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chalcopyrite mineral
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 96 (2012) 310–315
Contents lists available at SciVerse ScienceDirect
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa
Electron paramagnetic resonance, optical absorption and Raman spectral studies on a pyrite/chalcopyrite mineral G. Udayabhaskar Reddy a, K. Seshamaheswaramma a, Yoshinobu Nakamura b, S. Lakshmi Reddy a,⇑, Ray L. Frost c,⇑, Tamio Endo d a
Dept. of Physics, S.V.D. College, Kadapa 516 003, India Dept. of Applied Chemistry, School of Engineering, The University of Tokyo, Japan Inorganic Materials Research Program, Queensland University of Technology, 2 George Street, Brisbane, G.P.O. Box 2434, Queensland 4001, Australia d Faculty of Engineering, Mie University, TSU, Mie 514 8507, Japan b c
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
" Pyrite has cubic crystal structure
with cell constant 5.45 Å. " EPR studies on chalcopyrite suggest
high percentage of Cu. " Optical absorption spectrum of
chalcopyrite is due to Fe(II) and Cu(II) which are in distorted octahedral symmetry. " Raman studies on chalcopyrite indicate that it has sulfate and water fundamentals.
a r t i c l e
i n f o
Article history: Received 24 October 2011 Received in revised form 6 March 2012 Accepted 25 April 2012 Available online 3 May 2012 Keywords: Pyrite/chalcopyrite XRD EPR Optical absorption spectra NIR spectra Raman spectrum
a b s t r a c t Pyrite and chalcopyrite mineral samples from Mangampet barite mine, Kadapa, Andhra Pradesh, India are used in the present study. XRD data indicate that the pyrite mineral has a face centered cubic lattice structure with lattice constant 5.4179 Å. Also it possesses an average particle size of 91.9 nm. An EPR study on the powdered samples confirms the presence of iron in pyrite and iron and Mn(II) in chalcopyrite. The optical absorption spectrum of chalcopyrite indicates presence of copper which is in a distorted octahedral environment. NIR results confirm the presence of water fundamentals and Raman spectrum reveals the presence of water and sulfate ions. Ó 2012 Elsevier B.V. All rights reserved.
Introduction Minerals of geological interest are structurally and chemically complex when compared with pure inorganic solids. Many spec-
troscopic methods have been utilized to study the presence of metal ions, their oxidation state, energetics, structure property relationships, etc., Mossbauer, optical absorption, XPS, NMR and EPR spectroscopic studies are employed for structural information.
⇑ Corresponding authors. Address: Queensland University of Technology, Faculty of Science and Technology, 2 George St., Brisbane, Queensland 4001, Australia. Tel.: +61 7 3138 2407; fax: +61 7 3138 1804 (R.L. Frost). E-mail addresses: [email protected] (S. Lakshmi Reddy), [email protected] (R.L. Frost). 1386-1425/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.saa.2012.04.093
311
G. Udayabhaskar Reddy et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 96 (2012) 310–315 Table 1 Comparison of energies of the bands with their assignments for Cu(II) in rhombic octahedral coordination with ground state 2A1g(dx2 y2 ). Sample
2
cm Antlerite Cu3SO4(OH)4 Turquoise CuAl6(PO4)(OH)84H2O ZPPH (ZnKPO4)6H2O Atacamite Cu2(OH)3Cl Libethenite Cu2PO4OH Chalcopyrite (Cu,Fe)S2
2
1
2
B1g(dXY)
A1g(dZ 2 )
1
2
B2g(dXZ) 1
B3g(dYZ) 1
Reference
nm
cm
Nm
cm
Nm
cm
Nm
8475
1180
910
1290 1242 1121 1190
1060 668 1040 971 846 928
10990
7750 8049 8920 8400
9435 14970 9613 10296 11820 10780
12117 11083 14925 16025
825 902 670 624
16390 18354 13330 15380 20450 27320
610 545 750 650 489 366
The pyrite group of minerals has the general formula AX2, where A can be Fe, Zn, Hg, Au, Co, Cu, Mn, Ni, Ir, Pd, Pt or Ru and X can be S, As, Sb, Bi, Se and Te. Among them pyrite is the common mineral. Pyrite is an iron sulfide with the formula FeS2 with S–S band. It is a semiconductor with band gap of 0.95 eV [1]. Pyrite readily changes by oxidation to iron sulfate or to the hydrated oxide of iron. The unit cell is a face centered cubic sub lattice into which sulfur ions are embedded. Pyrite is similar in structure to fluorite and NaCl with cell edge constant a = 5.42 Å. Molecular sul+2 fide ion (S2 2 ) is oriented along the axis of third order, while Fe ions are at the center of the octahedron [2]. Each iron atom is surrounded by six sulfur atoms at the corners of the octahedron [3]. Measurements of X-ray absorption edges on chalcopyrite suggest that copper is present in two valence states, so that resonance between Cu+ + Fe3+ and Cu2+ + Fe2+ may be presumed [4]. Mössbauer spectra of naturally occurring chalcopyrite have been studied over a temperature range 300–448 K [5]. Mössbauer studies on natural Egyptian chalcopyrite have reveal that most of the iron is in Fe+2 state [6]. X ray diffraction (XRD) on natural and synthetic pyrite minerals was also reported [7]. Absorption spectra of CuFeS2 and Fe-Doped CuAlS2 and CuGaS2 have been studied [8]. Optical absorption, electron paramagnetic resonance (EPR) and Raman spectral studies have not been studied on pyrite or chalcopyrite mineral originating from the barite mine of Magampet, Kodur of Kadapa district, India. In this study we report XRD, EPR, optical absorption and Raman spectral studies and relate these studies to the structure of the mineral. Experimental Pyrite and chalcopyrite are brownish yellow in color. It is evident from the chemical analysis that the mineral pyrite contains 56.8 wt.% of iron and chalcopyrite contains 26.8 wt.% of iron and 28.2 wt.