Mossbauer study on microwave synthesized
Hyperfine Interact (2008) 186:113–120
DOI 10.1007/s10751-008-9840-4
Mössbauer study on microwave synthesized (Cu,Fe)
sulfide composites and correlation with natural
mineral—cubanite
Sarita Pareek · Anwar Rais ·
Amita Tripathi · Usha Chandra
Published online: 10 October 2008
© Springer Science + Business Media B.V. 2008
Abstract In nature, the mineral ‘Cubanite’ with composition CuFe2 S3 occurs in
orthorhombic structure in the matrix of chalcopyrite and pyrrhotite. Synthesis of
this mineral in the laboratory conditions has not been reported yet. An attempt
to synthesize the orthorhombic Cu–Fe–Sulfide (cubanite) by resistive as well as
microwave heating technique is reported. MW-heated sample shows the presence
of orthorhombic component along with isocubanite and pyrrhotite. The synthesized
samples were studied in details through XRD and Mössbauer spectroscopy. The
synthesis process, responsible for different proportions of the minerals, may indicate
the conditions of their genesis in nature. Formation of isocubanite (cubic cubanite)
seems unavoidable under the conditions of synthesis. The striking indistinguishable
character of cubanite and chalcopyrite could be challenged through Mössbauer hyperfine parameter—the hyperfine field of chalcopyrite (∼37 T) being quite different
from that of cubanite (∼33 T).
Keywords Orthorhombic cubanite · Isocubanite · Mössbauer spectroscopy ·
Cu–Fe–S · Chalcopyrite–pyrrhotite
1 Introduction
In mineral deposits, cubanite often occurs as a lath in a chalcopyrite matrix. Intergrowth of cubanite and chalcopyrite has been described from a number of localities
S. Pareek · U. Chandra (B)
High-Pressure Physics Laboratory, Department of Physics,
University of Rajasthan, Jaipur 302055, India
e-mail: ushac_jp1@sancharnet.in
A. Rais
Geological Survey of India, Jaipur 302017, India
A. Tripathi
Mössbauer Lab., Department of Physics, J. N. V. University,
Jodhpur 342001, India
114
S. Pareek et al.
by Schwartz [1]. The origin of the certain of these intergrowths is clearly due to the
breaking down of a solid solution—via exsolution of chalcopyrite into orthorhombic
cubanite with its characteristics striation. The crystal structure of natural cubanite
(orthorhombic Pcmn, a = 6.467 Å, b = 11.11 Å and c = 6.23 Å) have been described
widely by several investigators [2]. Both Cu and Fe are tetrahedrally coordinated by
four S atoms; One third of the S has 2 Cu and 2 Fe neighbours and the other 2/3 of
S have 3 Fe and one Cu. The Fe atoms are bonded together in pairs to give rise to
ferromagnetism.
Various techniques like 57 Fe Mössbauer spectroscopy, X-ray diffraction, electrical
resistivity etc., used under high pressure on the natural specimen show an insulator–
metal phase transition in the range 3.4–5.8 GPa coinciding with structural transition
from an orthorhombic to a hexagonal structure [3–5]. The room temperature data
shows that the metallization process concurs with a gradual transition from a magnetically ordered phase at low pressure to a non-magnetic (or paramagnetic phase)at
high-pressure. When orthorhombic cubanite is heated above 200◦ C in vacuum, an
irreversible order–disorder transition to a cubic phase (known as isocubanite—
an isomorphous phase of cubanite) is observed. In nature, a sub-solidus process
occurs with transformation of cubic (iso) cubanite to orthorhombic structure. Such
transformation during synthesis may be decided by the physico-chemical conditions
existing in the laboratory. No successful attempt to synthesize cubanite in the
orthorhombic structure under laboratory conditions has been reported.
In this report, an attempt to synthesize and correlate the conditions of the natural
genesis of the mineral in orthorhombic structure through resistive and microwave
heating methods is elaborated under compositional variations as well as under various conditions of heating and cooling cycles. The composite samples thus obtained
are analyzed through XRD and 57 Fe Mössbauer Spectroscopy.
