Abstract. 57Fe M6ssbauer spectra of the two silicate min- erals balangeroite (BAL) and carlosturanite (CST) have been collected at 80 and 295 K under normal ...
Phys Chem Minerals (1994) 21:222-227
PHYSlCICHEMllTRY NMIllERAIS 9
S p r i n g e r - V e r l a g 1994
57Fe Miissbauer Study of the Asbestiform Silicates Balangeroite and Carlosturanite Antonio Deriu 1, Giovanni Ferraris 2, Elena Belluso 2 1Dipartimento di Fisica, Universit~ di Parma Viale delle Scienze, 43100 Parma, Italy 2 Dipartimento di Scienze Mineralogiche e Petrologiche, Universit~t di Torino, Via Valperga Caluso 37, 10125 Torino, Italy Received: May 15, 1993/Revised, accepted April 2, 1994
Abstract. 57Fe M6ssbauer spectra of the two silicate minerals balangeroite (BAL) and carlosturanite (CST) have been collected at 80 and 295 K under normal and magic angle geometry. For both minerals the spectra have been fitted with two ferrous and two ferric doublets; Fe 2+ accounts for 80 and 62% of Fetot in Bal and CST, respectively. The number of doublets used to fit the spectra supports the hypotheses that: (i) in the serpentine-like structure of CST iron occupies only octahedra which lie between the tetrahedral silicate strips; (ii) the octahedral framework of BAL (actually monoclinic) is satisfactorily described with an orthorhombic sub-cell.
rahedra occupy [001] channels (Ferraris et al. 1987). In the monoclinic cell with a = 19.163 (2), b = 19.224 (2), c = 9.599(3) ]k, 7 = 89-50(1) ~ (c unique axis; space group P2/ n), obtained from powder neutron-diffraction data (Belluso and Ferraris 1991), there are four formula units with composition MzlO3(OH)2o(Si~O12)2; M (octahedral cations) ~~ Mgo.6z + Feo.19 2 + + Feo.lo 3 + + Mno.os + m i nor A1, Ca, Cr, Ti. The monoclinic symmetry is mainly due to the arrangement of the silicate chain, while the octahedral framework can be satisfactorily described by reference to a smaller (1/6) orthorhombic cell (a = 13.63, b = 13.51, c = 3 . 2 0 A; space group Pnnm). The content
Introduction Balangeroite (BAL) and carlosturanite (CST) are two asbestiform silicates abundantly and widely present in the serpentinites of the western Alps (Belluso and Ferraris 1991; Compagnoni etal. 1983; Compagnoni etal. 1985). Macroscopically they are very similar to longfibre chrysotile with which they are usually intergrown and have been confused in the past. The medical hazards of fibrous minerals is connected not only with the needle-like nature of the inhaled fibres, but also with their chemical and structural properties (Guthrie 1992; Skinner et al. 1988). A study of potential toxicity suggests that BAL and CST may interact in a number o f ways in vivo, because of the presence of Fe z+ and Fe 3+ (Astolfi et al. 1991). With the purpose of better characterizing the crystal chemistry of BAL and CST, a M6ssbauer study of these two minerals is presented in this paper.
Balangeroite The crystal structure of BAL (Fig. 1) is based on an octahedral framework where single chains o f silicate tet-
Correspondence to: A. Deriu
Fig. 1. Perspective view of the balangeroite structure along [001]. The single chains of silicate tetrahedra run in channels left in an octahedral framework. This framework is obtained through the connection of [001] chains of octahedra which are grouped in three (walls) and four (bundles). The depth of shading increases in the order M(1), M(2), M(3) and M(4) for the four independent octahedral sites within the orthorhombic sub-cell. Both the monoclinic (full line) and the orthorhombie (dashed line) cells are shown
223
Fig. 2. Perspective view of the carlosturanite structure along [010]. Fe atoms occupy the dotted octahedra between the strips of Si-tetrahedra. Open andfilled circles show the oxygen atoms of HzO and OH, respectively
o f this sub-cell, w h e r e the silicate t e t r a h e d r a a p p e a r diso r d e r e d , is M14Oa(OH)13.33(Sis.33016 ) with o n l y f o u r i n d e p e n d e n t o c t a h e d r a l sites.
