 38.89 36.71 36.31 39.82 38.32
5.74 5.21 5.41 6.61 6.01 8.24 6.65 7.14
39.23 39.04 39.90 44.84 40.29 39.42 40.43 37.96
0.29 0.20 0.62 0.20 0.22 1.20 0.24 1.33
1.47 1.62 1.80 1.50 1.59
0.39 0.38 0.26 0.29 0.35 0.30 0.37 0.31
0.15 0.19 0.16 0.15 0.19 0.22 0.23 0.22
0.57 0.50 0.36 0.35 0.46 3.14 0.37 0.36
by XRD. XRD showed that all chrysotiles contained a small amount of lizardite as an associated serpentine mineral. Chrysotile asbestos was decomposed, then quantitative analyses of specimens were performed using standard chemical analytical procedures. The results of chemical analyses are given in Table 2. 57 Fe Mössbauer spectra were recorded in the transmission mode at 20 °C using standard Wissel (Starnberg, Germany) instrumental configuration. 57Co in a rhodium matrix was used as the Mössbauer source. The velocity scale and all the data refer to the metallic a-Fe absorber at 20 °C. A quantitative analysis of the recorded spectra was made using the MossWin program. Fourier transform infrared (FT-IR) spectra were recorded at RT using a Perkin–Elmer spectrometer (model 2000). The FT-IR spectrometer was linked to a PC with an installed IRDM (IR data manager) program to process the recorded spectra. The specimens were pressed into small discs using a spectroscopically pure KBr matrix. A thermal field emission scanning electron microscope (FESEM, model JSM-7000F manufactured by Jeol Ltd.) was used. FESEM was linked to the EDS/INCA 350 (energy dispersive X-ray analyzer) manufactured by Oxford Instruments Ltd. The specimens were not coated with an electrically conductive surface layer.
3. Results and discussion 3.1. Mössbauer spectroscopy Generally, asbestiform minerals contain various inorganic impurities either as associated minerals or located in the crystal structure of asbestos. The chemical compositions (%) of the chrysotiles investigated are given in Table 1. The total iron content varied
2.06
from 5.21% to 8.24 %, expressed as Fe2O3. Iron ions in chrysotiles can be incorporated into their crystal lattice, or can be present in associated minerals. Mössbauer spectra of chrysotiles from different localities are shown in Figs. 2–4, while the calculated Mössbauer parameters are given in Table 2. The main feature of all Mössbauer spectra of chrysotiles is the presence of Fe2+ and Fe3+ ions in the crystal lattice of chrysotile, as well as in magnetite as an associated mineral. Two magnetic splitting components were recorded in the Mössbauer spectra and assigned to magnetite, but not fully stoichiometric (Fe3xO4). Slightly decreased HMF values could have been influenced by the incorporation of other metal cation impurities into the crystal structure of magnetite. The inner part of the spectra was resolved for two quadrupole doublets. The quadrupole doublets with parameters d = 1.11–1.27 mm s1 and D = 2.41– 2.84 mm s1 can be assigned to Fe2+ ions in octahedral positions. Since Fe2+ and Mg2+ ions are isomorphous, it is obvious that Fe2+ is substituted for Mg in the brucite layer of chrysotile. The second doublet with parameters d = 0.30–0.40 mm s1 and D = 0.57– 0.70 mm s1 is due to the presence of Fe3+ ions in the crystal structure of chrysotile. The width C of Fe3+ lines was enlarged in relation to Fe2+ lines and varied between C = 0.50 and 0.74 mm s1. This C broadening suggests the presence of two or more Fe3+ doublets (superposition). In clay minerals (also layer silicates), Fe3+ ions in the tetrahedral position have Mössbauer parameters of d/ D = 0.22/0.33 [10]. The substitution of Fe3+ for Si4+ in tetrahedral positions can also be supposed in the case of chrysotile. If Fe3+ ions are substituted for Mg2+ + ions, an excess of charge (1+) appears and in that case Me3+ ions (generally, it will be Fe3+ ions) should be substituted for Si4+ ions due to the compensation of charges. The resulting Mössbauer spectrum should be characterized by Fe3+ in the tetrahedral position.