2 Experimental procedure
Stoichiometric proportion of Cu,Fe and S were heated up to 800◦ C in an evacuated
quartz tube using a resistive furnace under controlled heating rate. The samples thus
heated were:
(a)
(b)
(c)
(d)
Cu–Fe–S (1:2:3) cooled slowly up to RT—sample ‘a’
Cu–Fe–S (1:2:3) quenched in ice water—sample ‘b’
Cu–Fe–S (1:2:3) cooled down to 400◦ C and then quenched—sample ‘c’.
sample ‘d’—Cu–Fe–S (1:2:1.5), in an evacuated quartz tube was heated using
domestic microwave oven at full power of 600 W for few seconds. Sample was
slowly cooled down to room temperature.
All the samples were characterized by recording X-ray diffraction patterns using
Philips diffractometers with Fe Kα and Cu Kα radiations respectively. The composite
pattern was analyzed through specific program for the respective mineral compositions (for chalcopyrite (CuFeS2 ), pyrrhotite (Fe1−x S) and cubanite–orthorhombic as
well as cubic) using standard tables. The Mössbauer measurements were done using
a 10 mCi Co57 (Rh) source at constant acceleration mode with absorber at room
temperature. Complementing the XRD analysis, the composite spectra were fitted
Mössbauer study on microwave synthesized sulfide composites
Fig. 1 a X-ray diffraction
pattern of Cu–Fe–S (1:2:3)
resistive heated up to 800◦ C
and slowly cooled to room
temperature using Cu Kα
radiation. b X-ray diffraction
patterns of microwave heated
samples Cu–Fe–S (1:2:1.5) and
Cu–Fe–S (1:2:3) respectively
using Fe Kα radiation. The
arrows display the peaks due
to orthorhombic cubanite
115
a
b 600
Intensity (arb.Units)
500
400
C u F e 2 S 1 .5
300
C u F e 2S 3
200
100
30
40
50
60
Two Theta
70
80
90
with two sextets and a doublet with a reasonable goodness of fit parameter and line
width using the analysis program developed by Jernberg and Sundquist [6].
3 Results and discussions
The dependency of the synthesis process is well depicted in the Fig. 1. The X-ray pattern of sample ‘a’ has been analyzed as a mixture of isocubanite, pyrrhotite (Fe1−x S)
and chalcopyrite (CuFeS2 ) while the microwave heated sample CuFe2 S1.5 showed the
presence of orthorhombic cubanite (having distinct peak with d = 3.22 Å) marked
by the arrows along with good amount of pyrrhotite. On the contrary, microwave
heated CuFe2 S3 (with excess sulphur) displays very few peaks representing the
presence of isocubanite/chalcopyrite and little amount of pyrrhotite. XRD pattern
of the sample ‘b’ indicates only chalcopyrite and pyrrhotite while sample ‘c’ shows
good amount of chalcopyrite/isocubanite with less quantity of pyrrhotite. Cabri [2] by
studying in details the orthorhombic cubanite heated at 200◦ C in vacuum, established
similarity in ‘d’ values of the high temperature phase(isocubanite) and chalcopyrite.
Isocubanite, a cubic polymorph of cubanite with a = 5.303 Å shows strongest lines in
the X-ray diffraction pattern [3.04(100), 2.65(65), 1.87(34), 1.59(13)] which matches
well with X-ray lines of chalcopyrite [7] [3.03(100), 1.86(31) and 1.59(18)]. The extra
peak observed at d = 2.65 Å, therefore should be a signature of isocubanite.
116
S. Pareek et al.
Fig. 2 Room temperature Mössbauer spectra of resistively heated Cu–Fe–S (1:2:3) a cooled slowly
to RT b quenched in ice water and c cooled down to 400◦ C then quenched to RT. d Microwave
heated Cu–Fe–S (1:2:1.5). The velocity scale is relative to metallic Fe
Mössbauer spectroscopy is a sensitive technique to distinguish the minerals
through its parameters. Greenwood and Whitfield [8] studying the controversial
minerals (cubanite and chalcopyrite) through this technique found that the magnetic
Mössbauer study on microwave synthesized sulfide composites
Fig. 2 (continued)
117
d
hyperfine field of chalcopyrite is larger (∼36 T) as compared to cubanite (∼33 T).
