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Mössbauer study of vacancies in natural pyrrhotite
Journal of Alloys and Compounds 289 (1999) 36–41
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¨ Mossbauer study of vacancies in natural pyrrhotite J.W.A. Kondoro Department of Physics, University of Dar-Es-Salaam, P.O. Box 35063, Dar-Es-Salaam, Tanzania Received 2 March 1998; received in revised form 3 January 1999
Abstract ¨ Mossbauer spectra corresponding to natural hexagonal and monoclinic pyrrhotite from Lake Victoria Goldfield in Tanzania (East Africa) were recorded between room temperature and 4.2 K. There is good agreement between experiment and the literature. The ¨ Mossbauer spectra of samples of monoclinic pyrrhotite are relatively narrow, but those corresponding to hexagonal pyrrhotite are very broad and asymmetric due to the vacancy distribution. The intensity ratios of the inequivalent Fe sites in monoclinic pyrrhotite are in the ratio of about 2:1:2:2 in the order of increasing hyperfine field, but those corresponding to hexagonal pyrrhotite do not have any particular pattern due to varying at.% Fe in the analysed material which is, in turn, related to the vacancy distribution. 1999 Elsevier Science S.A. All rights reserved. ¨ spectroscopy; Sulfides; Iron sulfides Keywords: Mossbauer
1. Introduction Pyrrhotites are phases of iron sulphides characterised by varied composition (46.5–50 at.% Fe) and a NiAs-type structure which can be modified by vacancy distribution depending on the at.% Fe [1,2]. At room temperature, stable phases of natural pyrrhotite are troilite (FeS), hexagonal pyrrhotite Fe 12x S and monoclinic pyrrhotite [2]. The room temperature troilite is antiferromagnetic and has a distorted NiAs structure, 2C superstructure. Monoclinic Fe 7 S 8 is ferrimagnetic and has a 4C superstructure of NiAs type which is derived from FeS by subtraction of one Fe per eight (FeS) units. The resultant structure contains layers of Fe sites separated from layers of Fe sites with vacancies by sulphur atoms, thereby lowering the symmetry of the system from hexagonal to monoclinic [3]. Hexagonal pyrrhotites are iron sulphides with 47.37–47.83 at.% Fe, which is the region of at.% Fe between troilite and monoclinic pyrrhotite. These pyrrhotites are either stoichiometric phases Fe 9 S 10 , Fe 10 S 11 , Fe 11 S 12 or a mixture of the stoichiometric phases with troilite or with monoclinic pyrrhotite [2,4,5]. It has been observed that the presence of vacancies or vacancy layers in the Fe–S system dictates the structural and magnetic properties of the various phases [6,7]. For example, ferrimagnetism is attributed to ordered vacancies and anti-ferromagnetism is related to disordered vacancies [1,5,8]. Besides the magnetic properties, which are linked
to vacancy distribution through structural properties of the ¨ sulphides, there are also variations of Mossbauer parameters which seem to be associated with structural properties. ¨ Mossbauer studies of synthetic Fe 12x S (0.004 # x # 0.143) show slight linewidth broadening for 0.004 # x # 0.042 which increases significantly for 0.054 # x # 0.079 before decreasing to normal at 0.106 # x # 0.143 [9]. Linewidth ¨ broadening of Mossbauer spectra corresponding to hexagonal pyrrhotite have been observed in several other studies of pyrrhotite (e.g., Refs. [5,8,9]), although not much attention has been given to this effect. ¨ The broadened Mossbauer peaks in synthetic Fe 12x S (0.054 # x # 0.079) and some natural iron sulphides [10] could be a result of vacancy distribution or a mixture of ¨ phases or both. Mossbauer spectra of some samples of natural iron sulphides taken from cores drilled in various areas of Lake Victoria Gold field in Tanzania show linewidth broadening. This study presents an analysis of ¨ linewidth broadening of the Mossbauer spectra.
2. Experimental Rock samples were taken from cores drilled in various areas of Lake Victoria Goldfield in Tanzania, East Africa. The samples were ground in an agate mortar into very fine ¨ powder which was later analysed using Mossbauer spectroscopy. The spectra of powdered samples were collected
0925-8388 / 99 / $ – see front matter 1999 Elsevier Science S.A. All rights reserved. PII: S0925-8388( 99 )00170-X
J.W. A. Kondoro / Journal of Alloys and Compounds 289 (1999) 36 – 41
with a conventional constant acceleration spectrometer with a triangular reference signal using 15 mCi 57 Co in a ¨ rhodium matrix as a g-radiation source. Mossbauer measurements were conducted between room temperature and 4.2 K.