% of copper [9]. X-ray powder diffraction pattern of pyrite is recorded using Philips X-ray diffractometer operated in reflection geometry at 30 mA, 40 kV with Cu-Ka (k = 1.54060 Å) source at 25 °C in the range of 10–75°. Data are collected using a continuous scan rate of 1°/ 2 min1, which are then, refined into 2 theta steps of 0.02°. EPR spectra of pyrite and chalcopyrite powdered samples are recorded both at room (RT) and liquid nitrogen temperature (LNT) on JEOL JES TE100 ESR spectrometer operating at X band frequency (t = 9.40531 GHz for pyrite and 9.40620 GHz for chalcopyrite) having a 100 kHz field modulation to obtain first derivative EPR spectrum. DPPH with a g value of 2.0036 is used for g factor calculation. Optical absorption spectrum of the chalcopyrite sample is recorded at room temperature on Carey 5E UV–Vis–NIR spectrophotometer in mull form in the range 200–2000 nm. The chalcopyrite powdered mineral sample was placed and oriented on the stage of an Olympus BHSM microscope, equipped with 10 and 50 objectives and a Renishaw 1000 Raman microscope system. Raman spectra were excited by He–Ne laser
[32] [33] [34] [35] [37] Present work
(633 nm) at a resolution of 2 cm1 in the range between 100 and 4000 cm1. Band component analysis was undertaken using the Jandel ‘‘PEAKFIT’’ software package which enabled the type of fitting function to be selected and specific parameters to be fixed or varied accordingly. Band fitting was carried out using a Lorentz–Gauss cross product function with a minimum number of component bands used for the fitting process. The Lorentz–Gauss ratio was maintained at values greater than 0.7 and fitting was undertaken until reproducible results were obtained with squared correlations of r2 greater than 0.9975.
Theory Mn(II), being a d5 ion, has total spin S = 5/2. This state splits into three Kramers’ doublets, ±5/2>, ±3/2> and ±1/2> separated by 4D and 2D respectively where D is the zero-field splitting parameter. The deviation from axial symmetry leads to a term known as E in the spin Hamiltonian. The value of E can be easily calculated from single crystal measurements. A non-zero value of E results in making the spectrum unsymmetrical about the central sextet. Cu(II) has an electronic configuration [Ar] 3d9. In an octahedral crystal field, the corresponding ground state electronic configuration is t2g6eg3 which yields 2Eg term. The excited electronic configuration t2g5eg4 corresponds to 2T2g term. Hence single electron transition 2Eg ? 2T2g is expected in an octahedral crystal field. Normally, the ground 2Eg state splits due to Jahn–Teller effect and hence lowering of symmetry is expected for Cu(II) ion. This state splits into 2B1g(dx2 y2 ) and 2A1g(dz2 ) states in tetragonal symmetry and the excited term 2T2g also splits into 2B2g(dxy) and 2Eg(dxz, dyz) levels. In rhombic field, 2Eg ground state splits into 2A1g(dx2 y2 ) and 2A2g(dz2 ) whereas 2T2g splits into 2B1g(dxy), 2B2g(dxz) and 2 B3g(dyz) states. Thus three bands are expected for tetragonal (C4v) symmetry and four bands are expected for rhombic (D2h) symmetry [10].
Fig. 1. Powder X-ray diffraction pattern [Cu Ka] for pyrite mineral at 25 °C.
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G. Udayabhaskar Reddy et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 96 (2012) 310–315
Here ‘d’ is bulk density (pyrite = 4.84 g/cm3). The calculated value of porosity percentage is 3.43. The grain size of the compound is evaluated from the line broadening of the peaks using Debye–Scherrer equation
DðhklÞ ¼ Fig. 2. EPR spectrum of pyrite mineral at RT (m = 9.40531 GHz).
The ground state configuration of Fe(II) ion is 3d6. In an octahedral field, assuming high spin state, the configuration is expressed as t 42g e2g . This configuration gives rise to electronic states 5T2g, 3Eg, 3 T2g and some more triplets and singlets of which 5T2g is the ground state. The other excited configurations, such as t32g e3g gives rise to a several triplet and singlet states and one quintet state designated as 5Eg. Thus the spin allowed transition 5T2g ? 5Eg, is expected to be strong and all other spin forbidden transitions are very weak [11,12]. Thus, the 5T2g ? 5Eg transition gives rise to an intense, but broad absorption band. Often this band splits into two in an octahedral environment. If the splitting is of the order of 2000 cm1, then it is due to static distortion of octahedron [13–15]. However, an intermediate value between 100 and 2000 cm1 indicates a dynamic Jahn–Teller effect in the excited 5 Eg state [16,17]. In the latter case, the energy level is split symmetrically to the center of gravity and the average of these values of these bands is to be taken as 10Dq value. Results and discussion X-ray diffraction results Fig. 1 shows the X-ray diffraction pattern of pyrite mineral recorded on Philips diffractrometer up to 25 °C. The peaks are characterized by using Scherrer formula. The powder diffraction pattern is similar to that of the spectra reported for pyrite [18]. This calculated unit cell value of 5.419 Å well agreed with reported value on pyrite sample [19]. This confirms that the sample is pyrite with cubic octahedral structure. The X-ray density ‘dx’ is calculated ZM using the formula [20] dx ¼ Na 3. Here ‘‘Z’’(4) represents the number of molecules in a unit cell of the pyrite lattice, M (119.98 g) is the molecular weight of the mineral, N is the Avogadro’s number and ‘a’ the lattice constant of the sample. The calculated value of X-ray density is 5.01195 g/cm3. This indicates that X-ray density depends on mass. The percentage porosity of each sample was calculated using the relation [21].