Figure 2 and Table 1 show the Mössbauer spectra and the analyzed parameters
of the resistive as well as microwave heated samples. Isomer shifts (IS) 0.60 and
0.20 mm/s are typical values for high spin Fe2+ and Fe3+ respectively in metal–
iron sulfide compounds when iron is tetrahedrally coordinated by sulfur [9]. Using
XRD analysis, sample ‘a’ could be resolved into two magnetic sextets with IS =
0.23 and 0.46 mm/s respectively and a nonmagnetic doublet with IS of 0.46 mm/s
and QS of 0.61 mm/s assigned to chalcopyrite (Cu+1 Fe3+ S2 ), pyrrhotite(Fe1−x S)
and cubanite (CuFe2 S3 ) respectively. If one considers the value of hyperfine field
alone, the magnetic sextet with field ∼33 T should be assigned to cubanite instead
to chalcopyrite, however the IS of 0.23 mm/s seems to be typical value of Fe3+ at
the tetrahedral symmetry. The nonmagnetic component can also be fitted with two
singlets having IS = 0.15 and 0.772 mm/s respectively with relaxation line broadening
implying the electrons moving rapidly between Fe2+ and Fe3+ ions in the cubanite
structure. The fitting of the Mössbauer spectra of sample ‘b’ and ‘c’ was not easy
since the phases were not very well formed. Nevertheless one can see the prominent
chalcopyrite field of 33.9 T with IS = 0.26 mm/s as well as a broad non-magnetic
isocubanite with IS = 0.50 mm/s and pyrrhotite for sample ‘b’. In sample ‘c’, the
chalcopyrite phase was missing. Sextet related to pyrrhotite phase with IS of 0.782
and 0.551 mm/s belonging to Fe2+ were seen along with very broad non-magnetic
doublet. Both these quenched samples have shown large proportion of magnetic
components while the spectrum of the slowly cooled sample resembled that of
orthorhombic cubanite at high pressure (4.5 GPa) [4]. The microwave heated sample
‘d’, analyzed as composed of orthorhombic cubanite showed a sextet with 34.3 T
along with pyrrhotite and non-magnetic doublet (Fig. 2d). Orthorhombic cubanite
transforms into cubic cubanite above 200◦ C. The transition is accompanied by the
decrease in the saturation magnetization resulting due to the random redistribution
of iron and copper atoms in the lattice. In mineral deposits, cubanite often occurs in a
chalcopyrite matrix. The textural relationship suggests a sub solidus process whereby
exsolution of cubic cubanite occurs in the host chalcopyrite.
118
Table 1 Room temperature Mössbauer parameters of resistively heated samples Cu–Fe–S (1:2:3) at various conditions of synthesis as well as microwave heated
Cu–Fe–S (1:2:1.5)
Sample description
Components
Isomer shift
metallic Fe (mm/s)
Quadrupole
splitting (mm/s)
Width (mm/s)
Intensity %
Magnetic hyperfine
field (Tesla)
Sample ‘a’—resistive
heated-slowly cooled
Magnetic sextet P1
0.235
−0.011
0.440
40.70
33.95
Magnetic sextet P2
Nonmagnetic doublet
Magnetic sextet P1
0.466
0.460
0.218
−0.166
0.611
−0.093
0.910
0.600
0.354
19.94
39.36
17.96
27.10
–
33.98
Magnetic sextet P2
Nonmagnetic doublet
Magnetic sextet P1
0.623
0.502
0.782
0.027
0.869
−0.277
1.299
0.507
0.398
66.67
15.37
19.98
26.50
–
30.50
Magnetic sextet P2
Nonmagnetic doublet
Magnetic sextet P1
0.551
0.395
0.152
0.194
0.417
0.227
0.794
1.803
0.363
44.53
35.70
50.40
25.94
–
34.33
Magnetic sextet P2
Nonmagnetic doublet
0.728
0.427
0.352
0.571
0.303
0.461
36.96
12.64
30.36
–
Sample ‘b’—resistive
heated- quenched
Sample ‘c’—resistive
heated- quenched at 400◦ C
Sample ‘d’—microwave
heated CuFe2 S1.5
S. Pareek et al.
Mössbauer study on microwave synthesized sulfide composites
a
758000
756000
754000
752000
Counts
Fig. 3 a Room temperature
Mössbauer spectra (raw data)
of sample ‘b’ with composition
Cu– Fe–S (1:2:3) resistively
heated but quenched in ice (b)
the sample annealed at 180 C
for 20 days. The spectra were
taken at velocity range
related to inner two lines
of metallic Fe
119
750000
748000
746000
744000
742000
740000
0
50
100
150
200
250
channel
b
1374000
anealed at 180C for 20 days
1372000
1370000
counts
1368000
1366000
1364000
1362000
1360000
1358000
1356000
0
50
100
150
200
250
channel
Figure 3 shows the raw Mössbauer patterns (unanalyzed) of the sample ‘b’ after
annealing at 180◦ C for 20 days. The new inner peaks emerge at the expenses of
outer peaks consisting of chalcopyrite and pyrrhotite [2], thus demonstrating nicely
exsolution process occurring in nature. The optical reflection technique also confirms
the exsolution of chalcopyrite into cubanite and the presence of cubanite in the
sample.