3. Results and discussion ¨ The six-line pattern of the Mossbauer spectra shown in Fig. 1 corresponds to a typical monoclinic pyrrhotite sample from Lake Victoria Goldfield recorded at various temperatures as indicated. The spectra recorded between room temperature and 77 K were fitted with four sextets
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and those recorded at 4.2 K were fitted with five sextets as explained in detail elsewhere [11]. Several room temperature studies of single crystals of Fe 7 S 8 have shown that monoclinic pyrrhotite has four inequivalent Fe sites [10– 12]. Two Fe sites, Fe-1 and Fe-3, occur together in the same layers and the other two Fe sites, Fe-2 and Fe-4, occur together along with vacancies. The room temperature hyperfine fields corresponding to the four Fe sites are 30.0, 29.5, 25.3 and 22.8 T in the intensity ratio 2:1:2:2 for Fe-1, Fe-2, Fe-3 and Fe-4, respectively [11]. The hyperfine parameters of monoclinic pyrrhotite found in this study, shown in Table 1, are in good agreement with literature values [11,13]. The doublets in the room ¨ temperature Mossbauer spectra shown in Fig. 1 have
¨ Fig. 1. Mossbauer spectrum of monoclinic pyrrhotite at various temperatures.
J.W. A. Kondoro / Journal of Alloys and Compounds 289 (1999) 36 – 41
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Table 1 ¨ Mossbauer parameters of monoclinic pyrrhotite samples at various temperatures T (K)
HF (T)
IS (mm / s)
30.37 29.11 25.05 22.56
0.96 0.24 0.58 0.57 0.56 0.56
0 55.24 51.81 33.83 31.41 26.72 26.07 20.86
0.16 0.69 0.05 0.61 0.62 0.72 0.82 0.60
RT
4.2 K
QS (mm / s)
FWHM (mm / s)
Intensity (%)
Phase
2.78 0.64 0.02 0.08 0.04 0.05
0.18 0.12 0.33 0.37 0.44 0.44
11.51 15.08 23.44 9.68 25.53 24.13
Ferrihydrite
0.67 20.09 0.05 20.04 0.13 20.05 0.03 0.12
0.07 0.18 0.17 0.35 0.32 0.30 0.29 0.31
3.37 8.33 9.18 24.83 11.41 18.22 11.16 15.90
hyperfine parameters similar to chlorite [14,15]. However, as the temperature is lowered to 4.2 K the doublets disappear while the outer sextets which are weak at room temperature appear well developed at 4.2 K. Since the ¨ Mossbauer parameters of the outer sextets correspond to ferrihydrite, the doublets are likely to be superparamagnetic ferrihydrite. ¨ Typical Mossbauer spectra of a sample containing mostly hexagonal pyrrhotite recorded at various temperatures are shown in Fig. 2 and the corresponding parameters are given in Table 2. The linewidths are broader than those corresponding to monoclinic pyrrhotite and the peaks are rather asymmetric such that peak 6.peak 1, peak 2.peak 5 and peak 3.peak 4, where the lines are numbered from 1 to 6 in the order of increasing velocity. The full-width-athalf-maximum (FWHM) of the outer peaks of the ¨ Mossbauer spectra of hexagonal pyrrhotite are up to about 0.6 mm / s as shown in Table 2, similar to values obtained for metal–metalloid glasses (e.g., Ref. [16]). ¨ The broadening of the linewidths in the Mossbauer spectra of hexagonal pyrrhotite could be due to the distribution of vacancies in the vacancy layers thereby changing the short range order around the different Fe sites. The random distribution of vacancies produces several inequivalent Fe sites which in turn produce a ¨ structureless Mossbauer spectrum as shown in Fig. 2. Due ¨ to the structureless form of the resulting Mossbauer spectra, almost any number of sextets could be used to fit the spectra as already observed elsewhere [9]. A slight decrease in the at.% Fe in FeS, for example in the case of Fe 12x S (0.004 # x # 0.042), leads to a slight linewidth broadening and when the at.% Fe decreases to midway between FeS and Fe 7 S 8 , the Fe 12x S (0.054 # x # 0.079) linewidth worsens. However, the situation improves when the at.% Fe approaches Fe 7 S 8 [9]. This could imply that the reduction in the at.% Fe leads to vacancies which are randomly distributed around Fe sites. When the at.% Fe in
Monoclinic pyrrhotite
Ferrihydrite Monoclinic pyrrhotite
iron sulphide is close either to FeS or Fe 7 S 8 the chaos in the system could be viewed as a disturbance of either the FeS or Fe 7 S 8 structural arrangement.When the at.% Fe in iron sulphide is midway between FeS and Fe 7 S 8 , then the chaos is at its peak because the structural arrangement resembles neither FeS nor Fe 7 S 8 . Whereas the intensity of the Fe sites in monoclinic pyrrhotite are almost in the ratio 2:1:2:2 in the order of decreasing hyperfine field, Fe sites in hexagonal pyrrhotite do not show any particular pattern as already reported in previous studies [10]; see Table 3. Since the intensity ratio of 2:1:2:2 for the inequivalent Fe sites in Fe 7 S 8 is likely to vary when the iron concentration is changed as already observed elsewhere [13], then the difference in the intensity ratios of the inequivalent Fe sites between monoclinic and hexagonal pyrrhotite could be due to the difference in at.