d 100: Percentage porosityðp%Þ ¼ 1 dx
0:9k b1=2 cos h
Here D is the average particle size of the crystal, k is the wavelength of incident X ray, h is the corresponding Bragg angle, b1=2 is the full width at the half maximum height (FWHM) of the peak. The average particle size of the crystal is calculated as 91.6 nm. EPR results The EPR spectrum of the pyrite mineral sample recorded at room temperature is shown in Fig. 2. A single peak with g = 2.38 is observed in the spectrum at room temperature. Any additional peaks are not seen even at low temperature. This might be due to the very high concentration of iron present in the mineral. Fig. 3 shows the EPR spectrum of chalcopyrite mineral recorded at room temperature in the range 0–500 mT. It shows various resonances with g values of 3.77, 3.19, 2.46, 2.46, 2.18 and a sextant hyperfine structure with g value of 1.997. The expanded version of the sextant of the sample is shown in Fig. 4. The spectrum consists of a high intense sextet with g = 1.997 and A = 7.90 mT. This indicates that more Mn(II) ions are present in the octahedral environment. Further the presence of resolved hyperfine structure at g = 1.997 strongly indicates that Mn(II) ions are isolated or significantly distant from each other in octahedral environment. The strong resonance line in lower field with g = 3.20 and other weak resonances with g values 3.77, 2.46 and 2.18 are also due Mn(II) in distorted octahedral crystalline field in the chalcopyrite. The lack of hyperfine splitting at g = 3.20, 3.77 resonance lines is due to fluctuations of the ligand field parameters in the vicinity of Mn(II) ion and random distribution of the structural distortions [22]. The hyperfine constant ‘A’ value provides a qualitative measure of the ionic nature of bonding with Mn(II) ion. The percentage of covalency of Mn–ligand bond has been calculated using ‘A’ (8.0 mT) value obtained from the EPR spectrum and Matumura’s plot [23]. Also the approximate value of hyperfine constant (A) is calculated by using covalency (C) equations [24,25].
Aiso ¼ ð2:04C 04:5Þ 104 cm1 . The value obtained is 91 104 cm1. This calculated value agrees well with the observed hyperfine constant for the sample indicating the ionic character for metal ligand bond in the mineral
Fig. 3. Powdered EPR spectrum of chalcopyrite mineral at RT (t = 9.40619).
G. Udayabhaskar Reddy et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 96 (2012) 310–315
313
Fig. 4. Expanded version of chalcopyrite mineral at RT (t = 9.40620).
Fig. 6. NIR spectrum of chalcopyrite mineral at room temperature.
Optical absorption spectral analysis
Fig. 5. Optical absorption spectrum of chalcopyrite at room temperature.
under study. The number of ligands around Mn(II) ion is estimated using the covalency [24] equation for C
C¼
1 ½1 0:16ðX p X q Þ 0:035ðX p X q Þ2 n
Here XP and Xq represent electro-negativities of metal and ligand. Assuming Xp = XMn = 1.6 and Xq = XS = 3.5, the number of ligands (n) obtained are 16. This suggests that Mn(II) may be surrounded by four SO2 4 .
Optical absorption spectrum of chalcopyrite mineral is recorded in mull form from 200 to 800 nm and its peak fit analysis is shown in Fig. 5 It consists of bands at 16025, 27320, 35715, 36765, 44445 and 48310 cm1 whereas NIR spectrum is recorded in mull form in the range of 800–1500 nm, its peak fit analysis is shown in Fig. 6. It shows energies at 10780, 8400, 8245, 7215, 6955, 6760 cm1. Ferrous and ferric ion complexes derive strong bands in NIR spectrum. The bands at 8400, 10780, 16025 and 27320 cm1 in the UV–Vis, NIR regions are assigned to Cu(II) in rhombic symmetry. The general ordering of the energy levels for rhombic symmetry is as fol2 lows [26] A1g(dx2 y 2 ) < 2A2g(dz ) < 2B1g(dxy) < 2B2g(dxz) < 2B3g(dyz). Accordingly, the optical absorption bands observed in chalcopyrite mineral are 8400, 10780, 16025 and 27320 cm1 (Table 1). These energies are comparable with the other data reported for copper containing samples [27–31]. The appearance of two sharp bands at 8245 cm1 and 10780 cm1 indicate ferrous iron in the chalcopyrite mineral. The average of these bands 9513 cm1 is taken as 10 Dq band for Fe(II) ion and is assigned to 5T2g ? 5Eg(D) transition. Accordingly the Dq value obtained is 951 cm1. The splitting of 10 Dq band (10780 8245 = 2535 cm1) indicates that it is due to Jahn–Teller effect in the excited 5Eg state. The bands observed at 35715, 36765, 44445 and 48310 cm1 might be charge transfer bands. Fig. 5 clearly indicates charge transfer energies at 35715, 36765, 44445 and 48310 cm1. These bands are assigned to charge transfer transitions to metal ligand. NIR results The NIR spectrum shown in Fig. 6 consists of group of three sharp and strong bands located at 7215, 6955 and 6755 cm1.These energies are not d–d transitions. The OH- stretching mode gives rise to the most common features in near infrared region. Hydroxyl exists as part of the structure and the stretching
314
G. Udayabhaskar Reddy et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 96 (2012) 310–315
Fig. 7. Raman spectrum of chalcopyrite mineral at room temperature in the range 100–600 cm1.