Table 1. Average values (wt% with e.s.d.'s in parentheses) for electron probe analyses of BAL (5 analyses) and CST (11 analyses); water not analysed. The number of cations pfu. is calculated on the basis of 8 Si for BAL and 45 oxygens for CST BAL
CST
SiO2 A1203 TiO2 Cr203 FeO MnO MgO CaO
28.5(2) 26.3(6) 3.7(2) 32.0(5) -
35.4(8) 1.23(7) 3.7(2) 0.20(6) 3.5(2) 0.97(9) 39.4(9) 0.12(7)
Total
90.5
84.52
Si A1 Ti Cr Fe z+ Mn Mg Ca
8.00 6.17 0.87 13.40 -
11.27 0.46 0.88 0.04 0.93 0.27 18.70 0.04
Carlosturanite
C S T is m o n o c l i n i c w i t h a cell [ a = 36.70(3), b = 9.41 (2), c = 7.291 (5) ~ , fl = 101.1 (1)~ space g r o u p Cm] which, a l o n g [100], is 7 times t h a t o f s e r p e n t i n e (lizardite). A m o d e l for its c r y s t a l s t r u c t u r e (Fig. 2) can be o b t a i n e d f r o m the s e r p e n t i n e layer b y s u b s t i t u t i n g [Si207] - 6 groups with tetrahedrally arranged [(OH)sH20] -6 g r o u p s (Mellini et al. 1985). This s u b s t i t u t i o n b r e a k s the t e t r a h e d r a l l a y e r into t w o [010] strips p e r cell, w h i c h contains two formula units with composition M z l [ S i a z O 2 s ( O H ) 4 ] ( O H ) 3 o ' H 2 0 ; M ( o c t a h e d r a l cation) ~ M g o . 9 0 + F e o . 0 5 + T i o . 0 2 + m i n o r M n a n d C r ; a small a m o u n t o f A1 c a n r e p l a c e Si. C S T c a n be c o n s i d ered a n h y d r a t e d M g - r i c h / S i - p o o r serpentine.
Experimental The samples of BAL (No. 3187, S. Vittore mine, Balangero, Italy) and of CST (No. 128/a, Grange Peyro at Sampeyre, Valle Varaita, Italy) used for this work were preliminarly analysed for chemical composition (Table 1) and purity. Optical and electron microscopy (SEM/EDS) and X-ray powder diffraction showed that foreign phases were neglible. The following conditions were used for electron probe analyses. BAL: SEM Cambridge S-360 equipped with EDS 860-500 Link System, 15 KV, 800 pA; standards, S i O 2 (Si), Fe203 (Fe), Mn (Mn), MgO (Mg); data reduction with ZAF4 (Duncumb and Reed 1968). CST: WDS ARL SEMQ, 15KV, 15 nA; standards, omphacite (Si, Na, Mg, A1), kaersutite (Ti, K, Fe), rhodonite (Mn), diopside (Ca), chromite (Cr); data reduction with MAGIC IV (Colby 1968). The content of octahedral cations in the orthorhombic sub-cell of BAL is (Mga.93Fe4.12Mno.58)z~3.63 (Table 1). The ferric iron has not been analysed in this sample which, however, has a composition very close to that of the sample reported by Compagnoni et al. 1983 where Fe2+/Fe 3+ =2.1. The content of octahedral cations in the unit cell of CST is (Mg37.goFel.86Til.76Mno.sgCro.osCao.os)z41.72 (Table 1). In CST the ferric iron has never been analysed. 57Fe M6ssbauer spectra were measured at 295 K using a 20 mCi nominal 57Co source in Rh matrix under normal and magic angle
54~ ' (Gol'danskii 1964) geometry. Since the two sets of data show no substantial differences, at 80 K only spectra under normal geometry were collected. The spectra were recorded over 1024 channels of a computer controlled constant-acceleration spectrometer in the velocity range _+6 mm/s (calibrated with Fee). The unfolded spectra were fitted by a Zz minimization procedure with Lorentzian lineshapes of equal full line width at half height (F) (Table 2; isomer shift is relative to Fec0. Low- and room-temperature spectra show very close features (Table 2; Figs. 3 and 4) and differences can be explained by secondorder Doppler Effect (isomer shift, kS) and different deformation of coordination polyhedra (quadrupole splitting, QS). In particular, the relative areas of the peaks are fairly constants at different temperatures and geometries. For both minerals, the line intensities within each quadrupole doublet were initially allowed to vary in order to account for asymmetry between the high and low-velocity peaks. The ratio of the intensities between these two peaks was finally fixed in CST, where the counting statistics is poorer than in BAL. Under magic angle geometry, asymmetry is still present and just decreases slightly in CST where the effect is larger (Table 2). Therefore, this effect is only in part connected with texture in the absorbers and other