% Fe between them. Fe 7 S 8 is a phase of iron sulphide with 46.67 at.% Fe and when the at.% Fe is increased up to 47.37–47.83, then monoclinic changes to hexagonal pyrrhotite [2]. It has been reported that when small amounts of Fe are added to Fe 7 S 8 it enters preferentially site Fe-4 in the sublattice with vacancies [13] which could consequently change the ratio of the different Fe sites in pyrrhotite. This means that the intensities of the four inequivalent Fe sites in hexagonal pyrrhotite would vary according to the at.% Fe bound to S. It is very likely that the hexagonal pyrrhotite found in Lake Victoria Goldfield has varying at.% Fe bound to S which leads to the variation in the intensity ratios of the four inequivalent Fe sites in the analysed hexagonal pyrrhotite. A similar development is observed when S is replaced by Se in Fe 7 S 8 . A study of Fe 7 (S x Se 12x ) 8 shows that the linewidths are almost equally narrow at x 5 0 and x 5 1 but the linewidth almost doubles at x 5 0.5 [17]. In the case of Fe 7 S 8 , S atoms lie between layers containing Fe atoms only and those which have both Fe atoms and
J.W. A. Kondoro / Journal of Alloys and Compounds 289 (1999) 36 – 41
39
leads to a variation in the primitive unit cell axes in a similar pattern. The change of the at.% Fe between FeS and Fe 7 S 8 goes hand in hand with a change of the at.% S; therefore, linewidth broadening could be due to vacancy distribution in Fe layers as well as the arrangement of S atoms in the system, all of which affect the short range order of the Fe sites. Studies of the order–disorder transition in pyrrhotite with respect to temperature show a narrowing of linewidth as temperature increases from room temperature [6,7]. The linewidths of hexagonal pyrrhotite are about 0.7 mm / s and asymmetric as shown in Table 2. These phenomena have already been observed in transition metal–metalloid glasses (e.g., Ref. [18]), where they were attributed to correlations between site-to-site variation in isomer shift, hyperfine field and electric field gradient. Although the mode of formation of transition metal– metalloid glasses is different from that of hexagonal pyrrhotite, the random distribution of the inequivalent Fe sites in the material is the same and, consequently, the reflection which is the broadening of the linewidth and the asymmetric effect is the same. An attempt to correlate the atomic ratio Fe / S in iron sulphides FeS x (1.0 # x # 1.15) with the weighted hyperfine field, Bhf , from the literature [5,9,11,19,20] is shown in Fig. 3. The mean values of Bhf corresponding to the fitted inequivalent Fe sites of pyrrhotite have been weighted against the corresponding intensities for the pyrrhotite phase irrespective of the fitting model. The weighted mean hyperfine fields for FeS x (1.0 # x # 1.15) decrease when the at.% Fe decreases, as shown in Fig. 3. Since the correlation between Bhf and FeS x (1.0 # x # 1.15) is linear, it is very likely that the arrangement of S atoms and vacancies in the Fe plane, which influence the short range order of the Fe sites, are distributed randomly.
4. Conclusion
¨ Fig. 2. Mossbauer spectra of hexagonal pyrrhotite at various temperatures.
vacancies; therefore, the substitution of S for Se does not affect the arrangement of layers containing Fe atoms. However, the short range order around Fe atoms is affected because of the difference in radii between S and Se. Consequently, the variation of composition x in both Fe 12x S (0.004 # x # 0.143) [9] and Fe 7 (S x Se 12x ) 8 [17]
¨ The peaks of the Mossbauer spectra for hexagonal pyrrhotite are broader than those of monoclinic pyrrhotite. ¨ Besides, the Mossbauer spectra of hexagonal pyrrhotite are asymmetric such that peak 6.peak 1, peak 2.peak 5 and peak 3.peak 4, where the lines are numbered from 1 to 6 in order of increasing energy. The intensity of the Fe sites in monoclinic pyrrhotite are almost in the ratio 2:1:2:2 in the order of decreasing hyperfine field but those corresponding to hexagonal pyrrhotite do not show any particular pattern. The variation of the at.% Fe in iron sulphide as the structural system changes from FeS to Fe 7 S 8 varies the short range order of the Fe sites in the plane of Fe sites with vacancies and consequently affects the intensity ratios of the inequivalent Fe sites. ¨ The broadening of the linewidth in the Mossbauer spectra corresponding to hexagonal pyrrhotite could be related to the disturbance of the short range order around
J.W. A. Kondoro / Journal of Alloys and Compounds 289 (1999) 36 – 41
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Table 2 ¨ Mossbauer parameters of hexagonal pyrrhotite samples at various temperatures Temp.