M. Ristic´ et al. / Journal of Molecular Structure 993 (2011) 120–126
122 Table 2 Calculated Mössbauer parameters at 20 °C. Sample
Spectral line
d (mm s1)
D or Eq. (mm s1)
HMF (T)
C (mm s1)
Area (%)
Fe2+/Fe3+
5A Serbia
Q1 Q2 M1 M2
1.12 0.40 0.24 0.69
2.73 0.65 0.05 0.06
– – 48.7 46.1
0.31 0.55 0.33 0.44
51 27 10 12
1.89
6A Bosnia and Herzegovina
Q1 Q2 M1 M2
1.13 0.35 0.24 0.74
2.84 0.68 0.00 0.00
– – 48.7 46.1
0.32 0.55 0.30 0.30
41 29 12 18
1.41
7A Canada
Q1 Q2 M1 M2
1.11 0.30 0.24 0.70
2.75 0.57 0.07 0.04
– – 48.6 46.0
0.31 0.55 0.30 0.37
26 24 22 28
1.08
8A Canada
Q1 Q2 M1 M2
1.13 0.32 0.24 0.69
2.74 0.70 0.05 0.06
– – 48.7 46.3
0.30 0.74 0.23 0.34
31 33 12 24
0.94
8Aa Canada
Q1 Q2 Q3 M1 M2
1.12 0.37 0.30 0.24 0.68
2.75 0.93 0.46 0.05 0.06
– – – 48.7 46.3
0.30 0.57 0.37 0.23 0.34
32 20 11 13 24
1.07
9A Zimbabwe
Q1 Q2 M1 M2
1.19 0.31 0.24 0.69
2.70 0.57 0.05 0.06
– – 48.7 46.3
0.48 0.55 0.30 0.40
15 49 14 22
0.31
10A Zimbabwe
Q1 Q2 M1 M2
1.27 0.32 0.31 0.70
2.41 0.62 0.05 0.06
– – 48.6 45.7
0.50 0.67 0.53 0.43
8 69 17 6
0.12
11A Russia
Q1 Q2 M1 M2
1.11 0.35 0.26 0.67
2.74 0.60 0.01 0.02
– – 48.9 45.7
0.34 0.50 0.23 0.48
31 32 12 25
0.97
12A Russia
Q1 Q2 M1 M2
1.13 0.31 0.30 0.65
2.74 0.60 0.01 0.02
– – 49.0 45.6
0.44 0.50 0.33 0.41
32 39 15 24
0.82
Errors: d = ± 0.01 mm s1, D or Eq = ± 0.01 mm s1; HMF = ± 0.2 T. Isomer shift, d, is given relative to a-Fe at 20 °C. a Fitted by using superposition of three inner doublets.
Fig. 2.
57
Fe Mössbauer spectra of chrysotiles from Serbia (5A), Bosnia and Herzegovina (6A) and Canada (7A, 8A), recorded at 20 °C.
M. Ristic´ et al. / Journal of Molecular Structure 993 (2011) 120–126
Fig. 3.
Fig. 4.
57
123
Fe Mössbauer spectra of chrysotiles from Zimbabwe (9A, 10A) and Russia (11A, 12A), recorded at 20 °C.
57
Fe Mössbauer spectrum of chrysotile from Canada (8A), fitted by superposition of three inner doublets and two sextets.