Fig. 8. Raman spectrum of chalcopyrite mineral at room temperature in the range 600–1700 cm1.
mode appears whenever water is present in any form [the range 3645–3677 cm1]. The mOH overtone (2mOH) gives rise to a band in the NIR spectrum [32]. Accordingly the band observed at 7215 cm1 (3615 2 = 7230 = 2mOH) is assigned to the first overtone of OH. The other two energies observed at 6955 and 6755 cm1 are also due to molecular water. The band at 5017 cm1 is the combination of the frequency lattice modes [33]. The water present in the compound may be as adsorbed water.
low intensity sharp Raman band is observed at 430 cm1 and this is assigned to m2(Ag)SO4. The low energy Raman bands below 400 cm1 are attributable to Fe–O vibrations. Accordingly in the present sample very sharp high intensity bands observed at 380, 345 cm1 and a broad and weak band observed at 150 cm1 are due to Fe–O vibrations.
Raman spectral analysis Raman spectrum of chalcopyrite mineral recorded at RT in the range 100–600 cm1 is shown in Fig. 7 and in the range 600– 1700 shown in Fig. 8. The free (SO4)2 ion has ideal tetrahedral (Td) symmetry and gives four fundamental vibrations at 981 (m1), 451(m2), 1104 (m3), 613(m4) cm1. Of these, m1 mode is non-degenerate, m2 mode is doubly degenerate and m3 and m4 modes are triply degenerate. In solid sulfate compounds the degeneracies are often removed by lowering of symmetry imposed by the molecular environment [34,35]. For ideal tetrahedral (Td) symmetry all modes are Raman active while m1 and m2 are IR inactive. An independent sharp band (shown in Fig. 8) observed at 1020 cm1 is due to m1(Ag)SO4. Highest intensity band with weak shoulders on either side is observed at 1330 cm1 which is due to 2m4(E)SO4. A couple of bands observed at 1495 and 1620 cm1 overlapped by one other are attributed to OH stretch m2(H2O). A
Conclusions (1) XRD data indicate that pyrite [FeS2], is a iron sulfide with cubic cell constant a = 5.4179 Å with average particle grain size of 9.16 Å. (2) The EPR studies on pyrite confirm the presence of iron which is in very high concentration. Copper manganese is also present in traces. (3) Optical absorption spectrum of chalcopyrite is due to Fe(II)/ Cu(II) and they are in distorted octahedral symmetry. (4) Mid-infrared spectral studies indicate combination overtones and combination tones of water fundamentals and sulfate. (5) Raman spectral studies on chalcopyrite mineral is due to sulfate and water molecules.
Acknowledgements The authors are thankful to UGC, New Delhi for financial assistance (Major research Project No.: 38-188/2009).
G. Udayabhaskar Reddy et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 96 (2012) 310–315
References [1] K. Ellemer, H. Tributch, in: Proceedings of the 12th Workshop on ‘Quantum Solar Energy Conversion’, 2000. [2] A.V. Milovsky, O.V. Kanonov, Minerology, Mir Publishers, 1985. [3] W.A. Deer, R.A. Howie, J. Zussman, ELBS ed., London, 1978, pp. 445. [4] C. Kurylenkxo, Bull. Soc. Franc. Min. Crist. 65 (1942) 45. [5] Deo. Raj, K. Chandra, S.P. Puri, J. Phys. Soc. Jpn. 24 (1968) 39. [6] N.A. Eissa, H.A. Sallam, M.M. El-Ockr, E.A. Mahoud, S.A. Saleh, J. Phys. Colloq. C6 (Supplement Article No: 12) 37 (1976) C6–C793. [7] S.S. Udubasa, S. Constantinescu, M.N. Grecu, N. Popescu-Pgrion, Gh. Udubasa, Gh.C. Popescu, F. Tolea, I.V. Popescu, Rom. J. Phys. 52 (2007) 93. [8] Teruo Teranishi, Katsuaki Sato, Ken’ichi Kondo, J. Phys. Soc. Jpn. 36 (1974) 1618. [9] M. Annesuddin, P.N. Char, M. Raza Hussain, E.R. Saxena, J. Therm. Anal. 26 (1983) 205. [10] M.A. Hitachmann, T.D. Waite, Inorg. Chem. 15 (1976) 2150. [11] G.H. Faye, Can. Min. 9 (1968) 403. [12] C.J. Ballhausen, Introduction to Ligand Field Theory, Mc-Graw Hill Book Company, New Delhi, 1962. [13] F.A. Cotton, Md Megers, J. Am. Chem. Soc. 82 (1960) 5023. [14] O.G. Holmes, D.S. McClure, J. Chem. Phys. 26 (1957) 1686. [15] G.D. Jones, Phys. Rev. 155 (1967) 259. [16] G.H. Faye, Can. J. Earth Sci. 5 (1968) 3. [17] W. Low, M. Weger, Phys. Rev. B 118 (1960) 1130. [18] J.F. Rilley, Am. Mineral. 53 (1968) 293.