HF (T)
IS (mm / s)
QS (mm / s)
FWHM (mm / s)
Intensity (%)
Phase
RT
0 0 27.51 25.07 23.23 21.03
1.10 1.02 0.68 0.70 0.69 0.68
2.72 1.55 0.03 0.02 0.03 0.02
0.30 0.20 0.42 0.42 0.47 0.19
11.41 12.67 22.96 18.82 16.19 11.37
Chlorite
0.0 0.0 32.02 29.26 27.17 25.15
1.22 0.96 0.81 0.81 0.80 0.79
2.95 1.63 0.02 0.02 0.02 0.03
0.28 0.18 0.41 0.51 0.44 0.54
10.25 10.42 31.47 17.64 15.25 14.96
Chlorite
0 0 33.01 30.44 28.18 25.63
1.22 0.66 0.74 0.72 0.71 0.74
3.17 1.83 0.01 0.02 0.02 0.01
0.16 0.16 0.35 0.46 0.46 0.47
8.89 9.46 32.05 16.49 17.85 15.25
Chlorite
77 K
4.2 K
Hexagonal pyrrhotite
Hexagonal pyrrhotite
Hexagonal pyrrhotite
Table 3 ¨ Mossbauer parameters of some hexagonal pyrrhotite samples Sample No. and location
HF (T)
QS (mm / s)
IS (mm / s)
I (%)
G (mm / s)
18 Old-mine (GEITA)
30.0 28.2 26.4 24.6
0.10 0.09 0.06 0.16
0.69 0.69 0.69 0.69
25 16 24 19
0.35 0.35 0.35 0.35
15 Mwamela I (NZEGA)
30.0 28.6 26.8 25.0
0.11 0.10 0.04 0.19
0.59 0.59 0.59 0.59
13 11 24 26
0.33 0.30 0.36 0.48
14 Mwamela II (NZEGA)
30.2 27.7 25.9 23.8
0.12 0.06 0.12 0.19
0.58 0.58 0.58 0.58
29 28 27 13
0.33 0.33 0.33 0.33
37 Wella (NZEGA)
30.0 27.4 25.8 23.8
0.11 0.05 0.14 0.23
0.69 0.69 0.69 0.69
34 26 24 16
0.30 0.30 0.30 0.30
Fe sites due to vacancies and the arrangement of S atoms in the pyrrhotite system. The observed asymmetric effect ¨ in the Mossbauer spectra of hexagonal pyrrhotite could be due to a correlation between the isomer shift and the hyperfine field.
Acknowledgements The author greatly appreciates the financial assistance of the DAAD.
Fig. 3. Variation of weighted hyperfine field, Bhf , with atomic ratio Fe / S in FeS x (1.0 # x # 1.15).
References [1] V.P. Gupta, A.K. Singh, K. Chandra, S.K. Jaireth, Phys. Status Solidi A 81 (1984) 281. [2] L.F. Power, H.A. Fine, Miner. Sci. Eng. 8 (1976) 107. [3] L.M. Levinson, D. Treves, J. Phys. Chem. Solids 29 (1968) 2227. [4] N. Morimoto, A. Gyobu, H. Mukaiyama, E. Izawa, Econ. Geol. 70 (1975) 824. [5] N.S. Ovanesyan, G. Trukhtanov, Yu.G. Odinets, G.V. Novikov, Sov. Phys. J. Exp. Theor. Phys. 33 (1971) 1193. [6] F. Li, H.F. Franzen, J. Solid State Chem. 124 (1996) 264. [7] F. Li, H.F. Franzen, J. Solid State Chem. 126 (1996) 108. [8] L.A. Marusak, L.N. Mulay, J. Appl. Phys. 50 (1979) 1865. [9] O. Kruse, Am. Miner. 75 (1990) 755.