4 Conclusion
The microwave synthesis with less sulfur content could show traces of orthorhombic
phase along with chalcopyrite and pyrrhotite explaining the exsolution of these
minerals to cubanite in natural environment. The synthesis process seems to be an
important parameter explaining the presence of mineral in the matrix of chalcopyrite
and pyrrhotite. Systematics needs to be developed to synthesize orthorhombic
120
S. Pareek et al.
minerals in the laboratory. Mössbauer spectroscopy seems to be a sensitive and
better technique to analyze the phases.
Acknowledgements We are grateful to DST, New Delhi and PLANEX (PRL), Ahmedabad
for the financial support; Magnetism and Mössbauer lab., U.O.R., Jaipur for X-ray diffraction;
GTZ, Germany for gifting Mössbauer set up. We thank referees for the valuable comments and
suggestions.
References
1. Schwartz, G.M.: Intergrowth of chalcopyrite and cubanite; experimental proof of the origin of
intergrowth and their bearing on the geologic thermometer. Econ. Geol. 22, 44–61 (1927)
2. Cabri, L.J., Hall, S.R., Szymanski, J.T., Stewart, J.M.: On the transformation of cubanite. Can.
Min. 12, 33–38 (1973), and references therein
3. Rosenberg, G.Kh., Pasternak, M.P., Hearne, G.R., McCammon, C.A.: High-pressure metallization and electronic–magnetic properties of hexagonal cubanite (CuFe2 S3 ). Phys. Chem. Miner.
24, 569–573 (1997)
4. McCammon, C.A.: Equation of state, bonding character and phase transition of cubanite,
CuFe2 S3 studied from 0 to 5 GPa. Amer. Miner. 80, 1–8 (1995)
5. McCammon, C.A., Zhang, J., Hazen, R.M., Finger, L.W.: High-pressure crystal chemistry of
cubanite CuFe2 S3 . Amer. Miner. 77, 937–944 (1992)
6. Jernberg, P., Sundquist, T.: Report UUIP-1090, Institute of Physics, University of Uppsala,
Uppsala, Sweden (1983)
7. Caye, R., Cerevelle, B., Cesborn, F., Oudin, E., Picot, P., Pillard, F.: Isocubanite, a new definition
of the cubic polymorph of cubanite CuFe2 S3 . Miner. Mag. 52, 509–514 (1988)
8. Greenwood, N.N., Whitfield, H.J.: Mössbauer effect studies on cubanite and related iron sulfides.
J. Chem. Soc. (A) 1697–1699 (1968)
9. Hoggins, J.T., Steinfink, H.: Empirical bonding relationship in metal–iron–sulfide compounds.
Inorg. Chem. 15, 1682–1685 (1976)
DOI 10.1007/s10751-008-9840-4
Mössbauer study on microwave synthesized (Cu,Fe)
sulfide composites and correlation with natural
mineral—cubanite
Sarita Pareek · Anwar Rais ·
Amita Tripathi · Usha Chandra
Published online: 10 October 2008
© Springer Science + Business Media B.V. 2008
Abstract In nature, the mineral ‘Cubanite’ with composition CuFe2 S3 occurs in
orthorhombic structure in the matrix of chalcopyrite and pyrrhotite. Synthesis of
this mineral in the laboratory conditions has not been reported yet. An attempt
to synthesize the orthorhombic Cu–Fe–Sulfide (cubanite) by resistive as well as
microwave heating technique is reported. MW-heated sample shows the presence
of orthorhombic component along with isocubanite and pyrrhotite. The synthesized
samples were studied in details through XRD and Mössbauer spectroscopy. The
synthesis process, responsible for different proportions of the minerals, may indicate
the conditions of their genesis in nature. Formation of isocubanite (cubic cubanite)
seems unavoidable under the conditions of synthesis. The striking indistinguishable
character of cubanite and chalcopyrite could be challenged through Mössbauer hyperfine parameter—the hyperfine field of chalcopyrite (∼37 T) being quite different
from that of cubanite (∼33 T).