During the Mössbauer investigation of some Ural serpentines Malysheva et al. [11] have assumed Fe3+ to be in the tetrahedral position and they obtained d/D = 0.31/0.31 mm s1 values. Blaauw et al. [12] recorded the RT Mössbauer spectra of Canadian chrysotiles from four mining localities. The authors have concluded that Fe2+ occupies only the octahedral (Mg2+) position in chrysotile, whereas Fe3+ is distributed between the octahedral and tetrahedral (Si4+) positions. For the isomer shift (relative to a-Fe) and the quadrupole splitting of Fe2+ in the octahedral position the d/D = 1.12/ 2.65 values were calculated, whereas for Fe3+ the values in octahedral positions d/D = 0.34/0.75 and for Fe3+ the values in tetrahedral positions d/D = 0.20/0.34 were calculated. In the present work we have also fitted Canadian chrysotile (sample 8A) for three quadrupole doublets (Fig. 3). The parameters d/D = 1.12/2.75 can be assigned to Fe2+ in octahedral positions, the parameters d/D = 0.30/
0.46 can be assigned to Fe3+ in the tetrahedral position and the parameters d/D = 0.37/0.93 can be assigned to Fe3+ in octahedral positions. Due to the variation in the Fe3þ oct chemical environment in the chrysotile structure the corresponding D value may vary. Gonzáles-Mancera et al. [13] used Mössbauer spectroscopy to investigate the iron state in serpentine minerals from Southern Mexico. In all samples Fe2+ and Fe3+ ions were interpreted as located only in octahedral positions. O’Hanley and Darby Dyar [14] used Mössbauer spectroscopy to show that the different parageneses of lizardite and chrysotile result from differential partitioning of Fe3+ and Fe2+ between different positions in these two minerals. Gunter et al. [15] also applied Mössbauer spectroscopy P to study Fe3+/ Fe ratios of some non-asbestiform and asbestiform amphiboles from Montana (USA). The quadrupole doublets were fitted applying quadrupole splitting distributions (QSD).
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On the basis of the areas under the Mössbauer peaks, the distribution of total iron between Fe(II) and Fe(III) states was calculated (Table 2). The Fe(II)/Fe(III) ratio in chrysotile asbestos strongly depends on the region of its origin. Generally, the Fe(II)/Fe(III) ratio in minerals provides important information about the partial pres-
sure of oxygen during crystallization, which is a parameter of geological significance [5]. The comparatively high Fe(II)/Fe(III) ratios, for example in the case of samples 5A (Serbia) and 6A (Bosnia), refer to the formation of chrysotile under high pressure and more reductive conditions. On the other hand, a low content of Fe(II)
Fig. 5. FT-IR spectra (RT) recorded at high wave numbers for chrysotiles from Serbia (5A), Bosnia and Herzegovina (6A), Canada (7A, 8A), Zimbabwe (9A, 10A) and Russia (11A, 12A).
Fig. 6. FT-IR spectra (RT) recorded in a lower IR region for chrysotiles from Serbia (5A), Bosnia and Herzegovina (6A), Canada (7A, 8A), Zimbabwe (9A, 10A) and Russia (11A, 12A).
M. Ristic´ et al. / Journal of Molecular Structure 993 (2011) 120–126
in samples 9A and 10A (Zimbabwe) permits us to conclude that these minerals have been formed under relatively low pressures and highly oxidative conditions.
3.2. FT-IR spectroscopy The FT-IR spectra of all chrysotile samples recorded at high wave numbers are shown in Fig. 5. Three IR bands are well visible, a strong one at 3688–3691 cm1, a weak one at 3644–3646 cm1, and a medium intensity but very broad band at 3450–3440 cm1. The IR band at 3688–3691 cm1 can be assigned to the surface
Fig. 7. FE-SEM images of selected chrysotiles from (a) Bosnia and Herzegovina (6A), (b) Zimbabwe (10A) and (c) Russia (12A).