J.W. A. Kondoro / Journal of Alloys and Compounds 289 (1999) 36 – 41 [10] J.W.A. Kondoro, C.A. Kiwanga, Appl. Radiat. Isot. 48 (1997) 555. [11] C. Jeandey, J.L. Oddou, J.L. Mattei, G. Fillion, Solid State Commun. 78 (1991) 195. [12] M. Tokonami, K. Nishiguchi, N. Morimoto, Am. Miner. 57 (1972) 1066. [13] J.R. Gosselin, M.G. Townsend, R.J. Tremblay, A.H. Webster, Mater. Sci. Bull. 10 (1975) 41. ¨ [14] J.M.D. Coey, in: G.J. Long (Ed.), Mossbauer Spectroscopy Applied to Inorganic Chemistry, Vol. I, Plenum Press, New York, 1984, p. 444. [15] J.G. Stevens, H. Pollak, L. Zhe, V.G. Stevens, R.M. White, J.L.
[16] [17] [18] [19] [20]
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¨ ¨ Gibson (Eds.), Mossbauer Mineral Data, Mossbauer Effect Data Centre, University of North Carolina, 1983. J.W. Kondoro, S.J. Campbell, U. Gonser, Phys. Status Solidi (a) 125 (1991) 67. T. Ericsson, O. Amcoff, P. Nordblad, Hyperfine Interactions 90 (1994) 515. M. Eibschutz, M.E. Lines, H.S. Chen, J. Non-Cryst. Solids 62 (1984) 1219. K. Ono, A. Ito, J. Phys. Soc. Jpn. 17 (1962). W. Keune, A.S. Camara, Phys. Status Solidi 27a (1975) 181.
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¨ Mossbauer study of vacancies in natural pyrrhotite J.W.A. Kondoro Department of Physics, University of Dar-Es-Salaam, P.O. Box 35063, Dar-Es-Salaam, Tanzania Received 2 March 1998; received in revised form 3 January 1999
Abstract ¨ Mossbauer spectra corresponding to natural hexagonal and monoclinic pyrrhotite from Lake Victoria Goldfield in Tanzania (East Africa) were recorded between room temperature and 4.2 K. There is good agreement between experiment and the literature. The ¨ Mossbauer spectra of samples of monoclinic pyrrhotite are relatively narrow, but those corresponding to hexagonal pyrrhotite are very broad and asymmetric due to the vacancy distribution. The intensity ratios of the inequivalent Fe sites in monoclinic pyrrhotite are in the ratio of about 2:1:2:2 in the order of increasing hyperfine field, but those corresponding to hexagonal pyrrhotite do not have any particular pattern due to varying at.% Fe in the analysed material which is, in turn, related to the vacancy distribution. 1999 Elsevier Science S.A. All rights reserved. ¨ spectroscopy; Sulfides; Iron sulfides Keywords: Mossbauer
1. Introduction Pyrrhotites are phases of iron sulphides characterised by varied composition (46.5–50 at.% Fe) and a NiAs-type structure which can be modified by vacancy distribution depending on the at.% Fe [1,2]. At room temperature, stable phases of natural pyrrhotite are troilite (FeS), hexagonal pyrrhotite Fe 12x S and monoclinic pyrrhotite [2]. The room temperature troilite is antiferromagnetic and has a distorted NiAs structure, 2C superstructure. Monoclinic Fe 7 S 8 is ferrimagnetic and has a 4C superstructure of NiAs type which is derived from FeS by subtraction of one Fe per eight (FeS) units. The resultant structure contains layers of Fe sites separated from layers of Fe sites with vacancies by sulphur atoms, thereby lowering the symmetry of the system from hexagonal to monoclinic [3]. Hexagonal pyrrhotites are iron sulphides with 47.37–47.83 at.% Fe, which is the region of at.% Fe between troilite and monoclinic pyrrhotite. These pyrrhotites are either stoichiometric phases Fe 9 S 10 , Fe 10 S 11 , Fe 11 S 12 or a mixture of the stoichiometric phases with troilite or with monoclinic pyrrhotite [2,4,5]. It has been observed that the presence of vacancies or vacancy layers in the Fe–S system dictates the structural and magnetic properties of the various phases [6,7]. For example, ferrimagnetism is attributed to ordered vacancies and anti-ferromagnetism is related to disordered vacancies [1,5,8]. Besides the magnetic properties, which are linked
to vacancy distribution through structural properties of the ¨ sulphides, there are also variations of Mossbauer parameters which seem to be associated with structural properties. ¨ Mossbauer studies of synthetic Fe 12x S (0.004 # x # 0.143) show slight linewidth broadening for 0.004 # x # 0.042 which increases significantly for 0.054 # x # 0.079 before decreasing to normal at 0.106 # x # 0.143 [9]. Linewidth ¨ broadening of Mossbauer spectra corresponding to hexagonal pyrrhotite have been observed in several other studies of pyrrhotite (e.g., Refs. [5,8,9]), although not much attention has been given to this effect. ¨ The broadened Mossbauer peaks in synthetic Fe 12x S (0.054 # x # 0.079) and some natural iron sulphides [10] could be a result of vacancy distribution or a mixture of ¨ phases or both. Mossbauer spectra of some samples of natural iron sulphides taken from cores drilled in various areas of Lake Victoria Gold field in Tanzania show linewidth broadening. This study presents an analysis of ¨ linewidth broadening of the Mossbauer spectra.