Keywords Orthorhombic cubanite · Isocubanite · Mössbauer spectroscopy ·
Cu–Fe–S · Chalcopyrite–pyrrhotite
1 Introduction
In mineral deposits, cubanite often occurs as a lath in a chalcopyrite matrix. Intergrowth of cubanite and chalcopyrite has been described from a number of localities
S. Pareek · U. Chandra (B)
High-Pressure Physics Laboratory, Department of Physics,
University of Rajasthan, Jaipur 302055, India
e-mail: ushac_jp1@sancharnet.in
A. Rais
Geological Survey of India, Jaipur 302017, India
A. Tripathi
Mössbauer Lab., Department of Physics, J. N. V. University,
Jodhpur 342001, India
114
S. Pareek et al.
by Schwartz [1]. The origin of the certain of these intergrowths is clearly due to the
breaking down of a solid solution—via exsolution of chalcopyrite into orthorhombic
cubanite with its characteristics striation. The crystal structure of natural cubanite
(orthorhombic Pcmn, a = 6.467 Å, b = 11.11 Å and c = 6.23 Å) have been described
widely by several investigators [2]. Both Cu and Fe are tetrahedrally coordinated by
four S atoms; One third of the S has 2 Cu and 2 Fe neighbours and the other 2/3 of
S have 3 Fe and one Cu. The Fe atoms are bonded together in pairs to give rise to
ferromagnetism.
Various techniques like 57 Fe Mössbauer spectroscopy, X-ray diffraction, electrical
resistivity etc., used under high pressure on the natural specimen show an insulator–
metal phase transition in the range 3.4–5.8 GPa coinciding with structural transition
from an orthorhombic to a hexagonal structure [3–5]. The room temperature data
shows that the metallization process concurs with a gradual transition from a magnetically ordered phase at low pressure to a non-magnetic (or paramagnetic phase)at
high-pressure. When orthorhombic cubanite is heated above 200◦ C in vacuum, an
irreversible order–disorder transition to a cubic phase (known as isocubanite—
an isomorphous phase of cubanite) is observed. In nature, a sub-solidus process
occurs with transformation of cubic (iso) cubanite to orthorhombic structure. Such
transformation during synthesis may be decided by the physico-chemical conditions
existing in the laboratory. No successful attempt to synthesize cubanite in the
orthorhombic structure under laboratory conditions has been reported.
In this report, an attempt to synthesize and correlate the conditions of the natural
genesis of the mineral in orthorhombic structure through resistive and microwave
heating methods is elaborated under compositional variations as well as under various conditions of heating and cooling cycles. The composite samples thus obtained
are analyzed through XRD and 57 Fe Mössbauer Spectroscopy.
2 Experimental procedure
Stoichiometric proportion of Cu,Fe and S were heated up to 800◦ C in an evacuated
quartz tube using a resistive furnace under controlled heating rate. The samples thus
heated were:
(a)
(b)
(c)
(d)
Cu–Fe–S (1:2:3) cooled slowly up to RT—sample ‘a’
Cu–Fe–S (1:2:3) quenched in ice water—sample ‘b’
Cu–Fe–S (1:2:3) cooled down to 400◦ C and then quenched—sample ‘c’.
sample ‘d’—Cu–Fe–S (1:2:1.5), in an evacuated quartz tube was heated using
domestic microwave oven at full power of 600 W for few seconds. Sample was
slowly cooled down to room temperature.
All the samples were characterized by recording X-ray diffraction patterns using
Philips diffractometers with Fe Kα and Cu Kα radiations respectively. The composite
pattern was analyzed through specific program for the respective mineral compositions (for chalcopyrite (CuFeS2 ), pyrrhotite (Fe1−x S) and cubanite–orthorhombic as
well as cubic) using standard tables. The Mössbauer measurements were done using
a 10 mCi Co57 (Rh) source at constant acceleration mode with absorber at room
temperature. Complementing the XRD analysis, the composite spectra were fitted
Mössbauer study on microwave synthesized sulfide composites
Fig. 1 a X-ray diffraction
pattern of Cu–Fe–S (1:2:3)
resistive heated up to 800◦ C
and slowly cooled to room
temperature using Cu Kα
radiation. b X-ray diffraction
patterns of microwave heated
samples Cu–Fe–S (1:2:1.5) and
Cu–Fe–S (1:2:3) respectively
using Fe Kα radiation. The
arrows display the peaks due
to orthorhombic cubanite
115
a
b 600
Intensity (arb.Units)
500
400
C u F e 2 S 1 .5
300
C u F e 2S 3
200
100
30
40
50
60
Two Theta
70
80
90
with two sextets and a doublet with a reasonable goodness of fit parameter and line
width using the analysis program developed by Jernberg and Sundquist [6].