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F i g . 4. M 6 s s b a u e r spectrum and residuals of fit for carlosturanite at 80 K (a) and at 2 9 5 K f o r n o r m a l (b) a n d m a g i c - a n g l e g e o m e t r y (e). T h e t w o f e r r o u s a n d t h e t w o f e r r i c d o u b l e t s u s e d f o r t h e fit a r e s h o w n
226 Table 2. Parameters derived from the fit of the M6ssbauer spectra of balangeroite (BAL) and carlosturanite (CST). Isomer shifts (IS), quadrupole splittings (QS) and full line widths (F) are in mm/s. The % area (A) of the doublets and the ratio (R) between the areas of the two members of each doublet are shown. IS is referred to Fecc For each doublet (D) data at 80 K, 295 K (normal geometry) and 295 K (magic angle) are reported on the 1st, 2nd and 3rd row, respectively
Fe 2+
BAL D1
D2
CST D1
D2
Fe 3+
A
R
IS
Qs
A
R
IS
QS
F
69.9 69.9 69.9 10.3 10.3 10.3
1.10 1.10 1.10 1.10 1.10 1.10
1,38 1,12 1.12 1.27 1.02 1.05
2.65 2.31 2.34 2.98 2.77 2.76
9.3 9.3 9.3 10.5 10.5 10.5
1.10 1.10 1.10 1.10 1.10 1.10
0.54 0.56 0.56 0.70 0.62 0.65
0.68 0.49 0.49 1.00 1.06 1.02
0.48 0.44 0.44 0.48 0.44 0.45
34.2 34.2 34.2 28.3 28.3 28.3
1.40 1.40 1.33 2.22 1.40 1.33
1.26 1.12 1.12 1.21 1.00 1.03
2.83 2.64 2.70 2.20 2.03 2.03
19.1 19.1 19.1 18.4 18.4 18.4
1.40 1.40 1.33 1.18 1.40 1.33
0.59 0.46 0.54 0.64 0.58 0.59
0.84 0.78 0.81 0.35 0.36 0.40
0.44 0.52 0.48 0.44 0.52 0.48
causes must be present, as recently summarized by Geiger et al. (1992), like overlap of doublets and the Gol'danskii-Karyagineffect (anisotropic recoil of nuclei). Besides, since the number of doublets used in the fit is by far smaller than the number of independent octahedral sites, next-nearest effects can be expected due to the chemical complexity of BAL and CST.
BaIangeroite After attempts of fitting the spectrum of BAL with three doublets (one for Fe 2+ and two for Fe3+), a second doublet for ferrous iron with large QS was introduced to account for the outer tail regions (Fig. 3). This fourth doublet, which accounts for only i/8 of total Fe 2 +, does not affect substantially the hyperfine parameters of the other doublets. Most of iron is ferrous (~80%); 20% of ferric iron is accounted for by two doublets showing very close relative areas (Table 2). The hyperfine parameters for the two Fe 2 + doublets (Table 2) are typical for octahedral coordination. The ferric IS values, instead, are unusually large for octahedral coordination, but similar values have been reported, e.g. in epidote and amphiboles (Mitra 1992).
Carlosturanite An inspection of the data, particularly at 80 K, shows that at least four (partially) resolved doublets contribute to the spectrum of CST. In fact, a model with two ferrous and two ferric ions was needed to obtain a satisfactory fit (Fig. 4). The absolute intensities of the peaks are much lower than those of BAL, due to the lower overall iron content in this mineral where, however, the percent amount of Fe 3+ (~38% of Fete0 is much higher than in BAL. Both for Fe 2+ and Fe 3+ the two relevant doublets have similar IS's, while the QS's are markedly different (Table 2). All hyperfine parameters show octahedral coordination, even if also for CST the ferric IS values are higher than usual.
Discussion T h e values o f the line w i d t h s (0.44 _< F_< 0.52 r a m / s , Table 2) o b t a i n e d f o r B A L a n d C S T are ~ 2 5 % l a r g e r t h a n the value o b t a i n e d for a F e e a b s o r b e r 12 ~tm t h i c k ( F =
0.36 m m / s ) , o n the s a m e e x p e r i m e n t a l a p p a r a t u s , a n d t h a n the value e x p e c t e d for single c r y s t a l l o g r a p h i c sites [ ~ 0.35 m m / s as m a x i m u m v a l u e ( M i t r a 1992)]. It seems, therefore, p r u d e n t to c o n c l u d e t h a t the d o u b l e t s used c a n i n c l u d e u n r e s o l v e d c o n t r i b u t i o n s f r o m different ind e p e n d e n t o c t a h e d r a a n d / o r n e x t - n e a r e s t - n e i g h b o u r effects in view o f the p s e u d o - s y m m e t r y a n d the c o m p l e x isomorphous substitutions.