2. Experimental Rock samples were taken from cores drilled in various areas of Lake Victoria Goldfield in Tanzania, East Africa. The samples were ground in an agate mortar into very fine ¨ powder which was later analysed using Mossbauer spectroscopy. The spectra of powdered samples were collected
0925-8388 / 99 / $ – see front matter 1999 Elsevier Science S.A. All rights reserved. PII: S0925-8388( 99 )00170-X
J.W. A. Kondoro / Journal of Alloys and Compounds 289 (1999) 36 – 41
with a conventional constant acceleration spectrometer with a triangular reference signal using 15 mCi 57 Co in a ¨ rhodium matrix as a g-radiation source. Mossbauer measurements were conducted between room temperature and 4.2 K.
3. Results and discussion ¨ The six-line pattern of the Mossbauer spectra shown in Fig. 1 corresponds to a typical monoclinic pyrrhotite sample from Lake Victoria Goldfield recorded at various temperatures as indicated. The spectra recorded between room temperature and 77 K were fitted with four sextets
37
and those recorded at 4.2 K were fitted with five sextets as explained in detail elsewhere [11]. Several room temperature studies of single crystals of Fe 7 S 8 have shown that monoclinic pyrrhotite has four inequivalent Fe sites [10– 12]. Two Fe sites, Fe-1 and Fe-3, occur together in the same layers and the other two Fe sites, Fe-2 and Fe-4, occur together along with vacancies. The room temperature hyperfine fields corresponding to the four Fe sites are 30.0, 29.5, 25.3 and 22.8 T in the intensity ratio 2:1:2:2 for Fe-1, Fe-2, Fe-3 and Fe-4, respectively [11]. The hyperfine parameters of monoclinic pyrrhotite found in this study, shown in Table 1, are in good agreement with literature values [11,13]. The doublets in the room ¨ temperature Mossbauer spectra shown in Fig. 1 have
¨ Fig. 1. Mossbauer spectrum of monoclinic pyrrhotite at various temperatures.
J.W. A. Kondoro / Journal of Alloys and Compounds 289 (1999) 36 – 41
38
Table 1 ¨ Mossbauer parameters of monoclinic pyrrhotite samples at various temperatures T (K)
HF (T)
IS (mm / s)
30.37 29.11 25.05 22.56
0.96 0.24 0.58 0.57 0.56 0.56
0 55.24 51.81 33.83 31.41 26.72 26.07 20.86
0.16 0.69 0.05 0.61 0.62 0.72 0.82 0.60
RT
4.2 K
QS (mm / s)
FWHM (mm / s)
Intensity (%)
Phase
2.78 0.64 0.02 0.08 0.04 0.05
0.18 0.12 0.33 0.37 0.44 0.44
11.51 15.08 23.44 9.68 25.53 24.13
Ferrihydrite
0.67 20.09 0.05 20.04 0.13 20.05 0.03 0.12
0.07 0.18 0.17 0.35 0.32 0.30 0.29 0.31
3.37 8.33 9.18 24.83 11.41 18.22 11.16 15.90
hyperfine parameters similar to chlorite [14,15]. However, as the temperature is lowered to 4.2 K the doublets disappear while the outer sextets which are weak at room temperature appear well developed at 4.2 K. Since the ¨ Mossbauer parameters of the outer sextets correspond to ferrihydrite, the doublets are likely to be superparamagnetic ferrihydrite. ¨ Typical Mossbauer spectra of a sample containing mostly hexagonal pyrrhotite recorded at various temperatures are shown in Fig. 2 and the corresponding parameters are given in Table 2. The linewidths are broader than those corresponding to monoclinic pyrrhotite and the peaks are rather asymmetric such that peak 6.peak 1, peak 2.peak 5 and peak 3.peak 4, where the lines are numbered from 1 to 6 in the order of increasing velocity. The full-width-athalf-maximum (FWHM) of the outer peaks of the ¨ Mossbauer spectra of hexagonal pyrrhotite are up to about 0.6 mm / s as shown in Table 2, similar to values obtained for metal–metalloid glasses (e.g., Ref. [16]). ¨ The broadening of the linewidths in the Mossbauer spectra of hexagonal pyrrhotite could be due to the distribution of vacancies in the vacancy layers thereby changing the short range order around the different Fe sites. The random distribution of vacancies produces several inequivalent Fe sites which in turn produce a ¨ structureless Mossbauer spectrum as shown in Fig. 2. Due ¨ to the structureless form of the resulting Mossbauer spectra, almost any number of sextets could be used to fit the spectra as already observed elsewhere [9]. A slight decrease in the at.% Fe in FeS, for example in the case of Fe 12x S (0.004 # x # 0.042), leads to a slight linewidth broadening and when the at.% Fe decreases to midway between FeS and Fe 7 S 8 , the Fe 12x S (0.054 # x # 0.079) linewidth worsens. However, the situation improves when the at.% Fe approaches Fe 7 S 8 [9]. This could imply that the reduction in the at.% Fe leads to vacancies which are randomly distributed around Fe sites. When the at.% Fe in