3 Results and discussions
The dependency of the synthesis process is well depicted in the Fig. 1. The X-ray pattern of sample ‘a’ has been analyzed as a mixture of isocubanite, pyrrhotite (Fe1−x S)
and chalcopyrite (CuFeS2 ) while the microwave heated sample CuFe2 S1.5 showed the
presence of orthorhombic cubanite (having distinct peak with d = 3.22 Å) marked
by the arrows along with good amount of pyrrhotite. On the contrary, microwave
heated CuFe2 S3 (with excess sulphur) displays very few peaks representing the
presence of isocubanite/chalcopyrite and little amount of pyrrhotite. XRD pattern
of the sample ‘b’ indicates only chalcopyrite and pyrrhotite while sample ‘c’ shows
good amount of chalcopyrite/isocubanite with less quantity of pyrrhotite. Cabri [2] by
studying in details the orthorhombic cubanite heated at 200◦ C in vacuum, established
similarity in ‘d’ values of the high temperature phase(isocubanite) and chalcopyrite.
Isocubanite, a cubic polymorph of cubanite with a = 5.303 Å shows strongest lines in
the X-ray diffraction pattern [3.04(100), 2.65(65), 1.87(34), 1.59(13)] which matches
well with X-ray lines of chalcopyrite [7] [3.03(100), 1.86(31) and 1.59(18)]. The extra
peak observed at d = 2.65 Å, therefore should be a signature of isocubanite.
116
S. Pareek et al.
Fig. 2 Room temperature Mössbauer spectra of resistively heated Cu–Fe–S (1:2:3) a cooled slowly
to RT b quenched in ice water and c cooled down to 400◦ C then quenched to RT. d Microwave
heated Cu–Fe–S (1:2:1.5). The velocity scale is relative to metallic Fe
Mössbauer spectroscopy is a sensitive technique to distinguish the minerals
through its parameters. Greenwood and Whitfield [8] studying the controversial
minerals (cubanite and chalcopyrite) through this technique found that the magnetic
Mössbauer study on microwave synthesized sulfide composites
Fig. 2 (continued)
117
d
hyperfine field of chalcopyrite is larger (∼36 T) as compared to cubanite (∼33 T).
Figure 2 and Table 1 show the Mössbauer spectra and the analyzed parameters
of the resistive as well as microwave heated samples. Isomer shifts (IS) 0.60 and
0.20 mm/s are typical values for high spin Fe2+ and Fe3+ respectively in metal–
iron sulfide compounds when iron is tetrahedrally coordinated by sulfur [9]. Using
XRD analysis, sample ‘a’ could be resolved into two magnetic sextets with IS =
0.23 and 0.46 mm/s respectively and a nonmagnetic doublet with IS of 0.46 mm/s
and QS of 0.61 mm/s assigned to chalcopyrite (Cu+1 Fe3+ S2 ), pyrrhotite(Fe1−x S)
and cubanite (CuFe2 S3 ) respectively. If one considers the value of hyperfine field
alone, the magnetic sextet with field ∼33 T should be assigned to cubanite instead
to chalcopyrite, however the IS of 0.23 mm/s seems to be typical value of Fe3+ at
the tetrahedral symmetry. The nonmagnetic component can also be fitted with two
singlets having IS = 0.15 and 0.772 mm/s respectively with relaxation line broadening
implying the electrons moving rapidly between Fe2+ and Fe3+ ions in the cubanite
structure. The fitting of the Mössbauer spectra of sample ‘b’ and ‘c’ was not easy
since the phases were not very well formed. Nevertheless one can see the prominent
chalcopyrite field of 33.9 T with IS = 0.26 mm/s as well as a broad non-magnetic
isocubanite with IS = 0.50 mm/s and pyrrhotite for sample ‘b’. In sample ‘c’, the
chalcopyrite phase was missing. Sextet related to pyrrhotite phase with IS of 0.782
and 0.551 mm/s belonging to Fe2+ were seen along with very broad non-magnetic
doublet. Both these quenched samples have shown large proportion of magnetic
components while the spectrum of the slowly cooled sample resembled that of
orthorhombic cubanite at high pressure (4.5 GPa) [4]. The microwave heated sample
‘d’, analyzed as composed of orthorhombic cubanite showed a sextet with 34.3 T
along with pyrrhotite and non-magnetic doublet (Fig. 2d). Orthorhombic cubanite
transforms into cubic cubanite above 200◦ C. The transition is accompanied by the
decrease in the saturation magnetization resulting due to the random redistribution
of iron and copper atoms in the lattice. In mineral deposits, cubanite often occurs in a
chalcopyrite matrix. The textural relationship suggests a sub solidus process whereby
exsolution of cubic cubanite occurs in the host chalcopyrite.