125
Mg–OH stretch, whereas the IR band at 3644–3646 cm1 can be assigned to the inner Mg–OH stretch [16]. The IR band at 3450– 3440 cm1 can be assigned to the –OH stretch of nonstructural water (moisture). Fig. 6 shows the FT-IR spectra of all chrysotile samples recorded in the lower MID-IR region. Three IR bands recorded in the region 1088 to 955 cm1 are typical of the Si– O–Si stretches in the silica network. The IR band at 1088– 1081 cm1 can be assigned to the out-of-plane symmetric stretching vibration of the silica sheet, whereas the two IR bands at 1010 to 1022 cm1 and 964 to 955 cm1 can be assigned to the inplane Si–O stretching, mainly parallel to the b- and a-axes, respectively [17]. The positions of the three IR bands at 1088, 1010 and 964 cm1 (sample 5A) differ from other samples (6A–12A). The distinct differences in the shape of the FT-IR spectra of chrysotile asbestos were also observed in the IR region 700 to 500 cm1. For example, chrysotile 5A showed in this region a strong IR band at 606 cm1, a weak one at 562 cm1, and three shoulders at 650, 618 and 582 cm1. In accordance with Foresti et al. [17], the shoulder at 650 cm1 can be assigned to outer Mg–OH vibration, the IR band at 606 cm1 to inner Mg–OH vibration, the shoulder at 582 cm1 to antisymmetric vibration and the weak peak at 562 cm1 to perpendicular Mg–O/Si–O bending vibration. In chrysotiles from 6A to 11A only a strong band at 605 cm1 and two shoulders at 665–657 cm1 and 561– 546 cm1 were visible. The IR band at 606 cm1 and the shoulder at 558 cm1 were recorded for chrysotile 12A. The shoulder at 470 to 480 cm1 and a strong band at 435 cm1 were recorded for chrysotiles 6A to 11A, whereas in sample 12A the shoulder 470 to 480 cm1 was not visible due to a shift to lower wave numbers. The IR band at 435 cm1 and the shoulder at 470 to 480 cm1 can be assigned to the bending vibrations of Si–O–Si and Mg–O bonds. Generally, the effects observed (the shifts of IR bands in chrysotile samples) can be related primarily to the incor-
Fig. 8. EDS spectrum of chrysotile from Canada (8A).
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M. Ristic´ et al. / Journal of Molecular Structure 993 (2011) 120–126
poration of Fe3+ and Al3+ impurities into the chrysotile crystal structure. The presence of a magnetite phase as associated with chrysotile could not be detected due to a small amount and strong overlapping with the bands of the dominant phase (chrysotile). Kloprogge et al. [18] used Raman spectroscopy to chrysotile mineral. The infrared and Raman spectra of different silicate minerals can be found in reference literature [19,20]. 3.3. FE-SEM + EDS FE-SEM inspection involved all chrysotile samples consisting of very elongated fibrils laterally arrayed in bundles. This is well visible in Fig. 7 which shows the FE-SEM images of selected samples from: (a) Bosnia and Herzegovina, (b) Zimbabwe, and (c) Russia. Fig. 8 shows EDS analysis of a selected area of sample 8A from Canada. Apart from Mg, Si and O, which are constitutive elements of the chrysotile crystal structure, the Fe and Al impurities in small amounts were detected. 4. Conclusions Chrysotile asbestos specimens from different geographic regions were characterized using 57Fe Mössbauer and FT-IR spectroscopies, FE-SEM and EDS. 57Fe Mössbauer spectra showed the presence of iron in the chrysotile crystal structure in oxidation states Fe2+ and Fe3+. Iron was also present in magnetite (Fe3xO4) as a mineral associated with chrysotile. Fe(II) occupied the octahedral Mg(II) positions in the chrysotile crystal structure. Fe(III) may occupy tetrahedral and octahedral positions. The Fe(II)/Fe(III) ratios obtained can be used to discuss the influence of reductive/oxidative conditions during the formation of chrysotiles. Differences
in the FT-IR spectra typical of silicate minerals were assigned to iron and aluminium impurities in the chrysotile crystal structure. FE-SEM showed very long laterally arrayed chrysotile fibers.
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