Monoclinic pyrrhotite
Ferrihydrite Monoclinic pyrrhotite
iron sulphide is close either to FeS or Fe 7 S 8 the chaos in the system could be viewed as a disturbance of either the FeS or Fe 7 S 8 structural arrangement.When the at.% Fe in iron sulphide is midway between FeS and Fe 7 S 8 , then the chaos is at its peak because the structural arrangement resembles neither FeS nor Fe 7 S 8 . Whereas the intensity of the Fe sites in monoclinic pyrrhotite are almost in the ratio 2:1:2:2 in the order of decreasing hyperfine field, Fe sites in hexagonal pyrrhotite do not show any particular pattern as already reported in previous studies [10]; see Table 3. Since the intensity ratio of 2:1:2:2 for the inequivalent Fe sites in Fe 7 S 8 is likely to vary when the iron concentration is changed as already observed elsewhere [13], then the difference in the intensity ratios of the inequivalent Fe sites between monoclinic and hexagonal pyrrhotite could be due to the difference in at.% Fe between them. Fe 7 S 8 is a phase of iron sulphide with 46.67 at.% Fe and when the at.% Fe is increased up to 47.37–47.83, then monoclinic changes to hexagonal pyrrhotite [2]. It has been reported that when small amounts of Fe are added to Fe 7 S 8 it enters preferentially site Fe-4 in the sublattice with vacancies [13] which could consequently change the ratio of the different Fe sites in pyrrhotite. This means that the intensities of the four inequivalent Fe sites in hexagonal pyrrhotite would vary according to the at.% Fe bound to S. It is very likely that the hexagonal pyrrhotite found in Lake Victoria Goldfield has varying at.% Fe bound to S which leads to the variation in the intensity ratios of the four inequivalent Fe sites in the analysed hexagonal pyrrhotite. A similar development is observed when S is replaced by Se in Fe 7 S 8 . A study of Fe 7 (S x Se 12x ) 8 shows that the linewidths are almost equally narrow at x 5 0 and x 5 1 but the linewidth almost doubles at x 5 0.5 [17]. In the case of Fe 7 S 8 , S atoms lie between layers containing Fe atoms only and those which have both Fe atoms and
J.W. A. Kondoro / Journal of Alloys and Compounds 289 (1999) 36 – 41
39
leads to a variation in the primitive unit cell axes in a similar pattern. The change of the at.% Fe between FeS and Fe 7 S 8 goes hand in hand with a change of the at.% S; therefore, linewidth broadening could be due to vacancy distribution in Fe layers as well as the arrangement of S atoms in the system, all of which affect the short range order of the Fe sites. Studies of the order–disorder transition in pyrrhotite with respect to temperature show a narrowing of linewidth as temperature increases from room temperature [6,7]. The linewidths of hexagonal pyrrhotite are about 0.7 mm / s and asymmetric as shown in Table 2. These phenomena have already been observed in transition metal–metalloid glasses (e.g., Ref. [18]), where they were attributed to correlations between site-to-site variation in isomer shift, hyperfine field and electric field gradient. Although the mode of formation of transition metal– metalloid glasses is different from that of hexagonal pyrrhotite, the random distribution of the inequivalent Fe sites in the material is the same and, consequently, the reflection which is the broadening of the linewidth and the asymmetric effect is the same. An attempt to correlate the atomic ratio Fe / S in iron sulphides FeS x (1.0 # x # 1.15) with the weighted hyperfine field, Bhf , from the literature [5,9,11,19,20] is shown in Fig. 3. The mean values of Bhf corresponding to the fitted inequivalent Fe sites of pyrrhotite have been weighted against the corresponding intensities for the pyrrhotite phase irrespective of the fitting model. The weighted mean hyperfine fields for FeS x (1.0 # x # 1.15) decrease when the at.% Fe decreases, as shown in Fig. 3. Since the correlation between Bhf and FeS x (1.0 # x # 1.15) is linear, it is very likely that the arrangement of S atoms and vacancies in the Fe plane, which influence the short range order of the Fe sites, are distributed randomly.