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[19] S.S. Udubase et al., Structural investigation on some sulphides from costesti, valea lul stan and jidoctita gold ores, 101. [20] B.D. Cullity, Elements of X-ray Diffraction, Addison Wesley, Reading, MA, 1959. [21] International Tables for X-ray Crystallography, vol. III, Kynoch Press, Birmingham, 1968, p. 210. [22] V. Carney, B. Petraova, M. Frumat, J. Non-Cryst. Solids 125 (1990) 17. [23] O. Matumura, J. Phys. Soc. Jpn. 14 (1959) 108. [24] E. Simanck, K.A. Muller, J. Phys. Chem. Solids 31 (1970) 1027. [25] A.M.F. Benial, V. Ramakrishnan, R. Murugesan, Spectrochim. Acta A 55 (1999) 2573. [26] M.A. Hitachmann, T.D. Waite, Inorg. Chem. 15 (1976) 2150. [27] R. Ramasubba Reddy, S. Lakshmi Reddy, G. Siva Reddy, B.J. Reddy, Cryst. Res. Technol. 37 (2002) 485. [28] K.B.N. Sharma, L.R. Murthy, B.J. Reddy, S. Vedanand, Phys. Lett. A 132 (1988) 293. [29] N. Madhu, A.V. Chandra Sekhar, B.J. Reddy, Y.P. Reddy, R.V.S.S.N. Ravikumar, Indian J. Chem. 38A (1999) 590. [30] S. Lakshmi Reddy, K. Ramesh, B.J. Reddy, 3rd Asia Pacific Physics Conference, 20–24 June, 1988, p. 994. [31] N.C. Gangi Reddy, R. Ramasubba Reddy, G. Siva Reddy, S. Lakshmi Reddy, B. Jagannatha Reddy, EPR, Optical absorption, MIR and Raman Spectral studies on Libethenite mineral, Cryst. Res. Technol. 41 (2006) 400. [32] G.R. Hunt, J.W. Salisbury, Mod. Geol. 1 (1970) 283. [33] G.R. Hunt, J.W. Salisbury, C. Lenhoff, Mod. Geol. 3 (1971) 1. [34] H.H. Adler, P.F. Kerr, Am. Mineral. 50 (1965) 132. [35] A. Hezel, S.D. Ross, Spectrochim. Acta A 22 (1966) 547.
Contents lists available at SciVerse ScienceDirect
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa
Electron paramagnetic resonance, optical absorption and Raman spectral studies on a pyrite/chalcopyrite mineral G. Udayabhaskar Reddy a, K. Seshamaheswaramma a, Yoshinobu Nakamura b, S. Lakshmi Reddy a,⇑, Ray L. Frost c,⇑, Tamio Endo d a
Dept. of Physics, S.V.D. College, Kadapa 516 003, India Dept. of Applied Chemistry, School of Engineering, The University of Tokyo, Japan Inorganic Materials Research Program, Queensland University of Technology, 2 George Street, Brisbane, G.P.O. Box 2434, Queensland 4001, Australia d Faculty of Engineering, Mie University, TSU, Mie 514 8507, Japan b c
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
" Pyrite has cubic crystal structure
with cell constant 5.45 Å. " EPR studies on chalcopyrite suggest
high percentage of Cu. " Optical absorption spectrum of
chalcopyrite is due to Fe(II) and Cu(II) which are in distorted octahedral symmetry. " Raman studies on chalcopyrite indicate that it has sulfate and water fundamentals.
a r t i c l e
i n f o
Article history: Received 24 October 2011 Received in revised form 6 March 2012 Accepted 25 April 2012 Available online 3 May 2012 Keywords: Pyrite/chalcopyrite XRD EPR Optical absorption spectra NIR spectra Raman spectrum
a b s t r a c t Pyrite and chalcopyrite mineral samples from Mangampet barite mine, Kadapa, Andhra Pradesh, India are used in the present study. XRD data indicate that the pyrite mineral has a face centered cubic lattice structure with lattice constant 5.4179 Å. Also it possesses an average particle size of 91.9 nm. An EPR study on the powdered samples confirms the presence of iron in pyrite and iron and Mn(II) in chalcopyrite. The optical absorption spectrum of chalcopyrite indicates presence of copper which is in a distorted octahedral environment. NIR results confirm the presence of water fundamentals and Raman spectrum reveals the presence of water and sulfate ions. Ó 2012 Elsevier B.V. All rights reserved.
Introduction Minerals of geological interest are structurally and chemically complex when compared with pure inorganic solids. Many spec-
troscopic methods have been utilized to study the presence of metal ions, their oxidation state, energetics, structure property relationships, etc., Mossbauer, optical absorption, XPS, NMR and EPR spectroscopic studies are employed for structural information.