118
Table 1 Room temperature Mössbauer parameters of resistively heated samples Cu–Fe–S (1:2:3) at various conditions of synthesis as well as microwave heated
Cu–Fe–S (1:2:1.5)
Sample description
Components
Isomer shift
metallic Fe (mm/s)
Quadrupole
splitting (mm/s)
Width (mm/s)
Intensity %
Magnetic hyperfine
field (Tesla)
Sample ‘a’—resistive
heated-slowly cooled
Magnetic sextet P1
0.235
−0.011
0.440
40.70
33.95
Magnetic sextet P2
Nonmagnetic doublet
Magnetic sextet P1
0.466
0.460
0.218
−0.166
0.611
−0.093
0.910
0.600
0.354
19.94
39.36
17.96
27.10
–
33.98
Magnetic sextet P2
Nonmagnetic doublet
Magnetic sextet P1
0.623
0.502
0.782
0.027
0.869
−0.277
1.299
0.507
0.398
66.67
15.37
19.98
26.50
–
30.50
Magnetic sextet P2
Nonmagnetic doublet
Magnetic sextet P1
0.551
0.395
0.152
0.194
0.417
0.227
0.794
1.803
0.363
44.53
35.70
50.40
25.94
–
34.33
Magnetic sextet P2
Nonmagnetic doublet
0.728
0.427
0.352
0.571
0.303
0.461
36.96
12.64
30.36
–
Sample ‘b’—resistive
heated- quenched
Sample ‘c’—resistive
heated- quenched at 400◦ C
Sample ‘d’—microwave
heated CuFe2 S1.5
S. Pareek et al.
Mössbauer study on microwave synthesized sulfide composites
a
758000
756000
754000
752000
Counts
Fig. 3 a Room temperature
Mössbauer spectra (raw data)
of sample ‘b’ with composition
Cu– Fe–S (1:2:3) resistively
heated but quenched in ice (b)
the sample annealed at 180 C
for 20 days. The spectra were
taken at velocity range
related to inner two lines
of metallic Fe
119
750000
748000
746000
744000
742000
740000
0
50
100
150
200
250
channel
b
1374000
anealed at 180C for 20 days
1372000
1370000
counts
1368000
1366000
1364000
1362000
1360000
1358000
1356000
0
50
100
150
200
250
channel
Figure 3 shows the raw Mössbauer patterns (unanalyzed) of the sample ‘b’ after
annealing at 180◦ C for 20 days. The new inner peaks emerge at the expenses of
outer peaks consisting of chalcopyrite and pyrrhotite [2], thus demonstrating nicely
exsolution process occurring in nature. The optical reflection technique also confirms
the exsolution of chalcopyrite into cubanite and the presence of cubanite in the
sample.
4 Conclusion
The microwave synthesis with less sulfur content could show traces of orthorhombic
phase along with chalcopyrite and pyrrhotite explaining the exsolution of these
minerals to cubanite in natural environment. The synthesis process seems to be an
important parameter explaining the presence of mineral in the matrix of chalcopyrite
and pyrrhotite. Systematics needs to be developed to synthesize orthorhombic
120
S. Pareek et al.
minerals in the laboratory. Mössbauer spectroscopy seems to be a sensitive and
better technique to analyze the phases.
Acknowledgements We are grateful to DST, New Delhi and PLANEX (PRL), Ahmedabad
for the financial support; Magnetism and Mössbauer lab., U.O.R., Jaipur for X-ray diffraction;
GTZ, Germany for gifting Mössbauer set up. We thank referees for the valuable comments and
suggestions.
References
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