4. Conclusion
¨ Fig. 2. Mossbauer spectra of hexagonal pyrrhotite at various temperatures.
vacancies; therefore, the substitution of S for Se does not affect the arrangement of layers containing Fe atoms. However, the short range order around Fe atoms is affected because of the difference in radii between S and Se. Consequently, the variation of composition x in both Fe 12x S (0.004 # x # 0.143) [9] and Fe 7 (S x Se 12x ) 8 [17]
¨ The peaks of the Mossbauer spectra for hexagonal pyrrhotite are broader than those of monoclinic pyrrhotite. ¨ Besides, the Mossbauer spectra of hexagonal pyrrhotite are asymmetric such that peak 6.peak 1, peak 2.peak 5 and peak 3.peak 4, where the lines are numbered from 1 to 6 in order of increasing energy. The intensity of the Fe sites in monoclinic pyrrhotite are almost in the ratio 2:1:2:2 in the order of decreasing hyperfine field but those corresponding to hexagonal pyrrhotite do not show any particular pattern. The variation of the at.% Fe in iron sulphide as the structural system changes from FeS to Fe 7 S 8 varies the short range order of the Fe sites in the plane of Fe sites with vacancies and consequently affects the intensity ratios of the inequivalent Fe sites. ¨ The broadening of the linewidth in the Mossbauer spectra corresponding to hexagonal pyrrhotite could be related to the disturbance of the short range order around
J.W. A. Kondoro / Journal of Alloys and Compounds 289 (1999) 36 – 41
40
Table 2 ¨ Mossbauer parameters of hexagonal pyrrhotite samples at various temperatures Temp.
HF (T)
IS (mm / s)
QS (mm / s)
FWHM (mm / s)
Intensity (%)
Phase
RT
0 0 27.51 25.07 23.23 21.03
1.10 1.02 0.68 0.70 0.69 0.68
2.72 1.55 0.03 0.02 0.03 0.02
0.30 0.20 0.42 0.42 0.47 0.19
11.41 12.67 22.96 18.82 16.19 11.37
Chlorite
0.0 0.0 32.02 29.26 27.17 25.15
1.22 0.96 0.81 0.81 0.80 0.79
2.95 1.63 0.02 0.02 0.02 0.03
0.28 0.18 0.41 0.51 0.44 0.54
10.25 10.42 31.47 17.64 15.25 14.96
Chlorite
0 0 33.01 30.44 28.18 25.63
1.22 0.66 0.74 0.72 0.71 0.74
3.17 1.83 0.01 0.02 0.02 0.01
0.16 0.16 0.35 0.46 0.46 0.47
8.89 9.46 32.05 16.49 17.85 15.25
Chlorite
77 K
4.2 K
Hexagonal pyrrhotite
Hexagonal pyrrhotite
Hexagonal pyrrhotite
Table 3 ¨ Mossbauer parameters of some hexagonal pyrrhotite samples Sample No. and location
HF (T)
QS (mm / s)
IS (mm / s)
I (%)
G (mm / s)
18 Old-mine (GEITA)
30.0 28.2 26.4 24.6
0.10 0.09 0.06 0.16
0.69 0.69 0.69 0.69
25 16 24 19
0.35 0.35 0.35 0.35
15 Mwamela I (NZEGA)
30.0 28.6 26.8 25.0
0.11 0.10 0.04 0.19
0.59 0.59 0.59 0.59
13 11 24 26
0.33 0.30 0.36 0.48
14 Mwamela II (NZEGA)
30.2 27.7 25.9 23.8
0.12 0.06 0.12 0.19
0.58 0.58 0.58 0.58
29 28 27 13
0.33 0.33 0.33 0.33
37 Wella (NZEGA)
30.0 27.4 25.8 23.8
0.11 0.05 0.14 0.23
0.69 0.69 0.69 0.69
34 26 24 16
0.30 0.30 0.30 0.30
Fe sites due to vacancies and the arrangement of S atoms in the pyrrhotite system. The observed asymmetric effect ¨ in the Mossbauer spectra of hexagonal pyrrhotite could be due to a correlation between the isomer shift and the hyperfine field.
Acknowledgements The author greatly appreciates the financial assistance of the DAAD.
Fig. 3. Variation of weighted hyperfine field, Bhf , with atomic ratio Fe / S in FeS x (1.0 # x # 1.15).
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