⇑ Corresponding authors. Address: Queensland University of Technology, Faculty of Science and Technology, 2 George St., Brisbane, Queensland 4001, Australia. Tel.: +61 7 3138 2407; fax: +61 7 3138 1804 (R.L. Frost). E-mail addresses: [email protected] (S. Lakshmi Reddy), [email protected] (R.L. Frost). 1386-1425/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.saa.2012.04.093
311
G. Udayabhaskar Reddy et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 96 (2012) 310–315 Table 1 Comparison of energies of the bands with their assignments for Cu(II) in rhombic octahedral coordination with ground state 2A1g(dx2 y2 ). Sample
2
cm Antlerite Cu3SO4(OH)4 Turquoise CuAl6(PO4)(OH)84H2O ZPPH (ZnKPO4)6H2O Atacamite Cu2(OH)3Cl Libethenite Cu2PO4OH Chalcopyrite (Cu,Fe)S2
2
1
2
B1g(dXY)
A1g(dZ 2 )
1
2
B2g(dXZ) 1
B3g(dYZ) 1
Reference
nm
cm
Nm
cm
Nm
cm
Nm
8475
1180
910
1290 1242 1121 1190
1060 668 1040 971 846 928
10990
7750 8049 8920 8400
9435 14970 9613 10296 11820 10780
12117 11083 14925 16025
825 902 670 624
16390 18354 13330 15380 20450 27320
610 545 750 650 489 366
The pyrite group of minerals has the general formula AX2, where A can be Fe, Zn, Hg, Au, Co, Cu, Mn, Ni, Ir, Pd, Pt or Ru and X can be S, As, Sb, Bi, Se and Te. Among them pyrite is the common mineral. Pyrite is an iron sulfide with the formula FeS2 with S–S band. It is a semiconductor with band gap of 0.95 eV [1]. Pyrite readily changes by oxidation to iron sulfate or to the hydrated oxide of iron. The unit cell is a face centered cubic sub lattice into which sulfur ions are embedded. Pyrite is similar in structure to fluorite and NaCl with cell edge constant a = 5.42 Å. Molecular sul+2 fide ion (S2 2 ) is oriented along the axis of third order, while Fe ions are at the center of the octahedron [2]. Each iron atom is surrounded by six sulfur atoms at the corners of the octahedron [3]. Measurements of X-ray absorption edges on chalcopyrite suggest that copper is present in two valence states, so that resonance between Cu+ + Fe3+ and Cu2+ + Fe2+ may be presumed [4]. Mössbauer spectra of naturally occurring chalcopyrite have been studied over a temperature range 300–448 K [5]. Mössbauer studies on natural Egyptian chalcopyrite have reveal that most of the iron is in Fe+2 state [6]. X ray diffraction (XRD) on natural and synthetic pyrite minerals was also reported [7]. Absorption spectra of CuFeS2 and Fe-Doped CuAlS2 and CuGaS2 have been studied [8]. Optical absorption, electron paramagnetic resonance (EPR) and Raman spectral studies have not been studied on pyrite or chalcopyrite mineral originating from the barite mine of Magampet, Kodur of Kadapa district, India. In this study we report XRD, EPR, optical absorption and Raman spectral studies and relate these studies to the structure of the mineral. Experimental Pyrite and chalcopyrite are brownish yellow in color. It is evident from the chemical analysis that the mineral pyrite contains 56.8 wt.% of iron and chalcopyrite contains 26.8 wt.% of iron and 28.2 wt.% of copper [9]. X-ray powder diffraction pattern of pyrite is recorded using Philips X-ray diffractometer operated in reflection geometry at 30 mA, 40 kV with Cu-Ka (k = 1.54060 Å) source at 25 °C in the range of 10–75°. Data are collected using a continuous scan rate of 1°/ 2 min1, which are then, refined into 2 theta steps of 0.02°. EPR spectra of pyrite and chalcopyrite powdered samples are recorded both at room (RT) and liquid nitrogen temperature (LNT) on JEOL JES TE100 ESR spectrometer operating at X band frequency (t = 9.40531 GHz for pyrite and 9.40620 GHz for chalcopyrite) having a 100 kHz field modulation to obtain first derivative EPR spectrum. DPPH with a g value of 2.0036 is used for g factor calculation. Optical absorption spectrum of the chalcopyrite sample is recorded at room temperature on Carey 5E UV–Vis–NIR spectrophotometer in mull form in the range 200–2000 nm. The chalcopyrite powdered mineral sample was placed and oriented on the stage of an Olympus BHSM microscope, equipped with 10 and 50 objectives and a Renishaw 1000 Raman microscope system. Raman spectra were excited by He–Ne laser
[32] [33] [34] [35] [37] Present work
(633 nm) at a resolution of 2 cm1 in the range between 100 and 4000 cm1. Band component analysis was undertaken using the Jandel ‘‘PEAKFIT’’ software package which enabled the type of fitting function to be selected and specific parameters to be fixed or varied accordingly. Band fitting was carried out using a Lorentz–Gauss cross product function with a minimum number of component bands used for the fitting process. The Lorentz–Gauss ratio was maintained at values greater than 0.7 and fitting was undertaken until reproducible results were obtained with squared correlations of r2 greater than 0.9975.
Theory Mn(II), being a d5 ion, has total spin S = 5/2. This state splits into three Kramers’ doublets, ±5/2>, ±3/2> and ±1/2> separated by 4D and 2D respectively where D is the zero-field splitting parameter. The deviation from axial symmetry leads to a term known as E in the spin Hamiltonian. The value of E can be easily calculated from single crystal measurements. A non-zero value of E results in making the spectrum unsymmetrical about the central sextet. Cu(II) has an electronic configuration [Ar] 3d9. In an octahedral crystal field, the corresponding ground state electronic configuration is t2g6eg3 which yields 2Eg term. The excited electronic configuration t2g5eg4 corresponds to 2T2g term. Hence single electron transition 2Eg ? 2T2g is expected in an octahedral crystal field. Normally, the ground 2Eg state splits due to Jahn–Teller effect and hence lowering of symmetry is expected for Cu(II) ion. This state splits into 2B1g(dx2 y2 ) and 2A1g(dz2 ) states in tetragonal symmetry and the excited term 2T2g also splits into 2B2g(dxy) and 2Eg(dxz, dyz) levels. In rhombic field, 2Eg ground state splits into 2A1g(dx2 y2 ) and 2A2g(dz2 ) whereas 2T2g splits into 2B1g(dxy), 2B2g(dxz) and 2 B3g(dyz) states. Thus three bands are expected for tetragonal (C4v) symmetry and four bands are expected for rhombic (D2h) symmetry [10].