Balangeroite I n the m o n o c l i n i c cell o f B A L 84 o c t a h e d r a are d i s t r i b u t e d a m o n g 24 i n d e p e n d e n t sites (18 in general p o s i t i o n w i t h m u l t i p l i c i t y 4, a n d 6 o n t w o f o l d axes w i t h m u l t i p l i c ity 2). H o w e v e r , as m e n t i o n e d above, b e c a u s e o f p s e u d o s y m m e t r y a n o r t h o r h o m b i c sub-cell can be used. This cell has a v o l u m e 6 times s m a l l e r a n d c o n t a i n s o n l y 14 o c t a h e d r a d i s t r i b u t e d a m o n g 4 i n d e p e n d e n t sites: one [M(1)] o n a centre o f s y m m e t r y with m u l t i p l i c i t y 2, a n d three [M(2), M(3), M(4)] on m i r r o r p l a n e s w i t h m u l t i plicity 4. Because o f l a c k o f suitable crystals, the s t r u c t u r a l m o d e l o f B A L is b a s e d on e l e c t r o n d i f f r a c t i o n d a t a a n d o n c o m p a r i s o n w i t h the i s o s t r u c t u r a l m i n e r a l gageite, w h e r e the o c t a h e d r a l c a t i o n s in the o r t h o r h o m b i c cell are (Mn9.65Mg3.77Zno.71Cao.o5Feo.04)s14.22. It has been s h o w n ( F e r r a r i s et al. 1987) t h a t in gageite M n fully occupies t w o o u t o f three i n d e p e n d e n t o c t a h e d r a w i t h m i r r o r s y m m e t r y ; the t h i r d o c t a h e d r o n o n a m i r r o r p l a n e a n d t h a t o n a centre o f s y m m e t r y a r e a b o u t 4 0 % filled with M n (the rest is Mg). O n c r y s t a l l o c h e m i c a l g r o u n d , m o s t o f the role p l a y e d b y M n in gageite c o u l d be p l a y e d b y Fe in B A L . O n this basis, the n u m b e r o f d o u b l e t s w h i c h are n e c e s s a r y to fit the M 6 s s b a u e r p a t t e r n (Fig. 3) c o u l d s h o w t h a t Fe in B A L (as M n in gageite) is m a i n l y p r e s e n t in two families o f o c t a h e d r a . Since in the real m o n o c l i n i c cell e a c h " i n d e p e n d e n t " o c t a h e d r o n o f the o r t h o r h o m b i c sub-cell is s p l i t t e d in six i n d e p e n d e n t , b u t p r e s u m a b l y v e r y similar, o c t a h e d r a , a s u b s t a n t i a l b r o a d ening o f the line arises.
227 The large differences between the QS values (Table 2) show that octahedra are differently distorted, in agreement with the structural results reported for gageite. According to the M6ssbauer data, Fe 3 + represents 20% of Fetot, a value lower than that (32%) reported by C o m p a g n o n i et al. 1983 for their sample. A reason for the difference can be the high uncertainty level arising from the presence of unresolved lines (Dollase 1975).
Carlosturanite C o m p a g n o n i et al. 1987 made the hypothesis that in CST the cations other than M g occupy only inter-strip octahedra (Fig. 2). The six dotted octahedra shown in Fig. 2 are less constrained by the silicate strips and are likely to be allowed enough expansion to receive Fe 2+, M n 2 +, and some vacancies which are needed to compensate higher charge cations like Ti 4+, Fe ~ + and, maybe, M n 3 +. A m o n g the six dotted octahedra, four (light) belong to a general position and two (dark) belong to a special position with mirror symmetry; altogether they account for 6 of the 42 octahedra which occur in the unit cell. The total n u m b e r is distributed a m o n g 14 independent octahedral sites: 7 in general positions (multiplicity 4) and 7 on mirror planes (multiplicity 2). As already discussed, the values of F suggest that each doublet used to fit the M6ssbauer spectrum of CST receives multiple contributions. However, it is worthy to stress that the n u m b e r of doublets (two each for Fe 2 + and Fe 3+) is in agreement with the hypothesis that Fe is concentrated within the dotted octahedra (Fig. 2) which belong to two independent sets only. The QS values (Table 2) indicate different distortions for the octahedral sites, in agreement with the different environment of the two independent sets of dotted octahedra (Fig. 2). In spite of the close chemical and structural relationship between CST and chrysotile (Mellini et al. 1985), the M6ssbauer spectrum of CST markedly differs from those reported for chrysotile (Blaauw et al. 1979) which have been fitted with one Fe 2 § and two F e 3 + doublets (one for tetrahedral Fe 3 +).