Fig. 1. Powder X-ray diffraction pattern [Cu Ka] for pyrite mineral at 25 °C.
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Here ‘d’ is bulk density (pyrite = 4.84 g/cm3). The calculated value of porosity percentage is 3.43. The grain size of the compound is evaluated from the line broadening of the peaks using Debye–Scherrer equation
DðhklÞ ¼ Fig. 2. EPR spectrum of pyrite mineral at RT (m = 9.40531 GHz).
The ground state configuration of Fe(II) ion is 3d6. In an octahedral field, assuming high spin state, the configuration is expressed as t 42g e2g . This configuration gives rise to electronic states 5T2g, 3Eg, 3 T2g and some more triplets and singlets of which 5T2g is the ground state. The other excited configurations, such as t32g e3g gives rise to a several triplet and singlet states and one quintet state designated as 5Eg. Thus the spin allowed transition 5T2g ? 5Eg, is expected to be strong and all other spin forbidden transitions are very weak [11,12]. Thus, the 5T2g ? 5Eg transition gives rise to an intense, but broad absorption band. Often this band splits into two in an octahedral environment. If the splitting is of the order of 2000 cm1, then it is due to static distortion of octahedron [13–15]. However, an intermediate value between 100 and 2000 cm1 indicates a dynamic Jahn–Teller effect in the excited 5 Eg state [16,17]. In the latter case, the energy level is split symmetrically to the center of gravity and the average of these values of these bands is to be taken as 10Dq value. Results and discussion X-ray diffraction results Fig. 1 shows the X-ray diffraction pattern of pyrite mineral recorded on Philips diffractrometer up to 25 °C. The peaks are characterized by using Scherrer formula. The powder diffraction pattern is similar to that of the spectra reported for pyrite [18]. This calculated unit cell value of 5.419 Å well agreed with reported value on pyrite sample [19]. This confirms that the sample is pyrite with cubic octahedral structure. The X-ray density ‘dx’ is calculated ZM using the formula [20] dx ¼ Na 3. Here ‘‘Z’’(4) represents the number of molecules in a unit cell of the pyrite lattice, M (119.98 g) is the molecular weight of the mineral, N is the Avogadro’s number and ‘a’ the lattice constant of the sample. The calculated value of X-ray density is 5.01195 g/cm3. This indicates that X-ray density depends on mass. The percentage porosity of each sample was calculated using the relation [21].
d 100: Percentage porosityðp%Þ ¼ 1 dx
0:9k b1=2 cos h
Here D is the average particle size of the crystal, k is the wavelength of incident X ray, h is the corresponding Bragg angle, b1=2 is the full width at the half maximum height (FWHM) of the peak. The average particle size of the crystal is calculated as 91.6 nm. EPR results The EPR spectrum of the pyrite mineral sample recorded at room temperature is shown in Fig. 2. A single peak with g = 2.38 is observed in the spectrum at room temperature. Any additional peaks are not seen even at low temperature. This might be due to the very high concentration of iron present in the mineral. Fig. 3 shows the EPR spectrum of chalcopyrite mineral recorded at room temperature in the range 0–500 mT. It shows various resonances with g values of 3.77, 3.19, 2.46, 2.46, 2.18 and a sextant hyperfine structure with g value of 1.997. The expanded version of the sextant of the sample is shown in Fig. 4. The spectrum consists of a high intense sextet with g = 1.997 and A = 7.90 mT. This indicates that more Mn(II) ions are present in the octahedral environment. Further the presence of resolved hyperfine structure at g = 1.997 strongly indicates that Mn(II) ions are isolated or significantly distant from each other in octahedral environment. The strong resonance line in lower field with g = 3.20 and other weak resonances with g values 3.77, 2.46 and 2.18 are also due Mn(II) in distorted octahedral crystalline field in the chalcopyrite. The lack of hyperfine splitting at g = 3.20, 3.77 resonance lines is due to fluctuations of the ligand field parameters in the vicinity of Mn(II) ion and random distribution of the structural distortions [22]. The hyperfine constant ‘A’ value provides a qualitative measure of the ionic nature of bonding with Mn(II) ion. The percentage of covalency of Mn–ligand bond has been calculated using ‘A’ (8.0 mT) value obtained from the EPR spectrum and Matumura’s plot [23]. Also the approximate value of hyperfine constant (A) is calculated by using covalency (C) equations [24,25].
Aiso ¼ ð2:04C 04:5Þ 104 cm1 . The value obtained is 91 104 cm1. This calculated value agrees well with the observed hyperfine constant for the sample indicating the ionic character for metal ligand bond in the mineral
Fig. 3. Powdered EPR spectrum of chalcopyrite mineral at RT (t = 9.40619).
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Fig. 4. Expanded version of chalcopyrite mineral at RT (t = 9.40620).
Fig. 6. NIR spectrum of chalcopyrite mineral at room temperature.
Optical absorption spectral analysis
Fig. 5. Optical absorption spectrum of chalcopyrite at room temperature.
under study. The number of ligands around Mn(II) ion is estimated using the covalency [24] equation for C
C¼
1 ½1 0:16ðX p X q Þ 0:035ðX p X q Þ2 n
Here XP and Xq represent electro-negativities of metal and ligand. Assuming Xp = XMn = 1.6 and Xq = XS = 3.5, the number of ligands (n) obtained are 16. This suggests that Mn(II) may be surrounded by four SO2 4 .
Optical absorption spectrum of chalcopyrite mineral is recorded in mull form from 200 to 800 nm and its peak fit analysis is shown in Fig. 5 It consists of bands at 16025, 27320, 35715, 36765, 44445 and 48310 cm1 whereas NIR spectrum is recorded in mull form in the range of 800–1500 nm, its peak fit analysis is shown in Fig. 6. It shows energies at 10780, 8400, 8245, 7215, 6955, 6760 cm1. Ferrous and ferric ion complexes derive strong bands in NIR spectrum. The bands at 8400, 10780, 16025 and 27320 cm1 in the UV–Vis, NIR regions are assigned to Cu(II) in rhombic symmetry. The general ordering of the energy levels for rhombic symmetry is as fol2 lows [26] A1g(dx2 y 2 ) < 2A2g(dz ) < 2B1g(dxy) < 2B2g(dxz) < 2B3g(dyz). Accordingly, the optical absorption bands observed in chalcopyrite mineral are 8400, 10780, 16025 and 27320 cm1 (Table 1). These energies are comparable with the other data reported for copper containing samples [27–31]. The appearance of two sharp bands at 8245 cm1 and 10780 cm1 indicate ferrous iron in the chalcopyrite mineral. The average of these bands 9513 cm1 is taken as 10 Dq band for Fe(II) ion and is assigned to 5T2g ? 5Eg(D) transition. Accordingly the Dq value obtained is 951 cm1. The splitting of 10 Dq band (10780 8245 = 2535 cm1) indicates that it is due to Jahn–Teller effect in the excited 5Eg state. The bands observed at 35715, 36765, 44445 and 48310 cm1 might be charge transfer bands. Fig. 5 clearly indicates charge transfer energies at 35715, 36765, 44445 and 48310 cm1. These bands are assigned to charge transfer transitions to metal ligand. NIR results The NIR spectrum shown in Fig. 6 consists of group of three sharp and strong bands located at 7215, 6955 and 6755 cm1.These energies are not d–d transitions. The OH- stretching mode gives rise to the most common features in near infrared region. Hydroxyl exists as part of the structure and the stretching
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Fig. 7. Raman spectrum of chalcopyrite mineral at room temperature in the range 100–600 cm1.
Fig. 8. Raman spectrum of chalcopyrite mineral at room temperature in the range 600–1700 cm1.
mode appears whenever water is present in any form [the range 3645–3677 cm1]. The mOH overtone (2mOH) gives rise to a band in the NIR spectrum [32]. Accordingly the band observed at 7215 cm1 (3615 2 = 7230 = 2mOH) is assigned to the first overtone of OH. The other two energies observed at 6955 and 6755 cm1 are also due to molecular water. The band at 5017 cm1 is the combination of the frequency lattice modes [33]. The water present in the compound may be as adsorbed water.
low intensity sharp Raman band is observed at 430 cm1 and this is assigned to m2(Ag)SO4. The low energy Raman bands below 400 cm1 are attributable to Fe–O vibrations. Accordingly in the present sample very sharp high intensity bands observed at 380, 345 cm1 and a broad and weak band observed at 150 cm1 are due to Fe–O vibrations.
Raman spectral analysis Raman spectrum of chalcopyrite mineral recorded at RT in the range 100–600 cm1 is shown in Fig. 7 and in the range 600– 1700 shown in Fig. 8. The free (SO4)2 ion has ideal tetrahedral (Td) symmetry and gives four fundamental vibrations at 981 (m1), 451(m2), 1104 (m3), 613(m4) cm1. Of these, m1 mode is non-degenerate, m2 mode is doubly degenerate and m3 and m4 modes are triply degenerate. In solid sulfate compounds the degeneracies are often removed by lowering of symmetry imposed by the molecular environment [34,35]. For ideal tetrahedral (Td) symmetry all modes are Raman active while m1 and m2 are IR inactive. An independent sharp band (shown in Fig. 8) observed at 1020 cm1 is due to m1(Ag)SO4. Highest intensity band with weak shoulders on either side is observed at 1330 cm1 which is due to 2m4(E)SO4. A couple of bands observed at 1495 and 1620 cm1 overlapped by one other are attributed to OH stretch m2(H2O). A
Conclusions (1) XRD data indicate that pyrite [FeS2], is a iron sulfide with cubic cell constant a = 5.4179 Å with average particle grain size of 9.16 Å. (2) The EPR studies on pyrite confirm the presence of iron which is in very high concentration. Copper manganese is also present in traces. (3) Optical absorption spectrum of chalcopyrite is due to Fe(II)/ Cu(II) and they are in distorted octahedral symmetry. (4) Mid-infrared spectral studies indicate combination overtones and combination tones of water fundamentals and sulfate. (5) Raman spectral studies on chalcopyrite mineral is due to sulfate and water molecules.
Acknowledgements The authors are thankful to UGC, New Delhi for financial assistance (Major research Project No.: 38-188/2009).
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