Conclusions There is an increasing evidence that the medical hazards of asbestos are connected not only with m o r p h o l o g y but also with chemistry; in particular, iron can be involved in the production of active oxygen species (Pezerat et al. 1989). The M6ssbauer spectra of BAL and CST show that in these asbestiform silicates both ferric and ferrous iron is present. While for BAL that was already known from chemical analyses, the presence o f Fe 3+ in CST was not yet proved. In fact, the ferric iron de-
tected in CST by E P R measurements (Astolfi et al. 1991) could be just a product of surface oxidation following grinding and/or immersion in a physiological solution. It can also be remarked that BAL and CST have M6ssbauer spectra clearly different from that of chrysotile, with which these minerals have been mistaken since ever, because of the strictly similar morphology.
Acknowledgments. Research included in the program of CS per la Geodinamica delle Catene Collisionali (C.N.R., Torino) and supported by M.U.R.S.T., C.N.R. and I.N.F.M. grants. S. Hafner kindly discussed preliminary results. References Astolfi A, Fubini B, Giamello E, Volante M, Belluso E, Ferraris G (1991) Asbestiform minerals associated with chrysotile from the western Alps (Piedmont - Italy): Chemical characteristics and possible related toxicity. In: Brown RC et al. (ed) Mechanisms in Fibre Carcinogenesis. Plenum Press, New York, pp 269-283 Belluso E, Ferraris G (1991) New data on balangeroite and carlosturanite from alpine serpentinites. Eur J Mineral 3: 55%566 Blaauw C, Stroink G, Leiper W (1979) M6ssbauer analysis of some canadian chrysotiles. Canad Mineral 17: 713-717 Colby JW (1968) Quantitative microprobe analysis of thin insulating film. In: Newkirk JB, Mallet GR, Pleiffer HG (eds). Plenum Press, New York, vol 11 : pp 287-305 Compagnoni R, Ferraris G, Fiora L (1983) Balangeroite, a new fibrous silicate related to gageite from Balangero, Italy. Amer Mineral 68:214-219 Compagnoni R, Ferraris G, Mellini M (1985) Carlosturanite, a new asbestiform rock-forming silicate from Val Varaita, Italy. Amer Mineral 70: 767-772 Dollase WA (1975) Statistical limitations of M6ssbauer spectral fitting. Amer Mineral 60 : 257-264 Duncumb P, Reed SJB (1968) The calculation of stopping power and backscatter effects in electron probe microanalysis. In: Heinrich KFJ (ed) Quantitative electron probe microanalysis. NBS spec. pub. 298:133-154 Ferraris G, Mellini M, Merlino S (1987) Electron-diffraction study of balangeroite and gageite: Crystal structures, polytypism, and fiber texture. Amer Mineral 72:382-391 Geiger CA, Armbruster Th, Lager GA, Jiang K, Lottermoser W, Amthauer G (1992) A combined temperature dependent 57Fe M6ssbauer and single crystal X-ray diffraction study of synthetic almandine: Evidence for the Gol'danskii-Karyagin effect. Phys Chem Minerals 19:121-126 Gol'danskii VI (1964) The M6ssbauer effect and its applications in chemistry. Van Nostrand, New York Guthrie GD Jr (1992) Biological effects of inhaled minerals. Amer Mineral 77 : 225-243 Mellini M, Eerraris G, Compagnoni R (1985) Carlosturanite: HRTEM evidence of a polysomatic series including serpentine. Amer Mineral 70: 773-781 Mitra S (1992) Applied M6ssbauer Spectroscopy. Pergamon Press, Oxford Pezerat M, Zalma R, Guignard J, Jaurand MC (1989) Production of oxygen radicals by the reduction of oxygen arising from the surface activity of mineral fibres. IARC Sci Publications 90:100-111 Skinner HCW, Ross M, Frondel C (1988) Asbestos and other fibrous materials. Oxford University Press, Oxford
